exit: fix race between wait_consider_task() and wait_task_zombie() (3245d6ac) · Commits · 戴 / test

wait_consider_task() checks EXIT_ZOMBIE after EXIT_DEAD/EXIT_TRACE and both checks can fail if we race with EXIT_ZOMBIE -> EXIT_DEAD/EXIT_TRACE change in between, gcc needs to reload p->exit_state after security_task_wait(). In this case ->notask_error will be wrongly cleared and do_wait() can hang forever if it was the last eligible child. Many thanks to Arne who carefully investigated the problem. Note: this bug is very old but it was pure theoretical until commit ("wait: completely ignore the EXIT_DEAD tasks"). Before this commit "-O2" was probably enough to guarantee that compiler won't read ->exit_state twice. Signed-off-by:

Oleg Nesterov <oleg@redhat.com> Reported-by:

Arne Goedeke <el@laramies.com> Tested-by:

Arne Goedeke <el@laramies.com> Cc: <stable@vger.kernel.org> [3.15+] Signed-off-by:

Andrew Morton <akpm@linux-foundation.org> Signed-off-by:

Linus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

+9 −3

Original line number	Diff line number	Diff line
		@@ -1287,9 +1287,15 @@ static int wait_task_continued(struct wait_opts wo, struct task_struct p)
		static int wait_consider_task(struct wait_opts *wo, int ptrace,
		struct task_struct *p)
		{
		/*
		* We can race with wait_task_zombie() from another thread.
		* Ensure that EXIT_ZOMBIE -> EXIT_DEAD/EXIT_TRACE transition
		* can't confuse the checks below.
		*/
		int exit_state = ACCESS_ONCE(p->exit_state);
		int ret;

		if (unlikely(p->exit_state == EXIT_DEAD))
		if (unlikely(exit_state == EXIT_DEAD))
		return 0;

		ret = eligible_child(wo, p);
		@@ -1310,7 +1316,7 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace,
		return 0;
		}

		if (unlikely(p->exit_state == EXIT_TRACE)) {
		if (unlikely(exit_state == EXIT_TRACE)) {
		/*
		* ptrace == 0 means we are the natural parent. In this case
		* we should clear notask_error, debugger will notify us.
		@@ -1337,7 +1343,7 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace,
		}

		/* slay zombie? */
		if (p->exit_state == EXIT_ZOMBIE) {
		if (exit_state == EXIT_ZOMBIE) {
		/* we don't reap group leaders with subthreads */
		if (!delay_group_leader(p)) {
		/*

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