Introduce seccomp-assisted syscall filtering

[strace] / strace.c

/*
 * Copyright (c) 1991, 1992 Paul Kranenburg <pk@cs.few.eur.nl>
 * Copyright (c) 1993 Branko Lankester <branko@hacktic.nl>
 * Copyright (c) 1993, 1994, 1995, 1996 Rick Sladkey <jrs@world.std.com>
 * Copyright (c) 1996-1999 Wichert Akkerman <wichert@cistron.nl>
 * Copyright (c) 1999-2019 The strace developers.
 * All rights reserved.
 *
 * SPDX-License-Identifier: LGPL-2.1-or-later
 */

#include "defs.h"
#include <stdarg.h>
#include <limits.h>
#include <fcntl.h>
#include "ptrace.h"
#include <signal.h>
#include <sys/resource.h>
#include <sys/stat.h>
#ifdef HAVE_PATHS_H
# include <paths.h>
#endif
#include <getopt.h>
#include <pwd.h>
#include <grp.h>
#include <dirent.h>
#include <locale.h>
#include <sys/utsname.h>
#ifdef HAVE_PRCTL
# include <sys/prctl.h>
#endif

#include "kill_save_errno.h"
#include "filter_seccomp.h"
#include "largefile_wrappers.h"
#include "mmap_cache.h"
#include "number_set.h"
#include "ptrace_syscall_info.h"
#include "scno.h"
#include "printsiginfo.h"
#include "trace_event.h"
#include "xstring.h"
#include "delay.h"
#include "wait.h"

/* In some libc, these aren't declared. Do it ourself: */
extern char **environ;
extern int optind;
extern char *optarg;

#ifdef ENABLE_STACKTRACE
/* if this is true do the stack trace for every system call */
bool stack_trace_enabled;
#endif

#define my_tkill(tid, sig) syscall(__NR_tkill, (tid), (sig))

/* Glue for systems without a MMU that cannot provide fork() */
#if !defined(HAVE_FORK)
# undef NOMMU_SYSTEM
# define NOMMU_SYSTEM 1
#endif
#if NOMMU_SYSTEM
# define fork() vfork()
#endif

const unsigned int syscall_trap_sig = SIGTRAP | 0x80;

cflag_t cflag = CFLAG_NONE;
unsigned int followfork;
unsigned int ptrace_setoptions = PTRACE_O_TRACESYSGOOD | PTRACE_O_TRACEEXEC
                                 | PTRACE_O_TRACEEXIT;
unsigned int xflag;
bool debug_flag;
bool Tflag;
bool iflag;
bool count_wallclock;
unsigned int qflag;
static unsigned int tflag;
static bool rflag;
static bool print_pid_pfx;

/* -I n */
enum {
        INTR_NOT_SET        = 0,
        INTR_ANYWHERE       = 1, /* don't block/ignore any signals */
        INTR_WHILE_WAIT     = 2, /* block fatal signals while decoding syscall. default */
        INTR_NEVER          = 3, /* block fatal signals. default if '-o FILE PROG' */
        INTR_BLOCK_TSTP_TOO = 4, /* block fatal signals and SIGTSTP (^Z); default if -D */
        NUM_INTR_OPTS
};
static int opt_intr;
/* We play with signal mask only if this mode is active: */
#define interactive (opt_intr == INTR_WHILE_WAIT)

/*
 * daemonized_tracer supports -D option.
 * With this option, strace forks twice.
 * Unlike normal case, with -D *grandparent* process exec's,
 * becoming a traced process. Child exits (this prevents traced process
 * from having children it doesn't expect to have), and grandchild
 * attaches to grandparent similarly to strace -p PID.
 * This allows for more transparent interaction in cases
 * when process and its parent are communicating via signals,
 * wait() etc. Without -D, strace process gets lodged in between,
 * disrupting parent<->child link.
 */
static bool daemonized_tracer;

static int post_attach_sigstop = TCB_IGNORE_ONE_SIGSTOP;
#define use_seize (post_attach_sigstop == 0)

/* Show path associated with fd arguments */
unsigned int show_fd_path;

static bool detach_on_execve;

static int exit_code;
static int strace_child;
static int strace_tracer_pid;

static const char *username;
static uid_t run_uid;
static gid_t run_gid;

unsigned int max_strlen = DEFAULT_STRLEN;
static int acolumn = DEFAULT_ACOLUMN;
static char *acolumn_spaces;

/* Default output style for xlat entities */
enum xlat_style xlat_verbosity = XLAT_STYLE_ABBREV;

static const char *outfname;
/* If -ff, points to stderr. Else, it's our common output log */
static FILE *shared_log;
static bool open_append;

struct tcb *printing_tcp;
static struct tcb *current_tcp;

struct tcb_wait_data {
        enum trace_event te; /**< Event passed to dispatch_event() */
        int status;          /**< status, returned by wait4() */
        unsigned long msg;   /**< Value returned by PTRACE_GETEVENTMSG */
        siginfo_t si;        /**< siginfo, returned by PTRACE_GETSIGINFO */
};

static struct tcb **tcbtab;
static unsigned int nprocs;
static size_t tcbtabsize;

static struct tcb_wait_data *tcb_wait_tab;
static size_t tcb_wait_tab_size;


#ifndef HAVE_PROGRAM_INVOCATION_NAME
char *program_invocation_name;
#endif

unsigned os_release; /* generated from uname()'s u.release */

static void detach(struct tcb *tcp);
static void cleanup(int sig);
static void interrupt(int sig);

#ifdef HAVE_SIG_ATOMIC_T
static volatile sig_atomic_t interrupted, restart_failed;
#else
static volatile int interrupted, restart_failed;
#endif

static sigset_t timer_set;
static void timer_sighandler(int);

#ifndef HAVE_STRERROR

# if !HAVE_DECL_SYS_ERRLIST
extern int sys_nerr;
extern char *sys_errlist[];
# endif

const char *
strerror(int err_no)
{
        static char buf[sizeof("Unknown error %d") + sizeof(int)*3];

        if (err_no < 1 || err_no >= sys_nerr) {
                xsprintf(buf, "Unknown error %d", err_no);
                return buf;
        }
        return sys_errlist[err_no];
}

#endif /* HAVE_STERRROR */

static void
print_version(void)
{
        static const char features[] =
#ifdef ENABLE_STACKTRACE
                " stack-trace=" USE_UNWINDER
#endif
#ifdef USE_DEMANGLE
                " stack-demangle"
#endif
#if SUPPORTED_PERSONALITIES > 1
# if defined HAVE_M32_MPERS
                " m32-mpers"
# else
                " no-m32-mpers"
# endif
#endif /* SUPPORTED_PERSONALITIES > 1 */
#if SUPPORTED_PERSONALITIES > 2
# if defined HAVE_MX32_MPERS
                " mx32-mpers"
# else
                " no-mx32-mpers"
# endif
#endif /* SUPPORTED_PERSONALITIES > 2 */
                "";

        printf("%s -- version %s\n"
               "Copyright (c) 1991-%s The strace developers <%s>.\n"
               "This is free software; see the source for copying conditions.  There is NO\n"
               "warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.\n",
               PACKAGE_NAME, PACKAGE_VERSION, COPYRIGHT_YEAR, PACKAGE_URL);
        printf("\nOptional features enabled:%s\n",
               features[0] ? features : " (none)");
}

static void
usage(void)
{
#ifdef ENABLE_STACKTRACE
# define K_OPT "k"
#else
# define K_OPT ""
#endif

        printf("\
usage: strace [-ACdffhi" K_OPT "qqrtttTvVwxxyyzZ] [-I n] [-b execve] [-e expr]...\n\
              [-a column] [-o file] [-s strsize] [-X format] [-P path]...\n\
              [-p pid]... [--seccomp-bpf]\n\
              { -p pid | [-D] [-E var=val]... [-u username] PROG [ARGS] }\n\
   or: strace -c[dfwzZ] [-I n] [-b execve] [-e expr]... [-O overhead]\n\
              [-S sortby] [-P path]... [-p pid]... [--seccomp-bpf]\n\
              { -p pid | [-D] [-E var=val]... [-u username] PROG [ARGS] }\n\
\n\
Output format:\n\
  -A             open the file provided in the -o option in append mode\n\
  -a column      alignment COLUMN for printing syscall results (default %d)\n\
  -i             print instruction pointer at time of syscall\n\
"
#ifdef ENABLE_STACKTRACE
"\
  -k             obtain stack trace between each syscall\n\
"
#endif
"\
  -o file        send trace output to FILE instead of stderr\n\
  -q             suppress messages about attaching, detaching, etc.\n\
  -qq            suppress messages about process exit status as well.\n\
  -r             print relative timestamp\n\
  -s strsize     limit length of print strings to STRSIZE chars (default %d)\n\
  -t             print absolute timestamp\n\
  -tt            print absolute timestamp with usecs\n\
  -T             print time spent in each syscall\n\
  -v             verbose mode: print entities unabbreviated\n\
  -x             print non-ascii strings in hex\n\
  -xx            print all strings in hex\n\
  -X format      set the format for printing of named constants and flags\n\
  -y             print paths associated with file descriptor arguments\n\
  -yy            print protocol specific information associated with socket\n\
                 file descriptors\n\
\n\
Statistics:\n\
  -c             count time, calls, and errors for each syscall and report\n\
                 summary\n\
  -C             like -c but also print regular output\n\
  -O overhead    set overhead for tracing syscalls to OVERHEAD usecs\n\
  -S sortby      sort syscall counts by: time, calls, errors, name, nothing\n\
                 (default %s)\n\
  -w             summarise syscall latency (default is system time)\n\
\n\
Filtering:\n\
  -e expr        a qualifying expression: option=[!]all or option=[!]val1[,val2]...\n\
     options:    trace, abbrev, verbose, raw, signal, read, write, fault,\n\
                 inject, status, kvm\n\
  -P path        trace accesses to path\n\
  -z             print only syscalls that returned without an error code\n\
  -Z             print only syscalls that returned with an error code\n\
\n\
Tracing:\n\
  -b execve      detach on execve syscall\n\
  -D             run tracer process as a detached grandchild, not as parent\n\
  -f             follow forks\n\
  -ff            follow forks with output into separate files\n\
  -I interruptible\n\
     1:          no signals are blocked\n\
     2:          fatal signals are blocked while decoding syscall (default)\n\
     3:          fatal signals are always blocked (default if '-o FILE PROG')\n\
     4:          fatal signals and SIGTSTP (^Z) are always blocked\n\
                 (useful to make 'strace -o FILE PROG' not stop on ^Z)\n\
\n\
Startup:\n\
  -E var         remove var from the environment for command\n\
  -E var=val     put var=val in the environment for command\n\
  -p pid         trace process with process id PID, may be repeated\n\
  -u username    run command as username handling setuid and/or setgid\n\
\n\
Miscellaneous:\n\
  --seccomp-bpf  enable seccomp-bpf filtering\n\
  -d             enable debug output to stderr\n\
  -h, --help     print help message\n\
  -V, --version  print version\n\
"
/* ancient, no one should use it
-F -- attempt to follow vforks (deprecated, use -f)\n\
 */
, DEFAULT_ACOLUMN, DEFAULT_STRLEN, DEFAULT_SORTBY);
        exit(0);

#undef K_OPT
}

void ATTRIBUTE_NORETURN
die(void)
{
        if (strace_tracer_pid == getpid()) {
                cleanup(0);
                exit(1);
        }

        _exit(1);
}

static void
error_opt_arg(int opt, const char *arg)
{
        error_msg_and_help("invalid -%c argument: '%s'", opt, arg);
}

static const char *ptrace_attach_cmd;

static int
ptrace_attach_or_seize(int pid)
{
        int r;
        if (!use_seize)
                return ptrace_attach_cmd = "PTRACE_ATTACH",
                       ptrace(PTRACE_ATTACH, pid, 0L, 0L);
        r = ptrace(PTRACE_SEIZE, pid, 0L, (unsigned long) ptrace_setoptions);
        if (r)
                return ptrace_attach_cmd = "PTRACE_SEIZE", r;
        r = ptrace(PTRACE_INTERRUPT, pid, 0L, 0L);
        return ptrace_attach_cmd = "PTRACE_INTERRUPT", r;
}

static const char *
ptrace_op_str(unsigned int op)
{
        const char *str = xlookup(ptrace_cmds, op);
        if (str)
                return str;

        static char buf[sizeof(op) * 3];
        xsprintf(buf, "%u", op);
        return buf;
}

/*
 * Used when we want to unblock stopped traced process.
 * Should be only used with PTRACE_CONT, PTRACE_DETACH and PTRACE_SYSCALL.
 * Returns 0 on success or if error was ESRCH
 * (presumably process was killed while we talk to it).
 * Otherwise prints error message and returns -1.
 */
static int
ptrace_restart(const unsigned int op, struct tcb *const tcp, unsigned int sig)
{
        int err;

        errno = 0;
        ptrace(op, tcp->pid, 0L, (unsigned long) sig);
        err = errno;
        if (!err || err == ESRCH)
                return 0;

        /*
         * Why curcol != 0? Otherwise sometimes we get this:
         *
         * 10252 kill(10253, SIGKILL)              = 0
         *  <ptrace(SYSCALL,10252):No such process>10253 ...next decode...
         *
         * 10252 died after we retrieved syscall exit data,
         * but before we tried to restart it. Log looks ugly.
         */
        if (current_tcp && current_tcp->curcol != 0) {
                tprintf(" <Cannot restart pid %d with ptrace(%s): %s>\n",
                        tcp->pid, ptrace_op_str(op), strerror(err));
                line_ended();
        }
        errno = err;
        perror_msg("ptrace(%s,pid:%d,sig:%u)",
                   ptrace_op_str(op), tcp->pid, sig);
        return -1;
}

static void
set_cloexec_flag(int fd)
{
        int flags, newflags;

        flags = fcntl(fd, F_GETFD);
        if (flags < 0) {
                /* Can happen only if fd is bad.
                 * Should never happen: if it does, we have a bug
                 * in the caller. Therefore we just abort
                 * instead of propagating the error.
                 */
                perror_msg_and_die("fcntl(%d, F_GETFD)", fd);
        }

        newflags = flags | FD_CLOEXEC;
        if (flags == newflags)
                return;

        if (fcntl(fd, F_SETFD, newflags)) /* never fails */
                perror_msg_and_die("fcntl(%d, F_SETFD, %#x)", fd, newflags);
}

/*
 * When strace is setuid executable, we have to swap uids
 * before and after filesystem and process management operations.
 */
static void
swap_uid(void)
{
        int euid = geteuid(), uid = getuid();

        if (euid != uid && setreuid(euid, uid) < 0) {
                perror_msg_and_die("setreuid");
        }
}

static FILE *
strace_fopen(const char *path)
{
        FILE *fp;

        swap_uid();
        fp = fopen_stream(path, open_append ? "a" : "w");
        if (!fp)
                perror_msg_and_die("Can't fopen '%s'", path);
        swap_uid();
        set_cloexec_flag(fileno(fp));
        return fp;
}

static int popen_pid;

#ifndef _PATH_BSHELL
# define _PATH_BSHELL "/bin/sh"
#endif

/*
 * We cannot use standard popen(3) here because we have to distinguish
 * popen child process from other processes we trace, and standard popen(3)
 * does not export its child's pid.
 */
static FILE *
strace_popen(const char *command)
{
        FILE *fp;
        int pid;
        int fds[2];

        swap_uid();
        if (pipe(fds) < 0)
                perror_msg_and_die("pipe");

        set_cloexec_flag(fds[1]); /* never fails */

        pid = vfork();
        if (pid < 0)
                perror_msg_and_die("vfork");

        if (pid == 0) {
                /* child */
                close(fds[1]);
                if (fds[0] != 0) {
                        if (dup2(fds[0], 0))
                                perror_msg_and_die("dup2");
                        close(fds[0]);
                }
                execl(_PATH_BSHELL, "sh", "-c", command, NULL);
                perror_msg_and_die("Can't execute '%s'", _PATH_BSHELL);
        }

        /* parent */
        popen_pid = pid;
        close(fds[0]);
        swap_uid();
        fp = fdopen(fds[1], "w");
        if (!fp)
                perror_msg_and_die("fdopen");
        return fp;
}

static void
outf_perror(const struct tcb * const tcp)
{
        if (tcp->outf == stderr)
                return;

        /* This is ugly, but we don't store separate file names */
        if (followfork >= 2)
                perror_msg("%s.%u", outfname, tcp->pid);
        else
                perror_msg("%s", outfname);
}

ATTRIBUTE_FORMAT((printf, 1, 0))
static void
tvprintf(const char *const fmt, va_list args)
{
        if (current_tcp) {
                int n = vfprintf(current_tcp->outf, fmt, args);
                if (n < 0) {
                        /* very unlikely due to vfprintf buffering */
                        outf_perror(current_tcp);
                } else
                        current_tcp->curcol += n;
        }
}

void
tprintf(const char *fmt, ...)
{
        va_list args;
        va_start(args, fmt);
        tvprintf(fmt, args);
        va_end(args);
}

#ifndef HAVE_FPUTS_UNLOCKED
# define fputs_unlocked fputs
#endif

void
tprints(const char *str)
{
        if (current_tcp) {
                int n = fputs_unlocked(str, current_tcp->outf);
                if (n >= 0) {
                        current_tcp->curcol += strlen(str);
                        return;
                }
                /* very unlikely due to fputs_unlocked buffering */
                outf_perror(current_tcp);
        }
}

void
tprints_comment(const char *const str)
{
        if (str && *str)
                tprintf(" /* %s */", str);
}

void
tprintf_comment(const char *fmt, ...)
{
        if (!fmt || !*fmt)
                return;

        va_list args;
        va_start(args, fmt);
        tprints(" /* ");
        tvprintf(fmt, args);
        tprints(" */");
        va_end(args);
}

static void
flush_tcp_output(const struct tcb *const tcp)
{
        if (fflush(tcp->outf))
                outf_perror(tcp);
}

void
line_ended(void)
{
        if (current_tcp) {
                current_tcp->curcol = 0;
                flush_tcp_output(current_tcp);
        }
        if (printing_tcp) {
                printing_tcp->curcol = 0;
                printing_tcp = NULL;
        }
}

void
set_current_tcp(const struct tcb *tcp)
{
        current_tcp = (struct tcb *) tcp;

        /* Sync current_personality and stuff */
        if (current_tcp)
                set_personality(current_tcp->currpers);
}

void
printleader(struct tcb *tcp)
{
        /* If -ff, "previous tcb we printed" is always the same as current,
         * because we have per-tcb output files.
         */
        if (followfork >= 2)
                printing_tcp = tcp;

        if (printing_tcp) {
                set_current_tcp(printing_tcp);
                if (!tcp->staged_output_data && printing_tcp->curcol != 0 &&
                    (followfork < 2 || printing_tcp == tcp)) {
                        /*
                         * case 1: we have a shared log (i.e. not -ff), and last line
                         * wasn't finished (same or different tcb, doesn't matter).
                         * case 2: split log, we are the same tcb, but our last line
                         * didn't finish ("SIGKILL nuked us after syscall entry" etc).
                         */
                        tprints(" <unfinished ...>\n");
                        printing_tcp->curcol = 0;
                }
        }

        printing_tcp = tcp;
        set_current_tcp(tcp);
        current_tcp->curcol = 0;

        if (print_pid_pfx)
                tprintf("%-5d ", tcp->pid);
        else if (nprocs > 1 && !outfname)
                tprintf("[pid %5u] ", tcp->pid);

        if (tflag) {
                struct timespec ts;
                clock_gettime(CLOCK_REALTIME, &ts);

                if (tflag > 2) {
                        tprintf("%lld.%06ld ",
                                (long long) ts.tv_sec, (long) ts.tv_nsec / 1000);
                } else {
                        time_t local = ts.tv_sec;
                        char str[MAX(sizeof("HH:MM:SS"), sizeof(ts.tv_sec) * 3)];
                        struct tm *tm = localtime(&local);

                        if (tm)
                                strftime(str, sizeof(str), "%T", tm);
                        else
                                xsprintf(str, "%lld", (long long) local);
                        if (tflag > 1)
                                tprintf("%s.%06ld ",
                                        str, (long) ts.tv_nsec / 1000);
                        else
                                tprintf("%s ", str);
                }
        }

        if (rflag) {
                struct timespec ts;
                clock_gettime(CLOCK_MONOTONIC, &ts);

                static struct timespec ots;
                if (ots.tv_sec == 0)
                        ots = ts;

                struct timespec dts;
                ts_sub(&dts, &ts, &ots);
                ots = ts;

                tprintf("%s%6ld.%06ld%s ",
                        tflag ? "(+" : "",
                        (long) dts.tv_sec, (long) dts.tv_nsec / 1000,
                        tflag ? ")" : "");
        }

        if (iflag)
                print_instruction_pointer(tcp);
}

void
tabto(void)
{
        if (current_tcp->curcol < acolumn)
                tprints(acolumn_spaces + current_tcp->curcol);
}

/* Should be only called directly *after successful attach* to a tracee.
 * Otherwise, "strace -oFILE -ff -p<nonexistant_pid>"
 * may create bogus empty FILE.<nonexistant_pid>, and then die.
 */
static void
after_successful_attach(struct tcb *tcp, const unsigned int flags)
{
        tcp->flags |= TCB_ATTACHED | TCB_STARTUP | flags;
        tcp->outf = shared_log; /* if not -ff mode, the same file is for all */
        if (followfork >= 2) {
                char name[PATH_MAX];
                xsprintf(name, "%s.%u", outfname, tcp->pid);
                tcp->outf = strace_fopen(name);
        }

#ifdef ENABLE_STACKTRACE
        if (stack_trace_enabled)
                unwind_tcb_init(tcp);
#endif
}

static void
expand_tcbtab(void)
{
        /* Allocate some (more) TCBs (and expand the table).
           We don't want to relocate the TCBs because our
           callers have pointers and it would be a pain.
           So tcbtab is a table of pointers.  Since we never
           free the TCBs, we allocate a single chunk of many.  */
        size_t old_tcbtabsize;
        struct tcb *newtcbs;
        struct tcb **tcb_ptr;

        old_tcbtabsize = tcbtabsize;

        tcbtab = xgrowarray(tcbtab, &tcbtabsize, sizeof(tcbtab[0]));
        newtcbs = xcalloc(tcbtabsize - old_tcbtabsize, sizeof(newtcbs[0]));

        for (tcb_ptr = tcbtab + old_tcbtabsize;
            tcb_ptr < tcbtab + tcbtabsize; tcb_ptr++, newtcbs++)
                *tcb_ptr = newtcbs;
}

static struct tcb *
alloctcb(int pid)
{
        unsigned int i;
        struct tcb *tcp;

        if (nprocs == tcbtabsize)
                expand_tcbtab();

        for (i = 0; i < tcbtabsize; i++) {
                tcp = tcbtab[i];
                if (!tcp->pid) {
                        memset(tcp, 0, sizeof(*tcp));
                        list_init(&tcp->wait_list);
                        tcp->pid = pid;
#if SUPPORTED_PERSONALITIES > 1
                        tcp->currpers = current_personality;
#endif
                        nprocs++;
                        debug_msg("new tcb for pid %d, active tcbs:%d",
                                  tcp->pid, nprocs);
                        return tcp;
                }
        }
        error_msg_and_die("bug in alloctcb");
}

void *
get_tcb_priv_data(const struct tcb *tcp)
{
        return tcp->_priv_data;
}

int
set_tcb_priv_data(struct tcb *tcp, void *const priv_data,
                  void (*const free_priv_data)(void *))
{
        if (tcp->_priv_data)
                return -1;

        tcp->_free_priv_data = free_priv_data;
        tcp->_priv_data = priv_data;

        return 0;
}

void
free_tcb_priv_data(struct tcb *tcp)
{
        if (tcp->_priv_data) {
                if (tcp->_free_priv_data) {
                        tcp->_free_priv_data(tcp->_priv_data);
                        tcp->_free_priv_data = NULL;
                }
                tcp->_priv_data = NULL;
        }
}

static void
droptcb(struct tcb *tcp)
{
        if (tcp->pid == 0)
                return;

        if (cflag && debug_flag) {
                struct timespec dt;

                ts_sub(&dt, &tcp->stime, &tcp->atime);
                debug_func_msg("pid %d: %.9f seconds of system time spent "
                               "since attach", tcp->pid, ts_float(&dt));
        }

        int p;
        for (p = 0; p < SUPPORTED_PERSONALITIES; ++p)
                free(tcp->inject_vec[p]);

        free_tcb_priv_data(tcp);

#ifdef ENABLE_STACKTRACE
        if (stack_trace_enabled)
                unwind_tcb_fin(tcp);
#endif

#ifdef HAVE_LINUX_KVM_H
        kvm_vcpu_info_free(tcp);
#endif

        if (tcp->mmap_cache)
                tcp->mmap_cache->free_fn(tcp, __func__);

        nprocs--;
        debug_msg("dropped tcb for pid %d, %d remain", tcp->pid, nprocs);

        if (tcp->outf) {
                bool publish = true;
                if (!is_complete_set(status_set, NUMBER_OF_STATUSES)) {
                        publish = is_number_in_set(STATUS_DETACHED, status_set);
                        strace_close_memstream(tcp, publish);
                }

                if (followfork >= 2) {
                        if (tcp->curcol != 0 && publish)
                                fprintf(tcp->outf, " <detached ...>\n");
                        fclose(tcp->outf);
                } else {
                        if (printing_tcp == tcp && tcp->curcol != 0 && publish)
                                fprintf(tcp->outf, " <detached ...>\n");
                        flush_tcp_output(tcp);
                }
        }

        if (current_tcp == tcp)
                set_current_tcp(NULL);
        if (printing_tcp == tcp)
                printing_tcp = NULL;

        list_remove(&tcp->wait_list);

        memset(tcp, 0, sizeof(*tcp));
}

/* Detach traced process.
 * Never call DETACH twice on the same process as both unattached and
 * attached-unstopped processes give the same ESRCH.  For unattached process we
 * would SIGSTOP it and wait for its SIGSTOP notification forever.
 */
static void
detach(struct tcb *tcp)
{
        int error;
        int status;

        /*
         * Linux wrongly insists the child be stopped
         * before detaching.  Arghh.  We go through hoops
         * to make a clean break of things.
         */

        if (!(tcp->flags & TCB_ATTACHED))
                goto drop;

        /* We attached but possibly didn't see the expected SIGSTOP.
         * We must catch exactly one as otherwise the detached process
         * would be left stopped (process state T).
         */
        if (tcp->flags & TCB_IGNORE_ONE_SIGSTOP)
                goto wait_loop;

        error = ptrace(PTRACE_DETACH, tcp->pid, 0, 0);
        if (!error) {
                /* On a clear day, you can see forever. */
                goto drop;
        }
        if (errno != ESRCH) {
                /* Shouldn't happen. */
                perror_func_msg("ptrace(PTRACE_DETACH,%u)", tcp->pid);
                goto drop;
        }
        /* ESRCH: process is either not stopped or doesn't exist. */
        if (my_tkill(tcp->pid, 0) < 0) {
                if (errno != ESRCH)
                        /* Shouldn't happen. */
                        perror_func_msg("tkill(%u,0)", tcp->pid);
                /* else: process doesn't exist. */
                goto drop;
        }
        /* Process is not stopped, need to stop it. */
        if (use_seize) {
                /*
                 * With SEIZE, tracee can be in group-stop already.
                 * In this state sending it another SIGSTOP does nothing.
                 * Need to use INTERRUPT.
                 * Testcase: trying to ^C a "strace -p <stopped_process>".
                 */
                error = ptrace(PTRACE_INTERRUPT, tcp->pid, 0, 0);
                if (!error)
                        goto wait_loop;
                if (errno != ESRCH)
                        perror_func_msg("ptrace(PTRACE_INTERRUPT,%u)", tcp->pid);
        } else {
                error = my_tkill(tcp->pid, SIGSTOP);
                if (!error)
                        goto wait_loop;
                if (errno != ESRCH)
                        perror_func_msg("tkill(%u,SIGSTOP)", tcp->pid);
        }
        /* Either process doesn't exist, or some weird error. */
        goto drop;

 wait_loop:
        /* We end up here in three cases:
         * 1. We sent PTRACE_INTERRUPT (use_seize case)
         * 2. We sent SIGSTOP (!use_seize)
         * 3. Attach SIGSTOP was already pending (TCB_IGNORE_ONE_SIGSTOP set)
         */
        for (;;) {
                unsigned int sig;
                if (waitpid(tcp->pid, &status, __WALL) < 0) {
                        if (errno == EINTR)
                                continue;
                        /*
                         * if (errno == ECHILD) break;
                         * ^^^  WRONG! We expect this PID to exist,
                         * and want to emit a message otherwise:
                         */
                        perror_func_msg("waitpid(%u)", tcp->pid);
                        break;
                }
                if (!WIFSTOPPED(status)) {
                        /*
                         * Tracee exited or was killed by signal.
                         * We shouldn't normally reach this place:
                         * we don't want to consume exit status.
                         * Consider "strace -p PID" being ^C-ed:
                         * we want merely to detach from PID.
                         *
                         * However, we _can_ end up here if tracee
                         * was SIGKILLed.
                         */
                        break;
                }
                sig = WSTOPSIG(status);
                debug_msg("detach wait: event:%d sig:%d",
                          (unsigned) status >> 16, sig);
                if (use_seize) {
                        unsigned event = (unsigned)status >> 16;
                        if (event == PTRACE_EVENT_STOP /*&& sig == SIGTRAP*/) {
                                /*
                                 * sig == SIGTRAP: PTRACE_INTERRUPT stop.
                                 * sig == other: process was already stopped
                                 * with this stopping sig (see tests/detach-stopped).
                                 * Looks like re-injecting this sig is not necessary
                                 * in DETACH for the tracee to remain stopped.
                                 */
                                sig = 0;
                        }
                        /*
                         * PTRACE_INTERRUPT is not guaranteed to produce
                         * the above event if other ptrace-stop is pending.
                         * See tests/detach-sleeping testcase:
                         * strace got SIGINT while tracee is sleeping.
                         * We sent PTRACE_INTERRUPT.
                         * We see syscall exit, not PTRACE_INTERRUPT stop.
                         * We won't get PTRACE_INTERRUPT stop
                         * if we would CONT now. Need to DETACH.
                         */
                        if (sig == syscall_trap_sig)
                                sig = 0;
                        /* else: not sure in which case we can be here.
                         * Signal stop? Inject it while detaching.
                         */
                        ptrace_restart(PTRACE_DETACH, tcp, sig);
                        break;
                }
                /* Note: this check has to be after use_seize check */
                /* (else, in use_seize case SIGSTOP will be mistreated) */
                if (sig == SIGSTOP) {
                        /* Detach, suppressing SIGSTOP */
                        ptrace_restart(PTRACE_DETACH, tcp, 0);
                        break;
                }
                if (sig == syscall_trap_sig)
                        sig = 0;
                /* Can't detach just yet, may need to wait for SIGSTOP */
                error = ptrace_restart(PTRACE_CONT, tcp, sig);
                if (error < 0) {
                        /* Should not happen.
                         * Note: ptrace_restart returns 0 on ESRCH, so it's not it.
                         * ptrace_restart already emitted error message.
                         */
                        break;
                }
        }

 drop:
        if (!qflag && (tcp->flags & TCB_ATTACHED))
                error_msg("Process %u detached", tcp->pid);

        droptcb(tcp);
}

static void
process_opt_p_list(char *opt)
{
        while (*opt) {
                /*
                 * We accept -p PID,PID; -p "`pidof PROG`"; -p "`pgrep PROG`".
                 * pidof uses space as delim, pgrep uses newline. :(
                 */
                int pid;
                char *delim = opt + strcspn(opt, "\n\t ,");
                char c = *delim;

                *delim = '\0';
                pid = string_to_uint(opt);
                if (pid <= 0) {
                        error_msg_and_die("Invalid process id: '%s'", opt);
                }
                if (pid == strace_tracer_pid) {
                        error_msg_and_die("I'm sorry, I can't let you do that, Dave.");
                }
                *delim = c;
                alloctcb(pid);
                if (c == '\0')
                        break;
                opt = delim + 1;
        }
}

static void
attach_tcb(struct tcb *const tcp)
{
        if (ptrace_attach_or_seize(tcp->pid) < 0) {
                perror_msg("attach: ptrace(%s, %d)",
                           ptrace_attach_cmd, tcp->pid);
                droptcb(tcp);
                return;
        }

        after_successful_attach(tcp, TCB_GRABBED | post_attach_sigstop);
        debug_msg("attach to pid %d (main) succeeded", tcp->pid);

        static const char task_path[] = "/proc/%d/task";
        char procdir[sizeof(task_path) + sizeof(int) * 3];
        DIR *dir;
        unsigned int ntid = 0, nerr = 0;

        if (followfork && tcp->pid != strace_child &&
            xsprintf(procdir, task_path, tcp->pid) > 0 &&
            (dir = opendir(procdir)) != NULL) {
                struct_dirent *de;

                while ((de = read_dir(dir)) != NULL) {
                        if (de->d_fileno == 0)
                                continue;

                        int tid = string_to_uint(de->d_name);
                        if (tid <= 0 || tid == tcp->pid)
                                continue;

                        ++ntid;
                        if (ptrace_attach_or_seize(tid) < 0) {
                                ++nerr;
                                debug_perror_msg("attach: ptrace(%s, %d)",
                                                 ptrace_attach_cmd, tid);
                                continue;
                        }

                        after_successful_attach(alloctcb(tid),
                                                TCB_GRABBED | post_attach_sigstop);
                        debug_msg("attach to pid %d succeeded", tid);
                }

                closedir(dir);
        }

        if (!qflag) {
                if (ntid > nerr)
                        error_msg("Process %u attached"
                                  " with %u threads",
                                  tcp->pid, ntid - nerr + 1);
                else
                        error_msg("Process %u attached",
                                  tcp->pid);
        }
}

static void
startup_attach(void)
{
        pid_t parent_pid = strace_tracer_pid;
        unsigned int tcbi;
        struct tcb *tcp;

        if (daemonized_tracer) {
                pid_t pid = fork();
                if (pid < 0)
                        perror_func_msg_and_die("fork");

                if (pid) { /* parent */
                        /*
                         * Wait for grandchild to attach to straced process
                         * (grandparent). Grandchild SIGKILLs us after it attached.
                         * Grandparent's wait() is unblocked by our death,
                         * it proceeds to exec the straced program.
                         */
                        pause();
                        _exit(0); /* paranoia */
                }
                /* grandchild */
                /* We will be the tracer process. Remember our new pid: */
                strace_tracer_pid = getpid();
        }

        for (tcbi = 0; tcbi < tcbtabsize; tcbi++) {
                tcp = tcbtab[tcbi];

                if (!tcp->pid)
                        continue;

                /* Is this a process we should attach to, but not yet attached? */
                if (tcp->flags & TCB_ATTACHED)
                        continue; /* no, we already attached it */

                if (tcp->pid == parent_pid || tcp->pid == strace_tracer_pid) {
                        errno = EPERM;
                        perror_msg("attach: pid %d", tcp->pid);
                        droptcb(tcp);
                        continue;
                }

                attach_tcb(tcp);

                if (interrupted)
                        return;
        } /* for each tcbtab[] */

        if (daemonized_tracer) {
                /*
                 * Make parent go away.
                 * Also makes grandparent's wait() unblock.
                 */
                kill(parent_pid, SIGKILL);
                strace_child = 0;
        }
}

/* Stack-o-phobic exec helper, in the hope to work around
 * NOMMU + "daemonized tracer" difficulty.
 */
struct exec_params {
        int fd_to_close;
        uid_t run_euid;
        gid_t run_egid;
        char **argv;
        char *pathname;
        struct sigaction child_sa;
};
static struct exec_params params_for_tracee;

static void ATTRIBUTE_NOINLINE ATTRIBUTE_NORETURN
exec_or_die(void)
{
        struct exec_params *params = &params_for_tracee;

        if (params->fd_to_close >= 0)
                close(params->fd_to_close);
        if (!daemonized_tracer && !use_seize) {
                if (ptrace(PTRACE_TRACEME, 0L, 0L, 0L) < 0) {
                        perror_msg_and_die("ptrace(PTRACE_TRACEME, ...)");
                }
        }

        if (username != NULL) {
                /*
                 * It is important to set groups before we
                 * lose privileges on setuid.
                 */
                if (initgroups(username, run_gid) < 0) {
                        perror_msg_and_die("initgroups");
                }
                if (setregid(run_gid, params->run_egid) < 0) {
                        perror_msg_and_die("setregid");
                }
                if (setreuid(run_uid, params->run_euid) < 0) {
                        perror_msg_and_die("setreuid");
                }
        } else if (geteuid() != 0)
                if (setreuid(run_uid, run_uid) < 0) {
                        perror_msg_and_die("setreuid");
                }

        if (!daemonized_tracer) {
                /*
                 * Induce a ptrace stop. Tracer (our parent)
                 * will resume us with PTRACE_SYSCALL and display
                 * the immediately following execve syscall.
                 * Can't do this on NOMMU systems, we are after
                 * vfork: parent is blocked, stopping would deadlock.
                 */
                if (!NOMMU_SYSTEM)
                        kill(getpid(), SIGSTOP);
        } else {
                alarm(3);
                /* we depend on SIGCHLD set to SIG_DFL by init code */
                /* if it happens to be SIG_IGN'ed, wait won't block */
                wait(NULL);
                alarm(0);
        }

        if (params_for_tracee.child_sa.sa_handler != SIG_DFL)
                sigaction(SIGCHLD, &params_for_tracee.child_sa, NULL);

        debug_msg("seccomp filter %s",
                  seccomp_filtering ? "enabled" : "disabled");
        if (seccomp_filtering)
                init_seccomp_filter();
        execv(params->pathname, params->argv);
        perror_msg_and_die("exec");
}

/*
 * Open a dummy descriptor for use as a placeholder.
 * The descriptor is O_RDONLY with FD_CLOEXEC flag set.
 * A read attempt from such descriptor ends with EOF,
 * a write attempt is rejected with EBADF.
 */
static int
open_dummy_desc(void)
{
        int fds[2];

        if (pipe(fds))
                perror_func_msg_and_die("pipe");
        close(fds[1]);
        set_cloexec_flag(fds[0]);
        return fds[0];
}

/* placeholder fds status for stdin and stdout */
static bool fd_is_placeholder[2];

/*
 * Ensure that all standard file descriptors are open by opening placeholder
 * file descriptors for those standard file descriptors that are not open.
 *
 * The information which descriptors have been made open is saved
 * in fd_is_placeholder for later use.
 */
static void
ensure_standard_fds_opened(void)
{
        int fd;

        while ((fd = open_dummy_desc()) <= 2) {
                if (fd == 2)
                        break;
                fd_is_placeholder[fd] = true;
        }

        if (fd > 2)
                close(fd);
}

/*
 * Redirect stdin and stdout unless they have been opened earlier
 * by ensure_standard_fds_opened as placeholders.
 */
static void
redirect_standard_fds(void)
{
        int i;

        /*
         * It might be a good idea to redirect stderr as well,
         * but we sometimes need to print error messages.
         */
        for (i = 0; i <= 1; ++i) {
                if (!fd_is_placeholder[i]) {
                        close(i);
                        open_dummy_desc();
                }
        }
}

static void
startup_child(char **argv)
{
        strace_stat_t statbuf;
        const char *filename;
        size_t filename_len;
        char pathname[PATH_MAX];
        int pid;
        struct tcb *tcp;

        filename = argv[0];
        filename_len = strlen(filename);

        if (filename_len > sizeof(pathname) - 1) {
                errno = ENAMETOOLONG;
                perror_msg_and_die("exec");
        }
        if (strchr(filename, '/')) {
                strcpy(pathname, filename);
        }
#ifdef USE_DEBUGGING_EXEC
        /*
         * Debuggers customarily check the current directory
         * first regardless of the path but doing that gives
         * security geeks a panic attack.
         */
        else if (stat_file(filename, &statbuf) == 0)
                strcpy(pathname, filename);
#endif /* USE_DEBUGGING_EXEC */
        else {
                const char *path;
                size_t m, n, len;

                for (path = getenv("PATH"); path && *path; path += m) {
                        const char *colon = strchr(path, ':');
                        if (colon) {
                                n = colon - path;
                                m = n + 1;
                        } else
                                m = n = strlen(path);
                        if (n == 0) {
                                if (!getcwd(pathname, PATH_MAX))
                                        continue;
                                len = strlen(pathname);
                        } else if (n > sizeof(pathname) - 1)
                                continue;
                        else {
                                strncpy(pathname, path, n);
                                len = n;
                        }
                        if (len && pathname[len - 1] != '/')
                                pathname[len++] = '/';
                        if (filename_len + len > sizeof(pathname) - 1)
                                continue;
                        strcpy(pathname + len, filename);
                        if (stat_file(pathname, &statbuf) == 0 &&
                            /* Accept only regular files
                               with some execute bits set.
                               XXX not perfect, might still fail */
                            S_ISREG(statbuf.st_mode) &&
                            (statbuf.st_mode & 0111))
                                break;
                }
                if (!path || !*path)
                        pathname[0] = '\0';
        }
        if (stat_file(pathname, &statbuf) < 0) {
                perror_msg_and_die("Can't stat '%s'", filename);
        }

        params_for_tracee.fd_to_close = (shared_log != stderr) ? fileno(shared_log) : -1;
        params_for_tracee.run_euid = (statbuf.st_mode & S_ISUID) ? statbuf.st_uid : run_uid;
        params_for_tracee.run_egid = (statbuf.st_mode & S_ISGID) ? statbuf.st_gid : run_gid;
        params_for_tracee.argv = argv;
        /*
         * On NOMMU, can be safely freed only after execve in tracee.
         * It's hard to know when that happens, so we just leak it.
         */
        params_for_tracee.pathname = NOMMU_SYSTEM ? xstrdup(pathname) : pathname;

#if defined HAVE_PRCTL && defined PR_SET_PTRACER && defined PR_SET_PTRACER_ANY
        if (daemonized_tracer)
                prctl(PR_SET_PTRACER, PR_SET_PTRACER_ANY);
#endif

        pid = fork();
        if (pid < 0)
                perror_func_msg_and_die("fork");

        if ((pid != 0 && daemonized_tracer)
         || (pid == 0 && !daemonized_tracer)
        ) {
                /* We are to become the tracee. Two cases:
                 * -D: we are parent
                 * not -D: we are child
                 */
                exec_or_die();
        }

        /* We are the tracer */

        if (!daemonized_tracer) {
                strace_child = pid;
                if (!use_seize) {
                        /* child did PTRACE_TRACEME, nothing to do in parent */
                } else {
                        if (!NOMMU_SYSTEM) {
                                /* Wait until child stopped itself */
                                int status;
                                while (waitpid(pid, &status, WSTOPPED) < 0) {
                                        if (errno == EINTR)
                                                continue;
                                        perror_msg_and_die("waitpid");
                                }
                                if (!WIFSTOPPED(status) || WSTOPSIG(status) != SIGSTOP) {
                                        kill_save_errno(pid, SIGKILL);
                                        perror_msg_and_die("Unexpected wait status %#x",
                                                           status);
                                }
                        }
                        /* Else: NOMMU case, we have no way to sync.
                         * Just attach to it as soon as possible.
                         * This means that we may miss a few first syscalls...
                         */

                        if (ptrace_attach_or_seize(pid)) {
                                kill_save_errno(pid, SIGKILL);
                                perror_msg_and_die("attach: ptrace(%s, %d)",
                                                   ptrace_attach_cmd, pid);
                        }
                        if (!NOMMU_SYSTEM)
                                kill(pid, SIGCONT);
                }
                tcp = alloctcb(pid);
                after_successful_attach(tcp, TCB_SKIP_DETACH_ON_FIRST_EXEC
                                             | (NOMMU_SYSTEM ? 0
                                                : (TCB_HIDE_LOG
                                                   | post_attach_sigstop)));
        } else {
                /* With -D, we are *child* here, the tracee is our parent. */
                strace_child = strace_tracer_pid;
                strace_tracer_pid = getpid();
                tcp = alloctcb(strace_child);
                tcp->flags |= TCB_SKIP_DETACH_ON_FIRST_EXEC | TCB_HIDE_LOG;
                /*
                 * Attaching will be done later, by startup_attach.
                 * Note: we don't do after_successful_attach() here either!
                 */

                /* NOMMU BUG! -D mode is active, we (child) return,
                 * and we will scribble over parent's stack!
                 * When parent later unpauses, it segfaults.
                 *
                 * We work around it
                 * (1) by declaring exec_or_die() NORETURN,
                 * hopefully compiler will just jump to it
                 * instead of call (won't push anything to stack),
                 * (2) by trying very hard in exec_or_die()
                 * to not use any stack,
                 * (3) having a really big (PATH_MAX) stack object
                 * in this function, which creates a "buffer" between
                 * child's and parent's stack pointers.
                 * This may save us if (1) and (2) failed
                 * and compiler decided to use stack in exec_or_die() anyway
                 * (happens on i386 because of stack parameter passing).
                 *
                 * A cleaner solution is to use makecontext + setcontext
                 * to create a genuine separate stack and execute on it.
                 */
        }

        if (seccomp_filtering)
                tcp->flags |= TCB_SECCOMP_FILTER;

        /*
         * A case where straced process is part of a pipe:
         * { sleep 1; yes | head -n99999; } | strace -o/dev/null sh -c 'exec <&-; sleep 9'
         * If strace won't close its fd#0, closing it in tracee is not enough:
         * the pipe is still open, it has a reader. Thus, "head" will not get its
         * SIGPIPE at once, on the first write.
         *
         * Preventing it by redirecting strace's stdin/out.
         * (Don't leave fds 0 and 1 closed, this is bad practice: future opens
         * will reuse them, unexpectedly making a newly opened object "stdin").
         */
        redirect_standard_fds();
}

static void
test_ptrace_seize(void)
{
        int pid;

        /* Need fork for test. NOMMU has no forks */
        if (NOMMU_SYSTEM) {
                post_attach_sigstop = 0; /* this sets use_seize to 1 */
                return;
        }

        pid = fork();
        if (pid < 0)
                perror_func_msg_and_die("fork");

        if (pid == 0) {
                pause();
                _exit(0);
        }

        /* PTRACE_SEIZE, unlike ATTACH, doesn't force tracee to trap.  After
         * attaching tracee continues to run unless a trap condition occurs.
         * PTRACE_SEIZE doesn't affect signal or group stop state.
         */
        if (ptrace(PTRACE_SEIZE, pid, 0, 0) == 0) {
                post_attach_sigstop = 0; /* this sets use_seize to 1 */
        } else {
                debug_msg("PTRACE_SEIZE doesn't work");
        }

        kill(pid, SIGKILL);

        while (1) {
                int status, tracee_pid;

                errno = 0;
                tracee_pid = waitpid(pid, &status, 0);
                if (tracee_pid <= 0) {
                        if (errno == EINTR)
                                continue;
                        perror_func_msg_and_die("unexpected wait result %d",
                                                tracee_pid);
                }
                if (WIFSIGNALED(status))
                        return;

                error_func_msg_and_die("unexpected wait status %#x", status);
        }
}

static unsigned int
get_os_release(void)
{
        struct utsname u;
        if (uname(&u) < 0)
                perror_msg_and_die("uname");
        /*
         * u.release string consists of at most three parts
         * and normally has this form: "3.2.9[-some-garbage]",
         * "X.Y-something" means "X.Y.0".
         */
        const char *p = u.release;
        unsigned int rel = 0;
        for (unsigned int parts = 0; parts < 3; ++parts) {
                unsigned int n = 0;
                for (; (*p >= '0') && (*p <= '9'); ++p) {
                        n *= 10;
                        n += *p - '0';
                }
                rel <<= 8;
                rel |= n;
                if (*p == '.')
                        ++p;
        }
        return rel;
}

static void
set_sighandler(int signo, void (*sighandler)(int), struct sigaction *oldact)
{
        const struct sigaction sa = { .sa_handler = sighandler };
        sigaction(signo, &sa, oldact);
}

/*
 * Initialization part of main() was eating much stack (~0.5k),
 * which was unused after init.
 * We can reuse it if we move init code into a separate function.
 *
 * Don't want main() to inline us and defeat the reason
 * we have a separate function.
 */
static void ATTRIBUTE_NOINLINE
init(int argc, char *argv[])
{
        int c, i;
        int optF = 0, zflags = 0;

        if (!program_invocation_name || !*program_invocation_name) {
                static char name[] = "strace";
                program_invocation_name =
                        (argc > 0 && argv[0] && *argv[0]) ? argv[0] : name;
        }

        strace_tracer_pid = getpid();

        os_release = get_os_release();

        shared_log = stderr;
        set_sortby(DEFAULT_SORTBY);
        set_personality(DEFAULT_PERSONALITY);
        qualify("trace=all");
        qualify("abbrev=all");
        qualify("verbose=all");
        qualify("status=all");
#if DEFAULT_QUAL_FLAGS != (QUAL_TRACE | QUAL_ABBREV | QUAL_VERBOSE)
# error Bug in DEFAULT_QUAL_FLAGS
#endif
        qualify("signal=all");

        static const char optstring[] = "+"
#ifdef ENABLE_STACKTRACE
            "k"
#endif
            "a:Ab:cCdDe:E:fFhiI:o:O:p:P:qrs:S:tTu:vVwxX:yzZ";

        enum {
                SECCOMP_OPTION = 0x100
        };
        static const struct option longopts[] = {
                { "seccomp-bpf", no_argument, 0, SECCOMP_OPTION },
                { "help", no_argument, 0, 'h' },
                { "version", no_argument, 0, 'V' },
                { 0, 0, 0, 0 }
        };

        while ((c = getopt_long(argc, argv, optstring, longopts, NULL)) != EOF) {
                switch (c) {
                case 'a':
                        acolumn = string_to_uint(optarg);
                        if (acolumn < 0)
                                error_opt_arg(c, optarg);
                        break;
                case 'A':
                        open_append = true;
                        break;
                case 'b':
                        if (strcmp(optarg, "execve") != 0)
                                error_msg_and_die("Syscall '%s' for -b isn't supported",
                                        optarg);
                        detach_on_execve = 1;
                        break;
                case 'c':
                        if (cflag == CFLAG_BOTH) {
                                error_msg_and_help("-c and -C are mutually exclusive");
                        }
                        cflag = CFLAG_ONLY_STATS;
                        break;
                case 'C':
                        if (cflag == CFLAG_ONLY_STATS) {
                                error_msg_and_help("-c and -C are mutually exclusive");
                        }
                        cflag = CFLAG_BOTH;
                        break;
                case 'd':
                        debug_flag = 1;
                        break;
                case 'D':
                        daemonized_tracer = 1;
                        break;
                case 'e':
                        qualify(optarg);
                        break;
                case 'E':
                        if (putenv(optarg) < 0)
                                perror_msg_and_die("putenv");
                        break;
                case 'f':
                        followfork++;
                        break;
                case 'F':
                        optF = 1;
                        break;
                case 'h':
                        usage();
                        break;
                case 'i':
                        iflag = 1;
                        break;
                case 'I':
                        opt_intr = string_to_uint_upto(optarg, NUM_INTR_OPTS - 1);
                        if (opt_intr <= 0)
                                error_opt_arg(c, optarg);
                        break;
#ifdef ENABLE_STACKTRACE
                case 'k':
                        stack_trace_enabled = true;
                        break;
#endif
                case 'o':
                        outfname = optarg;
                        break;
                case 'O':
                        if (set_overhead(optarg) < 0)
                                error_opt_arg(c, optarg);
                        break;
                case 'p':
                        process_opt_p_list(optarg);
                        break;
                case 'P':
                        pathtrace_select(optarg);
                        break;
                case 'q':
                        qflag++;
                        break;
                case 'r':
                        rflag = 1;
                        break;
                case 's':
                        i = string_to_uint(optarg);
                        if (i < 0 || (unsigned int) i > -1U / 4)
                                error_opt_arg(c, optarg);
                        max_strlen = i;
                        break;
                case 'S':
                        set_sortby(optarg);
                        break;
                case 't':
                        tflag++;
                        break;
                case 'T':
                        Tflag = 1;
                        break;
                case 'u':
                        username = optarg;
                        break;
                case 'v':
                        qualify("abbrev=none");
                        break;
                case 'V':
                        print_version();
                        exit(0);
                        break;
                case 'w':
                        count_wallclock = 1;
                        break;
                case 'x':
                        xflag++;
                        break;
                case 'X':
                        if (!strcmp(optarg, "raw"))
                                xlat_verbosity = XLAT_STYLE_RAW;
                        else if (!strcmp(optarg, "abbrev"))
                                xlat_verbosity = XLAT_STYLE_ABBREV;
                        else if (!strcmp(optarg, "verbose"))
                                xlat_verbosity = XLAT_STYLE_VERBOSE;
                        else
                                error_opt_arg(c, optarg);
                        break;
                case 'y':
                        show_fd_path++;
                        break;
                case 'z':
                        clear_number_set_array(status_set, 1);
                        add_number_to_set(STATUS_SUCCESSFUL, status_set);
                        zflags++;
                        break;
                case 'Z':
                        clear_number_set_array(status_set, 1);
                        add_number_to_set(STATUS_FAILED, status_set);
                        zflags++;
                        break;
                case SECCOMP_OPTION:
                        seccomp_filtering = true;
                        break;
                default:
                        error_msg_and_help(NULL);
                        break;
                }
        }

        argv += optind;
        argc -= optind;

        if (argc < 0 || (!nprocs && !argc)) {
                error_msg_and_help("must have PROG [ARGS] or -p PID");
        }

        if (!argc && daemonized_tracer) {
                error_msg_and_help("PROG [ARGS] must be specified with -D");
        }

        if (seccomp_filtering) {
                if (nprocs && (!argc || debug_flag))
                        error_msg("--seccomp-bpf is not enabled for processes"
                                  " attached with -p");
                if (!followfork) {
                        error_msg("--seccomp-bpf implies -f");
                        followfork = 1;
                }
        }

        if (optF) {
                if (followfork) {
                        error_msg("deprecated option -F ignored");
                } else {
                        error_msg("option -F is deprecated, "
                                  "please use -f instead");
                        followfork = optF;
                }
        }

        if (followfork >= 2 && cflag) {
                error_msg_and_help("(-c or -C) and -ff are mutually exclusive");
        }

        if (count_wallclock && !cflag) {
                error_msg_and_help("-w must be given with (-c or -C)");
        }

        if (cflag == CFLAG_ONLY_STATS) {
                if (iflag)
                        error_msg("-%c has no effect with -c", 'i');
                if (stack_trace_enabled)
                        error_msg("-%c has no effect with -c", 'k');
                if (rflag)
                        error_msg("-%c has no effect with -c", 'r');
                if (tflag)
                        error_msg("-%c has no effect with -c", 't');
                if (Tflag)
                        error_msg("-%c has no effect with -c", 'T');
                if (show_fd_path)
                        error_msg("-%c has no effect with -c", 'y');
        }

#ifndef HAVE_OPEN_MEMSTREAM
        if (!is_complete_set(status_set, NUMBER_OF_STATUSES))
                error_msg_and_help("open_memstream is required to use -z, -Z, or -e status");
#endif

        if (zflags > 1)
                error_msg("Only the last of -z/-Z options will take effect. "
                          "See status qualifier for more complex filters.");

        acolumn_spaces = xmalloc(acolumn + 1);
        memset(acolumn_spaces, ' ', acolumn);
        acolumn_spaces[acolumn] = '\0';

        set_sighandler(SIGCHLD, SIG_DFL, &params_for_tracee.child_sa);

#ifdef ENABLE_STACKTRACE
        if (stack_trace_enabled)
                unwind_init();
#endif

        /* See if they want to run as another user. */
        if (username != NULL) {
                struct passwd *pent;

                if (getuid() != 0 || geteuid() != 0) {
                        error_msg_and_die("You must be root to use the -u option");
                }
                pent = getpwnam(username);
                if (pent == NULL) {
                        error_msg_and_die("Cannot find user '%s'", username);
                }
                run_uid = pent->pw_uid;
                run_gid = pent->pw_gid;
        } else {
                run_uid = getuid();
                run_gid = getgid();
        }

        if (followfork)
                ptrace_setoptions |= PTRACE_O_TRACECLONE |
                                     PTRACE_O_TRACEFORK |
                                     PTRACE_O_TRACEVFORK;

        if (seccomp_filtering)
                check_seccomp_filter();
        if (seccomp_filtering)
                ptrace_setoptions |= PTRACE_O_TRACESECCOMP;

        debug_msg("ptrace_setoptions = %#x", ptrace_setoptions);
        test_ptrace_seize();
        test_ptrace_get_syscall_info();

        /*
         * Is something weird with our stdin and/or stdout -
         * for example, may they be not open? In this case,
         * ensure that none of the future opens uses them.
         *
         * This was seen in the wild when /proc/sys/kernel/core_pattern
         * was set to "|/bin/strace -o/tmp/LOG PROG":
         * kernel runs coredump helper with fd#0 open but fd#1 closed (!),
         * therefore LOG gets opened to fd#1, and fd#1 is closed by
         * "don't hold up stdin/out open" code soon after.
         */
        ensure_standard_fds_opened();

        /* Check if they want to redirect the output. */
        if (outfname) {
                /* See if they want to pipe the output. */
                if (outfname[0] == '|' || outfname[0] == '!') {
                        /*
                         * We can't do the <outfname>.PID funny business
                         * when using popen, so prohibit it.
                         */
                        if (followfork >= 2)
                                error_msg_and_help("piping the output and -ff "
                                                   "are mutually exclusive");
                        shared_log = strace_popen(outfname + 1);
                } else if (followfork < 2) {
                        shared_log = strace_fopen(outfname);
                } else if (strlen(outfname) >= PATH_MAX - sizeof(int) * 3) {
                        errno = ENAMETOOLONG;
                        perror_msg_and_die("%s", outfname);
                }
        } else {
                /* -ff without -o FILE is the same as single -f */
                if (followfork >= 2)
                        followfork = 1;
        }

        if (!outfname || outfname[0] == '|' || outfname[0] == '!') {
                setvbuf(shared_log, NULL, _IOLBF, 0);
        }

        /*
         * argv[0]      -pPID   -oFILE  Default interactive setting
         * yes          *       0       INTR_WHILE_WAIT
         * no           1       0       INTR_WHILE_WAIT
         * yes          *       1       INTR_NEVER
         * no           1       1       INTR_WHILE_WAIT
         */

        if (daemonized_tracer && !opt_intr)
                opt_intr = INTR_BLOCK_TSTP_TOO;
        if (outfname && argc) {
                if (!opt_intr)
                        opt_intr = INTR_NEVER;
                if (!qflag)
                        qflag = 1;
        }
        if (!opt_intr)
                opt_intr = INTR_WHILE_WAIT;

        /*
         * startup_child() must be called before the signal handlers get
         * installed below as they are inherited into the spawned process.
         * Also we do not need to be protected by them as during interruption
         * in the startup_child() mode we kill the spawned process anyway.
         */
        if (argc) {
                startup_child(argv);
        }

        set_sighandler(SIGTTOU, SIG_IGN, NULL);
        set_sighandler(SIGTTIN, SIG_IGN, NULL);
        if (opt_intr != INTR_ANYWHERE) {
                if (opt_intr == INTR_BLOCK_TSTP_TOO)
                        set_sighandler(SIGTSTP, SIG_IGN, NULL);
                /*
                 * In interactive mode (if no -o OUTFILE, or -p PID is used),
                 * fatal signals are handled asynchronously and acted
                 * when waiting for process state changes.
                 * In non-interactive mode these signals are ignored.
                 */
                set_sighandler(SIGHUP, interactive ? interrupt : SIG_IGN, NULL);
                set_sighandler(SIGINT, interactive ? interrupt : SIG_IGN, NULL);
                set_sighandler(SIGQUIT, interactive ? interrupt : SIG_IGN, NULL);
                set_sighandler(SIGPIPE, interactive ? interrupt : SIG_IGN, NULL);
                set_sighandler(SIGTERM, interactive ? interrupt : SIG_IGN, NULL);
        }

        sigemptyset(&timer_set);
        sigaddset(&timer_set, SIGALRM);
        sigprocmask(SIG_BLOCK, &timer_set, NULL);
        set_sighandler(SIGALRM, timer_sighandler, NULL);

        if (nprocs != 0 || daemonized_tracer)
                startup_attach();

        /* Do we want pids printed in our -o OUTFILE?
         * -ff: no (every pid has its own file); or
         * -f: yes (there can be more pids in the future); or
         * -p PID1,PID2: yes (there are already more than one pid)
         */
        print_pid_pfx = (outfname && followfork < 2 && (followfork == 1 || nprocs > 1));
}

static struct tcb *
pid2tcb(const int pid)
{
        if (pid <= 0)
                return NULL;

#define PID2TCB_CACHE_SIZE 1024U
#define PID2TCB_CACHE_MASK (PID2TCB_CACHE_SIZE - 1)

        static struct tcb *pid2tcb_cache[PID2TCB_CACHE_SIZE];
        struct tcb **const ptcp = &pid2tcb_cache[pid & PID2TCB_CACHE_MASK];
        struct tcb *tcp = *ptcp;

        if (tcp && tcp->pid == pid)
                return tcp;

        for (unsigned int i = 0; i < tcbtabsize; ++i) {
                tcp = tcbtab[i];
                if (tcp->pid == pid)
                        return *ptcp = tcp;
        }

        return NULL;
}

static void
cleanup(int fatal_sig)
{
        unsigned int i;
        struct tcb *tcp;

        if (!fatal_sig)
                fatal_sig = SIGTERM;

        for (i = 0; i < tcbtabsize; i++) {
                tcp = tcbtab[i];
                if (!tcp->pid)
                        continue;
                debug_func_msg("looking at pid %u", tcp->pid);
                if (tcp->pid == strace_child) {
                        kill(tcp->pid, SIGCONT);
                        kill(tcp->pid, fatal_sig);
                }
                detach(tcp);
        }
}

static void
interrupt(int sig)
{
        interrupted = sig;
}

static void
print_debug_info(const int pid, int status)
{
        const unsigned int event = (unsigned int) status >> 16;
        char buf[sizeof("WIFEXITED,exitcode=%u") + sizeof(int)*3 /*paranoia:*/ + 16];
        char evbuf[sizeof(",EVENT_VFORK_DONE (%u)") + sizeof(int)*3 /*paranoia:*/ + 16];

        strcpy(buf, "???");
        if (WIFSIGNALED(status))
                xsprintf(buf, "WIFSIGNALED,%ssig=%s",
                                WCOREDUMP(status) ? "core," : "",
                                sprintsigname(WTERMSIG(status)));
        if (WIFEXITED(status))
                xsprintf(buf, "WIFEXITED,exitcode=%u", WEXITSTATUS(status));
        if (WIFSTOPPED(status))
                xsprintf(buf, "WIFSTOPPED,sig=%s",
                         sprintsigname(WSTOPSIG(status)));
        evbuf[0] = '\0';
        if (event != 0) {
                static const char *const event_names[] = {
                        [PTRACE_EVENT_CLONE] = "CLONE",
                        [PTRACE_EVENT_FORK]  = "FORK",
                        [PTRACE_EVENT_VFORK] = "VFORK",
                        [PTRACE_EVENT_VFORK_DONE] = "VFORK_DONE",
                        [PTRACE_EVENT_EXEC]  = "EXEC",
                        [PTRACE_EVENT_EXIT]  = "EXIT",
                        [PTRACE_EVENT_SECCOMP]  = "SECCOMP",
                        /* [PTRACE_EVENT_STOP (=128)] would make biggish array */
                };
                const char *e = "??";
                if (event < ARRAY_SIZE(event_names))
                        e = event_names[event];
                else if (event == PTRACE_EVENT_STOP)
                        e = "STOP";
                xsprintf(evbuf, ",EVENT_%s (%u)", e, event);
        }
        error_msg("[wait(0x%06x) = %u] %s%s", status, pid, buf, evbuf);
}

static struct tcb *
maybe_allocate_tcb(const int pid, int status)
{
        if (!WIFSTOPPED(status)) {
                if (detach_on_execve && pid == strace_child) {
                        /* example: strace -bexecve sh -c 'exec true' */
                        strace_child = 0;
                        return NULL;
                }
                /*
                 * This can happen if we inherited an unknown child.
                 * Example: (sleep 1 & exec strace true)
                 */
                error_msg("Exit of unknown pid %u ignored", pid);
                return NULL;
        }
        if (followfork) {
                /* We assume it's a fork/vfork/clone child */
                struct tcb *tcp = alloctcb(pid);
                after_successful_attach(tcp, post_attach_sigstop);
                if (!qflag)
                        error_msg("Process %d attached", pid);
                return tcp;
        } else {
                /*
                 * This can happen if a clone call misused CLONE_PTRACE itself.
                 *
                 * There used to be a dance around possible re-injection of
                 * WSTOPSIG(status), but it was later removed as the only
                 * observable stop here is the initial ptrace-stop.
                 */
                ptrace(PTRACE_DETACH, pid, NULL, 0L);
                error_msg("Detached unknown pid %d", pid);
                return NULL;
        }
}

/*
 * Under Linux, execve changes pid to thread leader's pid, and we see this
 * changed pid on EVENT_EXEC and later, execve sysexit.  Leader "disappears"
 * without exit notification.  Let user know that, drop leader's tcb, and fix
 * up pid in execve thread's tcb.  Effectively, execve thread's tcb replaces
 * leader's tcb.
 *
 * BTW, leader is 'stuck undead' (doesn't report WIFEXITED on exit syscall)
 * in multi-threaded programs exactly in order to handle this case.
 */
static struct tcb *
maybe_switch_tcbs(struct tcb *tcp, const int pid)
{
        /*
         * PTRACE_GETEVENTMSG returns old pid starting from Linux 3.0.
         * On 2.6 and earlier it can return garbage.
         */
        if (os_release < KERNEL_VERSION(3, 0, 0))
                return NULL;

        const long old_pid = tcb_wait_tab[tcp->wait_data_idx].msg;

        /* Avoid truncation in pid2tcb() param passing */
        if (old_pid <= 0 || old_pid == pid)
                return NULL;
        if ((unsigned long) old_pid > UINT_MAX)
                return NULL;
        struct tcb *execve_thread = pid2tcb(old_pid);
        /* It should be !NULL, but I feel paranoid */
        if (!execve_thread)
                return NULL;

        if (execve_thread->curcol != 0) {
                /*
                 * One case we are here is -ff, try
                 * "strace -oLOG -ff test/threaded_execve".
                 * Another case is demonstrated by
                 * tests/maybe_switch_current_tcp.c
                 */
                fprintf(execve_thread->outf, " <pid changed to %d ...>\n", pid);
                /*execve_thread->curcol = 0; - no need, see code below */
        }
        /* Swap output FILEs and memstream (needed for -ff) */
        FILE *fp = execve_thread->outf;
        execve_thread->outf = tcp->outf;
        tcp->outf = fp;
        if (execve_thread->staged_output_data || tcp->staged_output_data) {
                struct staged_output_data *staged_output_data;

                staged_output_data = execve_thread->staged_output_data;
                execve_thread->staged_output_data = tcp->staged_output_data;
                tcp->staged_output_data = staged_output_data;
        }

        /* And their column positions */
        execve_thread->curcol = tcp->curcol;
        tcp->curcol = 0;
        /* Drop leader, but close execve'd thread outfile (if -ff) */
        droptcb(tcp);
        /* Switch to the thread, reusing leader's outfile and pid */
        tcp = execve_thread;
        tcp->pid = pid;
        if (cflag != CFLAG_ONLY_STATS) {
                printleader(tcp);
                tprintf("+++ superseded by execve in pid %lu +++\n", old_pid);
                line_ended();
                /*
                 * Need to reopen memstream for thread
                 * as we closed it in droptcb.
                 */
                if (!is_complete_set(status_set, NUMBER_OF_STATUSES))
                        strace_open_memstream(tcp);
                tcp->flags |= TCB_REPRINT;
        }

        return tcp;
}

static struct tcb *
maybe_switch_current_tcp(void)
{
        struct tcb *tcp = maybe_switch_tcbs(current_tcp, current_tcp->pid);

        if (tcp)
                set_current_tcp(tcp);

        return tcp;
}

static void
print_signalled(struct tcb *tcp, const int pid, int status)
{
        if (pid == strace_child) {
                exit_code = 0x100 | WTERMSIG(status);
                strace_child = 0;
        }

        if (cflag != CFLAG_ONLY_STATS
            && is_number_in_set(WTERMSIG(status), signal_set)) {
                printleader(tcp);
                tprintf("+++ killed by %s %s+++\n",
                        sprintsigname(WTERMSIG(status)),
                        WCOREDUMP(status) ? "(core dumped) " : "");
                line_ended();
        }
}

static void
print_exited(struct tcb *tcp, const int pid, int status)
{
        if (pid == strace_child) {
                exit_code = WEXITSTATUS(status);
                strace_child = 0;
        }

        if (cflag != CFLAG_ONLY_STATS &&
            qflag < 2) {
                printleader(tcp);
                tprintf("+++ exited with %d +++\n", WEXITSTATUS(status));
                line_ended();
        }
}

static void
print_stopped(struct tcb *tcp, const siginfo_t *si, const unsigned int sig)
{
        if (cflag != CFLAG_ONLY_STATS
            && !hide_log(tcp)
            && is_number_in_set(sig, signal_set)) {
                printleader(tcp);
                if (si) {
                        tprintf("--- %s ", sprintsigname(sig));
                        printsiginfo(si);
                        tprints(" ---\n");
                } else
                        tprintf("--- stopped by %s ---\n", sprintsigname(sig));
                line_ended();

#ifdef ENABLE_STACKTRACE
                if (stack_trace_enabled)
                        unwind_tcb_print(tcp);
#endif
        }
}

static void
startup_tcb(struct tcb *tcp)
{
        debug_msg("pid %d has TCB_STARTUP, initializing it", tcp->pid);

        tcp->flags &= ~TCB_STARTUP;

        if (!use_seize) {
                debug_msg("setting opts 0x%x on pid %d",
                          ptrace_setoptions, tcp->pid);
                if (ptrace(PTRACE_SETOPTIONS, tcp->pid, NULL, ptrace_setoptions) < 0) {
                        if (errno != ESRCH) {
                                /* Should never happen, really */
                                perror_msg_and_die("PTRACE_SETOPTIONS");
                        }
                }
        }

        if ((tcp->flags & TCB_GRABBED) && (get_scno(tcp) == 1))
                tcp->s_prev_ent = tcp->s_ent;

        if (cflag) {
                tcp->atime = tcp->stime;
        }
}

static void
print_event_exit(struct tcb *tcp)
{
        if (entering(tcp) || filtered(tcp) || hide_log(tcp)
            || cflag == CFLAG_ONLY_STATS) {
                return;
        }

        if (followfork < 2 && printing_tcp && printing_tcp != tcp
            && printing_tcp->curcol != 0) {
                set_current_tcp(printing_tcp);
                tprints(" <unfinished ...>\n");
                flush_tcp_output(printing_tcp);
                printing_tcp->curcol = 0;
                set_current_tcp(tcp);
        }

        print_syscall_resume(tcp);

        if (!(tcp->sys_func_rval & RVAL_DECODED)) {
                /*
                 * The decoder has probably decided to print something
                 * on exiting syscall which is not going to happen.
                 */
                tprints(" <unfinished ...>");
        }

        printing_tcp = tcp;
        tprints(") ");
        tabto();
        tprints("= ?\n");
        if (!is_complete_set(status_set, NUMBER_OF_STATUSES)) {
                bool publish = is_number_in_set(STATUS_UNFINISHED, status_set);
                strace_close_memstream(tcp, publish);
        }
        line_ended();
}

static size_t
trace_wait_data_size(struct tcb *tcp)
{
        return sizeof(struct tcb_wait_data);
}

static struct tcb_wait_data *
init_trace_wait_data(void *p)
{
        struct tcb_wait_data *wd = p;

        memset(wd, 0, sizeof(*wd));

        return wd;
}

static struct tcb_wait_data *
copy_trace_wait_data(const struct tcb_wait_data *wd)
{
        struct tcb_wait_data *new_wd = xmalloc(sizeof(*new_wd));

        memcpy(new_wd, wd, sizeof(*wd));

        return new_wd;
}

static void
free_trace_wait_data(struct tcb_wait_data *wd)
{
        free(wd);
}

static void
tcb_wait_tab_check_size(const size_t size)
{
        while (size >= tcb_wait_tab_size) {
                tcb_wait_tab = xgrowarray(tcb_wait_tab,
                                          &tcb_wait_tab_size,
                                          sizeof(tcb_wait_tab[0]));
        }
}

static const struct tcb_wait_data *
next_event(void)
{
        if (interrupted)
                return NULL;

        invalidate_umove_cache();

        struct tcb *tcp = NULL;
        struct list_item *elem;

        static EMPTY_LIST(pending_tcps);
        /* Handle the queued tcbs before waiting for new events.  */
        if (!list_is_empty(&pending_tcps))
                goto next_event_get_tcp;

        static struct tcb *extra_tcp;
        static size_t wait_extra_data_idx;
        /* Handle the extra tcb event.  */
        if (extra_tcp) {
                tcp = extra_tcp;
                extra_tcp = NULL;
                tcp->wait_data_idx = wait_extra_data_idx;

                debug_msg("dequeued extra event for pid %u", tcp->pid);
                goto next_event_exit;
        }

        /*
         * Used to exit simply when nprocs hits zero, but in this testcase:
         *  int main(void) { _exit(!!fork()); }
         * under strace -f, parent sometimes (rarely) manages
         * to exit before we see the first stop of the child,
         * and we are losing track of it:
         *  19923 clone(...) = 19924
         *  19923 exit_group(1)     = ?
         *  19923 +++ exited with 1 +++
         * Exiting only when wait() returns ECHILD works better.
         */
        if (popen_pid != 0) {
                /* However, if -o|logger is in use, we can't do that.
                 * Can work around that by double-forking the logger,
                 * but that loses the ability to wait for its completion
                 * on exit. Oh well...
                 */
                if (nprocs == 0)
                        return NULL;
        }

        const bool unblock_delay_timer = is_delay_timer_armed();

        /*
         * The window of opportunity to handle expirations
         * of the delay timer opens here.
         *
         * Unblock the signal handler for the delay timer
         * iff the delay timer is already created.
         */
        if (unblock_delay_timer)
                sigprocmask(SIG_UNBLOCK, &timer_set, NULL);

        /*
         * If the delay timer has expired, then its expiration
         * has been handled already by the signal handler.
         *
         * If the delay timer expires during wait4(),
         * then the system call will be interrupted and
         * the expiration will be handled by the signal handler.
         */
        int status;
        struct rusage ru;
        int pid = wait4(-1, &status, __WALL, (cflag ? &ru : NULL));
        int wait_errno = errno;

        /*
         * The window of opportunity to handle expirations
         * of the delay timer closes here.
         *
         * Block the signal handler for the delay timer
         * iff it was unblocked earlier.
         */
        if (unblock_delay_timer) {
                sigprocmask(SIG_BLOCK, &timer_set, NULL);

                if (restart_failed)
                        return NULL;
        }

        size_t wait_tab_pos = 0;
        bool wait_nohang = false;

        /*
         * Wait for new events until wait4() returns 0 (meaning that there's
         * nothing more to wait for for now), or a second event for some tcb
         * appears (which may happen if a tracee was SIGKILL'ed, for example).
         */
        for (;;) {
                struct tcb_wait_data *wd;

                if (pid < 0) {
                        if (wait_errno == EINTR)
                                break;
                        if (wait_nohang)
                                break;
                        if (nprocs == 0 && wait_errno == ECHILD)
                                return NULL;
                        /*
                         * If nprocs > 0, ECHILD is not expected,
                         * treat it as any other error here:
                         */
                        errno = wait_errno;
                        perror_msg_and_die("wait4(__WALL)");
                }

                if (!pid)
                        break;

                if (pid == popen_pid) {
                        if (!WIFSTOPPED(status))
                                popen_pid = 0;
                        break;
                }

                if (debug_flag)
                        print_debug_info(pid, status);

                /* Look up 'pid' in our table. */
                tcp = pid2tcb(pid);

                if (!tcp) {
                        tcp = maybe_allocate_tcb(pid, status);
                        if (!tcp)
                                goto next_event_wait_next;
                }

                if (cflag) {
                        tcp->stime.tv_sec = ru.ru_stime.tv_sec;
                        tcp->stime.tv_nsec = ru.ru_stime.tv_usec * 1000;
                }

                tcb_wait_tab_check_size(wait_tab_pos);

                /* Initialise a new wait data structure.  */
                wd = tcb_wait_tab + wait_tab_pos;
                init_trace_wait_data(wd);
                wd->status = status;

                if (WIFSIGNALED(status)) {
                        wd->te = TE_SIGNALLED;
                } else if (WIFEXITED(status)) {
                        wd->te = TE_EXITED;
                } else {
                        /*
                         * As WCONTINUED flag has not been specified to wait4,
                         * it cannot be WIFCONTINUED(status), so the only case
                         * that remains is WIFSTOPPED(status).
                         */

                        const unsigned int sig = WSTOPSIG(status);
                        const unsigned int event = (unsigned int) status >> 16;

                        switch (event) {
                        case 0:
                                /*
                                 * Is this post-attach SIGSTOP?
                                 * Interestingly, the process may stop
                                 * with STOPSIG equal to some other signal
                                 * than SIGSTOP if we happened to attach
                                 * just before the process takes a signal.
                                 */
                                if (sig == SIGSTOP &&
                                    (tcp->flags & TCB_IGNORE_ONE_SIGSTOP)) {
                                        debug_func_msg("ignored SIGSTOP on "
                                                       "pid %d", tcp->pid);
                                        tcp->flags &= ~TCB_IGNORE_ONE_SIGSTOP;
                                        wd->te = TE_RESTART;
                                } else if (sig == syscall_trap_sig) {
                                        wd->te = TE_SYSCALL_STOP;
                                } else {
                                        /*
                                         * True if tracee is stopped by signal
                                         * (as opposed to "tracee received
                                         * signal").
                                         * TODO: shouldn't we check for
                                         * errno == EINVAL too?
                                         * We can get ESRCH instead, you know...
                                         */
                                        bool stopped = ptrace(PTRACE_GETSIGINFO,
                                                pid, 0, &wd->si) < 0;

                                        wd->te = stopped ? TE_GROUP_STOP
                                                         : TE_SIGNAL_DELIVERY_STOP;
                                }
                                break;
                        case PTRACE_EVENT_STOP:
                                /*
                                 * PTRACE_INTERRUPT-stop or group-stop.
                                 * PTRACE_INTERRUPT-stop has sig == SIGTRAP here.
                                 */
                                switch (sig) {
                                case SIGSTOP:
                                case SIGTSTP:
                                case SIGTTIN:
                                case SIGTTOU:
                                        wd->te = TE_GROUP_STOP;
                                        break;
                                default:
                                        wd->te = TE_RESTART;
                                }
                                break;
                        case PTRACE_EVENT_EXEC:
                                        /*
                                         * TODO: shouldn't we check for
                                         * errno == EINVAL here, too?
                                         * We can get ESRCH instead, you know...
                                         */
                                if (ptrace(PTRACE_GETEVENTMSG, pid, NULL,
                                    &wd->msg) < 0)
                                        wd->msg = 0;

                                wd->te = TE_STOP_BEFORE_EXECVE;
                                break;
                        case PTRACE_EVENT_EXIT:
                                wd->te = TE_STOP_BEFORE_EXIT;
                                break;
                        case PTRACE_EVENT_SECCOMP:
                                wd->te = TE_SECCOMP;
                                break;
                        default:
                                wd->te = TE_RESTART;
                        }
                }

                if (!wd->te)
                        error_func_msg("Tracing event hasn't been determined "
                                       "for pid %d, status %0#x", pid, status);

                if (!list_is_empty(&tcp->wait_list)) {
                        wait_extra_data_idx = wait_tab_pos;
                        extra_tcp = tcp;
                        debug_func_msg("queued extra pid %d", tcp->pid);
                } else {
                        tcp->wait_data_idx = wait_tab_pos;
                        list_append(&pending_tcps, &tcp->wait_list);
                        debug_func_msg("queued pid %d", tcp->pid);
                }

                wait_tab_pos++;

                if (extra_tcp)
                        break;

next_event_wait_next:
                pid = wait4(-1, &status, __WALL | WNOHANG, (cflag ? &ru : NULL));
                wait_errno = errno;
                wait_nohang = true;
        }

next_event_get_tcp:
        elem = list_remove_head(&pending_tcps);

        if (!elem) {
                tcb_wait_tab_check_size(0);
                memset(tcb_wait_tab, 0, sizeof(*tcb_wait_tab));
                tcb_wait_tab->te = TE_NEXT;

                return tcb_wait_tab;
        } else {
                tcp = list_elem(elem, struct tcb, wait_list);
                debug_func_msg("dequeued pid %d", tcp->pid);
        }

next_event_exit:
        /* Is this the very first time we see this tracee stopped? */
        if (tcp->flags & TCB_STARTUP)
                startup_tcb(tcp);

        clear_regs(tcp);

        /* Set current output file */
        set_current_tcp(tcp);

        return tcb_wait_tab + tcp->wait_data_idx;
}

static int
trace_syscall(struct tcb *tcp, unsigned int *sig)
{
        if (entering(tcp)) {
                int res = syscall_entering_decode(tcp);
                switch (res) {
                case 0:
                        return 0;
                case 1:
                        res = syscall_entering_trace(tcp, sig);
                }
                syscall_entering_finish(tcp, res);
                return res;
        } else {
                struct timespec ts = {};
                int res = syscall_exiting_decode(tcp, &ts);
                if (res != 0) {
                        res = syscall_exiting_trace(tcp, &ts, res);
                }
                syscall_exiting_finish(tcp);
                return res;
        }
}

/* Returns true iff the main trace loop has to continue. */
static bool
dispatch_event(const struct tcb_wait_data *wd)
{
        unsigned int restart_op;
        unsigned int restart_sig = 0;
        enum trace_event te = wd ? wd->te : TE_BREAK;
        /*
         * Copy wd->status to a non-const variable to workaround glibc bugs
         * around union wait fixed by glibc commit glibc-2.24~391
         */
        int status = wd ? wd->status : 0;

        if (current_tcp && has_seccomp_filter(current_tcp))
                restart_op = seccomp_filter_restart_operator(current_tcp);
        else
                restart_op = PTRACE_SYSCALL;

        switch (te) {
        case TE_BREAK:
                return false;

        case TE_NEXT:
                return true;

        case TE_RESTART:
                break;

        case TE_SECCOMP:
                if (!has_seccomp_filter(current_tcp)) {
                        /*
                         * We don't know if forks/clones have a seccomp filter
                         * when they are created, but we can detect it when we
                         * have a seccomp-stop.
                         * In such a case, if !seccomp_before_sysentry, we have
                         * already processed the syscall entry, so we avoid
                         * processing it a second time.
                         */
                        current_tcp->flags |= TCB_SECCOMP_FILTER;
                        restart_op = PTRACE_SYSCALL;
                        break;
                }

                if (seccomp_before_sysentry) {
                        restart_op = PTRACE_SYSCALL;
                        break;
                }
                ATTRIBUTE_FALLTHROUGH;

        case TE_SYSCALL_STOP:
                if (trace_syscall(current_tcp, &restart_sig) < 0) {
                        /*
                         * ptrace() failed in trace_syscall().
                         * Likely a result of process disappearing mid-flight.
                         * Observed case: exit_group() or SIGKILL terminating
                         * all processes in thread group.
                         * We assume that ptrace error was caused by process death.
                         * We used to detach(current_tcp) here, but since we no
                         * longer implement "detach before death" policy/hack,
                         * we can let this process to report its death to us
                         * normally, via WIFEXITED or WIFSIGNALED wait status.
                         */
                        return true;
                }
                if (has_seccomp_filter(current_tcp)) {
                        /*
                         * Syscall and seccomp stops can happen in different
                         * orders depending on kernel.  strace tests this in
                         * check_seccomp_order_tracer().
                         *
                         * Linux 3.5--4.7:
                         * (seccomp-stop before syscall-entry-stop)
                         *         +--> seccomp-stop ->-PTRACE_SYSCALL->-+
                         *         |                                     |
                         *     PTRACE_CONT                   syscall-entry-stop
                         *         |                                     |
                         * syscall-exit-stop <---PTRACE_SYSCALL-----<----+
                         *
                         * Linux 4.8+:
                         * (seccomp-stop after syscall-entry-stop)
                         *                 syscall-entry-stop
                         *
                         *         +---->-----PTRACE_CONT---->----+
                         *         |                              |
                         *  syscall-exit-stop               seccomp-stop
                         *         |                              |
                         *         +----<----PTRACE_SYSCALL---<---+
                         *
                         * Note in Linux 4.8+, we restart in PTRACE_CONT
                         * after syscall-exit to skip the syscall-entry-stop.
                         * The next seccomp-stop will be treated as a syscall
                         * entry.
                         *
                         * The line below implements this behavior.
                         * Note that exiting(current_tcp) actually marks
                         * a syscall-entry-stop because the flag was inverted
                         * in the above call to trace_syscall.
                         */
                        restart_op = exiting(current_tcp) ? PTRACE_SYSCALL : PTRACE_CONT;
                }
                break;

        case TE_SIGNAL_DELIVERY_STOP:
                restart_sig = WSTOPSIG(status);
                print_stopped(current_tcp, &wd->si, restart_sig);
                break;

        case TE_SIGNALLED:
                print_signalled(current_tcp, current_tcp->pid, status);
                droptcb(current_tcp);
                return true;

        case TE_GROUP_STOP:
                restart_sig = WSTOPSIG(status);
                print_stopped(current_tcp, NULL, restart_sig);
                if (use_seize) {
                        /*
                         * This ends ptrace-stop, but does *not* end group-stop.
                         * This makes stopping signals work properly on straced
                         * process (that is, process really stops. It used to
                         * continue to run).
                         */
                        restart_op = PTRACE_LISTEN;
                        restart_sig = 0;
                }
                break;

        case TE_EXITED:
                print_exited(current_tcp, current_tcp->pid, status);
                droptcb(current_tcp);
                return true;

        case TE_STOP_BEFORE_EXECVE:
                /* The syscall succeeded, clear the flag.  */
                current_tcp->flags &= ~TCB_CHECK_EXEC_SYSCALL;
                /*
                 * Check that we are inside syscall now (next event after
                 * PTRACE_EVENT_EXEC should be for syscall exiting).  If it is
                 * not the case, we might have a situation when we attach to a
                 * process and the first thing we see is a PTRACE_EVENT_EXEC
                 * and all the following syscall state tracking is screwed up
                 * otherwise.
                 */
                if (!maybe_switch_current_tcp() && entering(current_tcp)) {
                        int ret;

                        error_msg("Stray PTRACE_EVENT_EXEC from pid %d"
                                  ", trying to recover...",
                                  current_tcp->pid);

                        current_tcp->flags |= TCB_RECOVERING;
                        ret = trace_syscall(current_tcp, &restart_sig);
                        current_tcp->flags &= ~TCB_RECOVERING;

                        if (ret < 0) {
                                /* The reason is described in TE_SYSCALL_STOP */
                                return true;
                        }
                }

                if (detach_on_execve) {
                        if (current_tcp->flags & TCB_SKIP_DETACH_ON_FIRST_EXEC) {
                                current_tcp->flags &= ~TCB_SKIP_DETACH_ON_FIRST_EXEC;
                        } else {
                                detach(current_tcp); /* do "-b execve" thingy */
                                return true;
                        }
                }
                break;

        case TE_STOP_BEFORE_EXIT:
                print_event_exit(current_tcp);
                break;
        }

        /* We handled quick cases, we are permitted to interrupt now. */
        if (interrupted)
                return false;

        /* If the process is being delayed, do not ptrace_restart just yet */
        if (syscall_delayed(current_tcp)) {
                if (current_tcp->delayed_wait_data)
                        error_func_msg("pid %d has delayed wait data set"
                                       " already", current_tcp->pid);

                current_tcp->delayed_wait_data = copy_trace_wait_data(wd);

                return true;
        }

        if (ptrace_restart(restart_op, current_tcp, restart_sig) < 0) {
                /* Note: ptrace_restart emitted error message */
                exit_code = 1;
                return false;
        }
        return true;
}

static bool
restart_delayed_tcb(struct tcb *const tcp)
{
        struct tcb_wait_data *wd = tcp->delayed_wait_data;

        if (!wd) {
                error_func_msg("No delayed wait data found for pid %d",
                               tcp->pid);
                wd = init_trace_wait_data(alloca(trace_wait_data_size(tcp)));
        }

        wd->te = TE_RESTART;

        debug_func_msg("pid %d", tcp->pid);

        tcp->flags &= ~TCB_DELAYED;

        struct tcb *const prev_tcp = current_tcp;
        current_tcp = tcp;
        bool ret = dispatch_event(wd);
        current_tcp = prev_tcp;

        free_trace_wait_data(tcp->delayed_wait_data);
        tcp->delayed_wait_data = NULL;

        return ret;
}

static bool
restart_delayed_tcbs(void)
{
        struct tcb *tcp_next = NULL;
        struct timespec ts_now;

        clock_gettime(CLOCK_MONOTONIC, &ts_now);

        for (size_t i = 0; i < tcbtabsize; i++) {
                struct tcb *tcp = tcbtab[i];

                if (tcp->pid && syscall_delayed(tcp)) {
                        if (ts_cmp(&ts_now, &tcp->delay_expiration_time) > 0) {
                                if (!restart_delayed_tcb(tcp))
                                        return false;
                        } else {
                                /* Check whether this tcb is the next.  */
                                if (!tcp_next ||
                                    ts_cmp(&tcp_next->delay_expiration_time,
                                           &tcp->delay_expiration_time) > 0) {
                                        tcp_next = tcp;
                                }
                        }
                }
        }

        if (tcp_next)
                arm_delay_timer(tcp_next);

        return true;
}

/*
 * As this signal handler does a lot of work that is not suitable
 * for signal handlers, extra care must be taken to ensure that
 * it is enabled only in those places where it's safe.
 */
static void
timer_sighandler(int sig)
{
        delay_timer_expired();

        if (restart_failed)
                return;

        int saved_errno = errno;

        if (!restart_delayed_tcbs())
                restart_failed = 1;

        errno = saved_errno;
}

#ifdef ENABLE_COVERAGE_GCOV
extern void __gcov_flush(void);
#endif

static void ATTRIBUTE_NORETURN
terminate(void)
{
        int sig = interrupted;

        cleanup(sig);
        if (cflag)
                call_summary(shared_log);
        fflush(NULL);
        if (shared_log != stderr)
                fclose(shared_log);
        if (popen_pid) {
                while (waitpid(popen_pid, NULL, 0) < 0 && errno == EINTR)
                        ;
        }
        if (sig) {
                exit_code = 0x100 | sig;
        }
        if (exit_code > 0xff) {
                /* Avoid potential core file clobbering.  */
                struct_rlimit rlim = {0, 0};
                set_rlimit(RLIMIT_CORE, &rlim);

                /* Child was killed by a signal, mimic that.  */
                exit_code &= 0xff;
                signal(exit_code, SIG_DFL);
#ifdef ENABLE_COVERAGE_GCOV
                __gcov_flush();
#endif
                raise(exit_code);

                /* Unblock the signal.  */
                sigset_t mask;
                sigemptyset(&mask);
                sigaddset(&mask, exit_code);
#ifdef ENABLE_COVERAGE_GCOV
                __gcov_flush();
#endif
                sigprocmask(SIG_UNBLOCK, &mask, NULL);

                /* Paranoia - what if this signal is not fatal?
                   Exit with 128 + signo then.  */
                exit_code += 128;
        }
        exit(exit_code);
}

int
main(int argc, char *argv[])
{
        setlocale(LC_ALL, "");
        init(argc, argv);

        exit_code = !nprocs;

        while (dispatch_event(next_event()))
                ;
        terminate();
}