[patch 10/14] signal: don't remove SIGNAL_UNKILLABLE for traced tasks.

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From: Jamie Iles <jamie.iles@xxxxxxxxxx>
Subject: signal: don't remove SIGNAL_UNKILLABLE for traced tasks.

When forcing a signal, SIGNAL_UNKILLABLE is removed to prevent recursive
faults, but this is undesirable when tracing.  For example, debugging an
init process (whether global or namespace), hitting a breakpoint and
SIGTRAP will force SIGTRAP and then remove SIGNAL_UNKILLABLE.  Everything
continues fine, but then once debugging has finished, the init process is
left killable which is unlikely what the user expects, resulting in either
an accidentally killed init or an init that stops reaping zombies.

Link: http://lkml.kernel.org/r/20170815112806.10728-1-jamie.iles@xxxxxxxxxx
Signed-off-by: Jamie Iles <jamie.iles@xxxxxxxxxx>
Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 kernel/signal.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff -puN kernel/signal.c~signal-dont-remove-signal_unkillable-for-traced-tasks kernel/signal.c
--- a/kernel/signal.c~signal-dont-remove-signal_unkillable-for-traced-tasks
+++ a/kernel/signal.c
@@ -1194,7 +1194,11 @@ force_sig_info(int sig, struct siginfo *
 			recalc_sigpending_and_wake(t);
 		}
 	}
-	if (action->sa.sa_handler == SIG_DFL)
+	/*
+	 * Don't clear SIGNAL_UNKILLABLE for traced tasks, users won't expect
+	 * debugging to leave init killable.
+	 */
+	if (action->sa.sa_handler == SIG_DFL && !t->ptrace)
 		t->signal->flags &= ~SIGNAL_UNKILLABLE;
 	ret = specific_send_sig_info(sig, info, t);
 	spin_unlock_irqrestore(&t->sighand->siglock, flags);
_
--
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