From cd0ae687deadbb05ed373d675e59a36f8915c86d Mon Sep 17 00:00:00 2001 From: Rich Felker Date: Mon, 5 Feb 2018 11:31:11 -0500 Subject: [PATCH] revert regression in faccessat AT_EACCESS robustness commit f9fb20b42da0e755d93de229a5a737d79a0e8f60 switched from using a pipe for the result to conveying it via the child process exit status. Alexander Monakov pointed out that the latter could fail if the application is not expecting faccessat to produce a child and performs a wait operation with __WCLONE or __WALL, and that it is not clear whether it's guaranteed to work when SIGCHLD's disposition has been set to SIG_IGN. in addition, that commit introduced a bug that caused EACCES to be produced instead of EBUSY due to an exit path that was overlooked when the error channel was changed, and introduced a spurious retry loop around the wait operation. --- src/unistd/faccessat.c | 35 ++++++++++++++--------------------- 1 file changed, 14 insertions(+), 21 deletions(-) diff --git a/src/unistd/faccessat.c b/src/unistd/faccessat.c index 33478959..76bbd4c7 100644 --- a/src/unistd/faccessat.c +++ b/src/unistd/faccessat.c @@ -1,6 +1,5 @@ #include #include -#include #include #include "syscall.h" #include "pthread_impl.h" @@ -9,26 +8,19 @@ struct ctx { int fd; const char *filename; int amode; -}; - -static const int errors[] = { - 0, -EACCES, -ELOOP, -ENAMETOOLONG, -ENOENT, -ENOTDIR, - -EROFS, -EBADF, -EINVAL, -ETXTBSY, - -EFAULT, -EIO, -ENOMEM, - -EBUSY + int p; }; static int checker(void *p) { struct ctx *c = p; int ret; - int i; if (__syscall(SYS_setregid, __syscall(SYS_getegid), -1) || __syscall(SYS_setreuid, __syscall(SYS_geteuid), -1)) __syscall(SYS_exit, 1); ret = __syscall(SYS_faccessat, c->fd, c->filename, c->amode, 0); - for (i=0; i < sizeof errors/sizeof *errors - 1 && ret!=errors[i]; i++); - return i; + __syscall(SYS_write, c->p, &ret, sizeof ret); + return 0; } int faccessat(int fd, const char *filename, int amode, int flag) @@ -42,20 +34,21 @@ int faccessat(int fd, const char *filename, int amode, int flag) char stack[1024]; sigset_t set; pid_t pid; - int ret = -EBUSY; - struct ctx c = { .fd = fd, .filename = filename, .amode = amode }; + int status; + int ret, p[2]; + + if (pipe2(p, O_CLOEXEC)) return __syscall_ret(-EBUSY); + struct ctx c = { .fd = fd, .filename = filename, .amode = amode, .p = p[1] }; __block_all_sigs(&set); pid = __clone(checker, stack+sizeof stack, 0, &c); - if (pid > 0) { - int status; - do { - __syscall(SYS_wait4, pid, &status, __WCLONE, 0); - } while (!WIFEXITED(status) && !WIFSIGNALED(status)); - if (WIFEXITED(status)) - ret = errors[WEXITSTATUS(status)]; - } + __syscall(SYS_close, p[1]); + + if (pid<0 || __syscall(SYS_read, p[0], &ret, sizeof ret) != sizeof(ret)) + ret = -EBUSY; + __syscall(SYS_close, p[0]); + __syscall(SYS_wait4, pid, &status, __WCLONE, 0); __restore_sigs(&set); -- 2.40.0