linux-kernel - [PATCH] prevent sending wrong signals to a traced process whose tracer gets killed

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Message-Id: <1196774846.19677.3.camel@elijah.suse.cz>
Date:	Tue, 04 Dec 2007 14:27:25 +0100
From:	Petr Tesarik <ptesarik@...e.cz>
To:	Roland McGrath <roland@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Torvalds <torvalds@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org
Subject: [PATCH] prevent sending wrong signals to a traced process whose
	tracer gets killed

Hi,

I experienced troubles when tracing a process with strace. Sometimes,
when I killed the strace process (SIGKILL), the traced process was also
killed. I found out that it was getting SIGTRAP and, indeed, when the
traced process set up a signal handler for SIGTRAP, it no longer died.

I noticed that normally, when the traced process is continued (via
PTRACE_CONT or similar), the signal to be sent to it is stored in
current->exit_code, which is then examined by the arch-specific code and
usually leads to something like:

                send_sig(current->exit_code, current, 1);

The exit_code is set in ptrace_stop(), but the tracing process may go
away while the traced process waits for it, and in that case exit_code
is left as-is. I think we must set it to zero in ptrace_untrace().

Signed-off-by: Petr Tesarik <ptesarik@...e.cz>

 ptrace.c |    1 +
 1 file changed, 1 insertion(+)

diff -pru a/kernel/ptrace.c b/kernel/ptrace.c
--- a/kernel/ptrace.c	2007-12-04 14:12:51.000000000 +0100
+++ b/kernel/ptrace.c	2007-12-04 14:13:28.000000000 +0100
@@ -52,6 +52,7 @@ void ptrace_untrace(struct task_struct *
 {
 	spin_lock(&child->sighand->siglock);
 	if (child->state == TASK_TRACED) {
+		child->exit_code = 0;
 		if (child->signal->flags & SIGNAL_STOP_STOPPED) {
 			child->state = TASK_STOPPED;
 		} else {

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