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Subject[PATCH 1/3] 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.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
kernel/signal.c | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 800a18f..b9aebe1 100644
--- a/kernel/signal.c
+++ b/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);
}

/*
--
2.5.0

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