linux-kernel - [PATCH 1/2] ptrace: partially fix the do_wait(WEXITED) vs EXIT_DEAD->EXIT

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Date:	Wed, 4 Jan 2012 17:29:02 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Denys Vlasenko <vda.linux@...glemail.com>,
	Lukasz Michalik <lmi@....uni.wroc.pl>,
	"Dmitry V. Levin" <ldv@...linux.org>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: [PATCH 1/2] ptrace: partially fix the do_wait(WEXITED) vs
	EXIT_DEAD->EXIT_ZOMBIE race

Test-case:

	int main(void)
	{
		int pid, status;

		pid = fork();
		if (!pid) {
			for (;;) {
				if (!fork())
					return 0;
				if (waitpid(-1, &status, 0) < 0) {
					printf("ERR!! wait: %m\n");
					return 0;
				}
			}
		}

		assert(ptrace(PTRACE_ATTACH, pid, 0,0) == 0);
		assert(waitpid(-1, NULL, 0) == pid);

		assert(ptrace(PTRACE_SETOPTIONS, pid, 0,
					PTRACE_O_TRACEFORK) == 0);

		do {
			ptrace(PTRACE_CONT, pid, 0, 0);
			pid = waitpid(-1, NULL, 0);
		} while (pid > 0);

		return 1;
	}

It fails because ->real_parent sees its child in EXIT_DEAD state
while the tracer is going to change the state back to EXIT_ZOMBIE
in wait_task_zombie().

The offending commit is 823b018e which moved the EXIT_DEAD check,
but in fact we should not blame it. The original code was not
correct as well because it didn't take ptrace_reparented() into
account and because we can't really trust ->ptrace.

This patch adds the additional check to close this particular
race but it doesn't solve the whole problem. We simply can't
rely on ->ptrace in this case, it can be cleared if the tracer
is multithreaded by the exiting ->parent.

I think we should kill EXIT_DEAD altogether, we should always
remove the soon-to-be-reaped child from ->children or at least
we should never do the DEAD->ZOMBIE transition. But this is too
complex for 3.2.

Reported-and-tested-by: Denys Vlasenko <vda.linux@...glemail.com>
Tested-by: Lukasz Michalik <lmi@....uni.wroc.pl>
Acked-by: Tejun Heo <tj@...nel.org>
Cc: <stable@...nel.org>		[3.0+]
Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 kernel/exit.c |    9 ++++++++-
 1 files changed, 8 insertions(+), 1 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index d0b7d98..e6e01b9 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1540,8 +1540,15 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace,
 	}
 
 	/* dead body doesn't have much to contribute */
-	if (p->exit_state == EXIT_DEAD)
+	if (unlikely(p->exit_state == EXIT_DEAD)) {
+		/*
+		 * But do not ignore this task until the tracer does
+		 * wait_task_zombie()->do_notify_parent().
+		 */
+		if (likely(!ptrace) && unlikely(ptrace_reparented(p)))
+			wo->notask_error = 0;
 		return 0;
+	}
 
 	/* slay zombie? */
 	if (p->exit_state == EXIT_ZOMBIE) {
-- 
1.5.5.1


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