Delivering SIGKILL to init

[Peter Jeremy]

On Tue, 2006-Sep-19 17:55:50 +1000, Peter Jeremy wrote:
>Prompted by some discussion elsewhere, I've been trying to send
>SIGKILL to init.  If I ktrace kill(1), I can see "kill(1,9)" which
>returns 0 but the signal is never delivered.  If I sent (eg) SIGXCPU
>then init dies and the kernel panics (as expected).  I've looked
>through sys/kern/kern_* and can't see anywhere that special cases
>the delivery of SIGKILL to init.

For anyone else interested:

I was pointed to code in kern_sig.c:issignal() by a friend.  The
signal action handling switch (about line 2172 in -stable) ignores
any signals marked SIG_DFL for "system" processes (those with a
PID of 1 or less).  Since SIGKILL is marked SIG_DFL (because it
can't be changed), this means SIGKILL isn't delivered.  SIGXCPU
(and a variety of other signals) have a handler defined by init
so they aren't SIG_DFL and therefore are delivered.

-- 
Peter Jeremy
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