From: Eugene Syromyatnikov <evgsyr@gmail.com>
Date: Wed, 3 Jan 2018 16:36:25 +0000 (+0100)
Subject: Call get_scno during startup_tcb only for forcibly attached processes
X-Git-Tag: v4.21~229
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=576e2db4d0175334fccaf38e04bfc8537344015d;p=strace

Call get_scno during startup_tcb only for forcibly attached processes

Otherwise it makes little sense on most arches to try to get syscall
number.

* defs.h (TCB_GRABBED): New tcb flag.
* strace.c (attach_tcb): Set TCB_GRABBED for the tcb.
(startup_tcb): Call get_scno() only if process is grabbed and its
registers may contain syscall number information.

Reported-by: Dmitry V. Levin <ldv@altlinux.org>
Closes: https://github.com/strace/strace/issues/22
---

diff --git a/defs.h b/defs.h
index afb2cb0b..35761290 100644
--- a/defs.h
+++ b/defs.h
@@ -244,6 +244,8 @@ struct tcb {
 #define TCB_TAMPERED	0x40	/* A syscall has been tampered with */
 #define TCB_HIDE_LOG	0x80	/* We should hide everything (until execve) */
 #define TCB_SKIP_DETACH_ON_FIRST_EXEC	0x100	/* -b execve should skip detach on first execve */
+#define TCB_GRABBED	0x200 /* We grab the process and can catch it
+			       * in the middle of a syscall */
 
 /* qualifier flags */
 #define QUAL_TRACE	0x001	/* this system call should be traced */
diff --git a/strace.c b/strace.c
index 70906248..7491a2e9 100644
--- a/strace.c
+++ b/strace.c
@@ -1002,7 +1002,8 @@ attach_tcb(struct tcb *const tcp)
 		return;
 	}
 
-	tcp->flags |= TCB_ATTACHED | TCB_STARTUP | post_attach_sigstop;
+	tcp->flags |= TCB_ATTACHED | TCB_GRABBED | TCB_STARTUP |
+		      post_attach_sigstop;
 	newoutf(tcp);
 	debug_msg("attach to pid %d (main) succeeded", tcp->pid);
 
@@ -1033,8 +1034,8 @@ attach_tcb(struct tcb *const tcp)
 			debug_msg("attach to pid %d succeeded", tid);
 
 			struct tcb *tid_tcp = alloctcb(tid);
-			tid_tcp->flags |= TCB_ATTACHED | TCB_STARTUP |
-					  post_attach_sigstop;
+			tid_tcp->flags |= TCB_ATTACHED | TCB_GRABBED |
+					  TCB_STARTUP | post_attach_sigstop;
 			newoutf(tid_tcp);
 		}
 
@@ -2149,7 +2150,7 @@ startup_tcb(struct tcb *tcp)
 		}
 	}
 
-	if (get_scno(tcp) == 1)
+	if ((tcp->flags & TCB_GRABBED) && (get_scno(tcp) == 1))
 		tcp->s_prev_ent = tcp->s_ent;
 }