[PATCH 1/1] signal: kill the obsolete SIGNAL_UNKILLABLE check in complete_signal()

[Oleg Nesterov <oleg@redhat.com>]

complete_signal() checks SIGNAL_UNKILLABLE before it starts to destroy the
thread group, today this is unnecessary and even not 100% correct.

After the commit f008faff0e27 ("signals: protect init from unwanted signals
more") we rely on sig_task_ignored(), complete_signal(SIGKILL) can only see
a SIGNAL_UNKILLABLE task if we actually want to kill it. And note that after
the commit b3bfa0cba867 ("signals: protect cinit from blocked fatal signals")
we do not drop SIGKILL dequeued by /sbin/init.

And it does not look right. fatal_signal_pending() should always imply that
the whole thread group (except ->group_exit_task if it is not NULL) is killed,
this check breaks the rule.

This explains WARN_ON(!JOBCTL_STOP_PENDING) in task_participate_group_stop()
triggered by the test-case from Dmitry:

	int main()
	{
		int pid = 1;
		ptrace(PTRACE_ATTACH, pid, 0, 0);
		ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_EXITKILL);
		sleep(1);
		return 0;
	}

do_signal_stop()->signal_group_exit() returns false because SIGNAL_GROUP_EXIT
is not set, but task_set_jobctl_pending() checks fatal_signal_pending() and
does not set JOBCTL_STOP_PENDING.

The test-case above needs root and (correctly) crashes the kernel, but we can
trigger the same warning inside the container or using another test-case:

	static int init(void *arg)
	{
		for (;;)
			pause();
	}

	int main(void)
	{
		char stack[16 * 1024];

		for (;;) {
			int pid = clone(init, stack + sizeof(stack)/2,
					CLONE_NEWPID | SIGCHLD, NULL);
			assert(pid > 0);

			assert(ptrace(PTRACE_ATTACH, pid, 0, 0) == 0);
			assert(waitpid(-1, NULL, WSTOPPED) == pid);

			assert(ptrace(PTRACE_DETACH, pid, 0, SIGSTOP) == 0);
			assert(syscall(__NR_tkill, pid, SIGKILL) == 0);
			assert(pid == wait(NULL));
		}
	}

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/signal.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index d64efad..9b6f385 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -914,7 +914,7 @@ static void complete_signal(int sig, struct task_struct *p, int group)
 	 * then start taking the whole group down immediately.
 	 */
 	if (sig_fatal(p, sig) &&
-	    !(signal->flags & (SIGNAL_UNKILLABLE | SIGNAL_GROUP_EXIT)) &&
+	    !(signal->flags & SIGNAL_GROUP_EXIT) &&
 	    !sigismember(&t->real_blocked, sig) &&
 	    (sig == SIGKILL || !t->ptrace)) {
 		/*
-- 
1.5.5.1