[PATCH v2] If init dies, log a signal which killed it, if any.

[Denys Vlasenko <vda.linux@googlemail.com>]

I just received another user's pleas for help when their
init mysteriously died. I again explained that they need to check
whether it died because of bad instruction, a segv, or something else.
Which was an annoying detour into writing a trivial C program
to spawn his init and print its exit code:

http://lists.busybox.net/pipermail/busybox/2012-January/077172.html

I hear you saying "just test it under /bin/sh". Well, the crashing init
_was_ /bin/sh.

Which prompted me to make kernel do this first step automatically.
We can print exit code, which makes it possible to see that
death was from e.g. SIGILL without writing test programs.

The code is fairly self-explanatory. Compile-tested.

Changes in v.2: don't try to decode signal names, just print
exit status in hex.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
---
 kernel/exit.c |    7 +++++--
 1 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 294b170..0c540a5 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -710,8 +710,11 @@ static struct task_struct *find_new_reaper(struct task_struct *father)
 
 	if (unlikely(pid_ns->child_reaper == father)) {
 		write_unlock_irq(&tasklist_lock);
-		if (unlikely(pid_ns == &init_pid_ns))
-			panic("Attempted to kill init!");
+		if (unlikely(pid_ns == &init_pid_ns)) {
+			panic("Attempted to kill init! exitcode=%08x\n",
+				father->signal->group_exit_code ?:
+					father->exit_code);
+		}
 
 		zap_pid_ns_processes(pid_ns);
 		write_lock_irq(&tasklist_lock);
-- 
1.7.7.5