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

From: Oleg Nesterov
Date: Wed Nov 04 2015 - 13:23:06 EST

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);
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

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 (;;)

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

for (;;) {
int pid = clone(init, stack + sizeof(stack)/2,
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@xxxxxxxxxx>
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
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)) {

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