[PATCH] signal: Restore the stop PTRACE_EVENT_EXIT

[Eric W. Biederman]

In the middle of do_exit() there is there is a call
"ptrace_event(PTRACE_EVENT_EXIT, code);" That call places the process
in TACKED_TRACED aka "(TASK_WAKEKILL | __TASK_TRACED)" and waits for
for the debugger to release the task or SIGKILL to be delivered.

Skipping past dequeue_signal when we know a fatal signal has already
been delivered resulted in SIGKILL remaining pending and
TIF_SIGPENDING remaining set.  This in turn caused the
scheduler to not sleep in PTACE_EVENT_EXIT as it figured
a fatal signal was pending.  This also caused ptrace_freeze_traced
in ptrace_check_attach to fail because it left a per thread
SIGKILL pending which is what fatal_signal_pending tests for.

This difference in signal state caused strace to report
strace: Exit of unknown pid NNNNN ignored

Therefore update the signal handling state like dequeue_signal
would when removing a per thread SIGKILL, by removing SIGKILL
from the per thread signal mask and clearing TIF_SIGPENDING.

Reported-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Reported-by: Ivan Delalande <colona@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Fixes: 35634ffa1751 ("signal: Always notice exiting tasks")
Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
---

Ivan this change fixes the issues you reported to me.  Can you confirm?
Oleg this looks like the most conservative regression fix I can manage.

 kernel/signal.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 99fa8ff06fd9..57b7771e20d7 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2436,9 +2436,12 @@ bool get_signal(struct ksignal *ksig)
 	}
 
 	/* Has this task already been marked for death? */
-	ksig->info.si_signo = signr = SIGKILL;
-	if (signal_group_exit(signal))
+	if (signal_group_exit(signal)) {
+		ksig->info.si_signo = signr = SIGKILL;
+		sigdelset(&current->pending.signal, SIGKILL);
+		recalc_sigpending();
 		goto fatal;
+	}
 
 	for (;;) {
 		struct k_sigaction *ka;
-- 
2.17.1