From 788e05a67c343fa22f2ae1d3ca264e7f15c25eaf Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Fri, 7 Oct 2005 17:46:19 +0400 Subject: [PATCH] [PATCH] fix do_coredump() vs SIGSTOP race Let's suppose we have 2 threads in thread group: A - does coredump B - has pending SIGSTOP thread A thread B do_coredump: get_signal_to_deliver: lock(->sighand) ->signal->flags = SIGNAL_GROUP_EXIT unlock(->sighand) lock(->sighand) signr = dequeue_signal() ->signal->flags |= SIGNAL_STOP_DEQUEUED return SIGSTOP; do_signal_stop: unlock(->sighand) coredump_wait: zap_threads: lock(tasklist_lock) send SIGKILL to B // signal_wake_up() does nothing unlock(tasklist_lock) lock(tasklist_lock) lock(->sighand) re-check sig->flags & SIGNAL_STOP_DEQUEUED, yes set_current_state(TASK_STOPPED); finish_stop: schedule(); // ->state == TASK_STOPPED wait_for_completion(&startup_done) // waits for complete() from B, // ->state == TASK_UNINTERRUPTIBLE We can't wake up 'B' in any way: SIGCONT will be ignored because handle_stop_signal() sees ->signal->flags & SIGNAL_GROUP_EXIT. sys_kill(SIGKILL)->__group_complete_signal() will choose uninterruptible 'A', so it can't help. sys_tkill(B, SIGKILL) will be ignored by specific_send_sig_info() because B already has pending SIGKILL. This scenario is not possbile if 'A' does do_group_exit(), because it sets sig->flags = SIGNAL_GROUP_EXIT and delivers SIGKILL to subthreads atomically, holding both tasklist_lock and sighand->lock. That means that do_signal_stop() will notice !SIGNAL_STOP_DEQUEUED after re-locking ->sighand. And it is not possible to any other thread to re-add SIGNAL_STOP_DEQUEUED later, because dequeue_signal() can only return SIGKILL. I think it is better to change do_coredump() to do sigaddset(SIGKILL) and signal_wake_up() under sighand->lock, but this patch is much simpler. Signed-off-by: Oleg Nesterov Signed-off-by: Linus Torvalds --- kernel/signal.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/kernel/signal.c b/kernel/signal.c index 619b027e92..c135f5aa2c 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -578,7 +578,8 @@ int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info) * is to alert stop-signal processing code when another * processor has come along and cleared the flag. */ - tsk->signal->flags |= SIGNAL_STOP_DEQUEUED; + if (!(tsk->signal->flags & SIGNAL_GROUP_EXIT)) + tsk->signal->flags |= SIGNAL_STOP_DEQUEUED; } if ( signr && ((info->si_code & __SI_MASK) == __SI_TIMER) && -- 2.39.5