[PATCH] fix do_coredump() vs SIGSTOP race
Oleg Nesterov [Fri, 7 Oct 2005 13:46:19 +0000 (17:46 +0400)]
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 <oleg@tv-sign.ru>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/signal.c

index 619b027..c135f5a 100644 (file)
@@ -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) &&