On Wed, 4 Nov 2015 20:19:12 +0100 Oleg Nesterov <o...@redhat.com> wrote:
> 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));
> }
> }
I'm thinking this should be backported into -stable due to WARN_ONs and
kernel crashes. And as f008faff0e27 is from 2009, that means all
kernels.
Your thoughts on this?
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