kernel/signal.c: protect the traced SIGNAL_UNKILLABLE tasks from SIGKILL

The comment in sig_ignored() says "Tracers may want to know about even
ignored signals" but SIGKILL can not be reported to debugger and it is
just wrong to return 0 in this case: SIGKILL should only kill the
SIGNAL_UNKILLABLE task if it comes from the parent ns.

Change sig_ignored() to ignore ->ptrace if sig == SIGKILL and rely on
sig_task_ignored().

SISGTOP coming from within the namespace is not really right too but at
least debugger can intercept it, and we can't drop it here because this
will break "gdb -p 1": ptrace_attach() won't work.  Perhaps we will add
another ->ptrace check later, we will see.

Link: http://lkml.kernel.org/r/20171103184206.GB21036@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Tested-by: Kyle Huey <me@kylehuey.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/signal.c

@@ -94,13 +94,15 @@ static int sig_ignored(struct task_struct *t, int sig, bool force)
 	if (sigismember(&t->blocked, sig) || sigismember(&t->real_blocked, sig))
 		return 0;
 
-	if (!sig_task_ignored(t, sig, force))
-		return 0;
-
 	/*
-	 * Tracers may want to know about even ignored signals.
+	 * Tracers may want to know about even ignored signal unless it
+	 * is SIGKILL which can't be reported anyway but can be ignored
+	 * by SIGNAL_UNKILLABLE task.
 	 */
-	return !t->ptrace;
+	if (t->ptrace && sig != SIGKILL)
+		return 0;
+
+	return sig_task_ignored(t, sig, force);
 }
 
 /*