From eac1b5e57d7abc836e78fd3fbcf77dbeed01edc9 Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Thu, 21 Jul 2011 20:00:43 +0200 Subject: ptrace: do_wait(traced_leader_killed_by_mt_exec) can block forever Test-case: void *tfunc(void *arg) { execvp("true", NULL); return NULL; } int main(void) { int pid; if (fork()) { pthread_t t; kill(getpid(), SIGSTOP); pthread_create(&t, NULL, tfunc, NULL); for (;;) pause(); } pid = getppid(); assert(ptrace(PTRACE_ATTACH, pid, 0,0) == 0); while (wait(NULL) > 0) ptrace(PTRACE_CONT, pid, 0,0); return 0; } It is racy, exit_notify() does __wake_up_parent() too. But in the likely case it triggers the problem: de_thread() does release_task() and the old leader goes away without the notification, the tracer sleeps in do_wait() without children/tracees. Change de_thread() to do __wake_up_parent(traced_leader->parent). Since it is already EXIT_DEAD we can do this without ptrace_unlink(), EXIT_DEAD threads do not exist from do_wait's pov. Signed-off-by: Oleg Nesterov Acked-by: Tejun Heo --- fs/exec.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/fs/exec.c b/fs/exec.c index b08367abf30..d219541db06 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -967,6 +967,14 @@ static int de_thread(struct task_struct *tsk) BUG_ON(leader->exit_state != EXIT_ZOMBIE); leader->exit_state = EXIT_DEAD; + + /* + * We are going to release_task()->ptrace_unlink() silently, + * the tracer can sleep in do_wait(). EXIT_DEAD guarantees + * the tracer wont't block again waiting for this thread. + */ + if (unlikely(leader->ptrace)) + __wake_up_parent(leader, leader->parent); write_unlock_irq(&tasklist_lock); release_task(leader); -- cgit v1.2.3-70-g09d2