ptrace: partially fix the do_wait(WEXITED) vs EXIT_DEAD->EXIT_ZOMBIE race

oleg-nesterov · torvalds · commit 50b8d257486a · 2012-01-04T15:01:59.000-08:00
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 823b018 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@googlemail.com> Tested-by: Lukasz Michalik <lmi@ift.uni.wroc.pl> Acked-by: Tejun Heo <tj@kernel.org> Cc: <stable@kernel.org> [3.0+] Signed-off-by: Oleg Nesterov <oleg@redhat.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git 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) {
