linux-kernel - Re: [RFC][PATCH -mm] replace cad

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Date:	Thu, 21 Sep 2006 15:21:23 +0200
From:	Cedric Le Goater <clg@...ibm.com>
To:	Andrew Morton <akpm@...l.org>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [RFC][PATCH -mm] replace cad_pid by a struct pid

Hello,

Andrew Morton wrote:
> On Wed, 20 Sep 2006 11:38:35 +0200
> Cedric Le Goater <clg@...ibm.com> wrote:
> 
>> There are a few places in the kernel where the init task is
>> signaled. The ctrl+alt+del sequence is one them. It kills a task,
>> usually init, using a cached pid (cad_pid).
>>
>> This patch replaces the pid_t by a struct pid to avoid pid wrap around
>> problem. The struct pid is initialized at boot time in init() and can
>> be modified through systctl with
>>
>> 	/proc/sys/kernel/cad_pid
> 
> hm.  Is there any sane scenario in which C-A-D would be directed to any
> other process?

I don't know. I really looked hard to see where this was used and i didn't
find any distro using it. 
 
> What happens if/when the process which is identifier by
> /proc/sys/kernel/cad_pid exits?  User error, I guess...

kill_pid() fails.

>> +extern struct pid* cad_pid;
> 
> Whitespace violation detected!
> 
>> -	if (shuting_down || kill_proc(1, SIGINT, 1)) {
>> +	if (shuting_down || kill_cad_pid(SIGINT, 1)) {
> 
> So your patch actually makes functional changes: lots of random
> init-signallers gain extra functionality: the process which they signal can
> now be configured via /proc/sys/kernel/cad_pid.  But by default, things
> remain unchanged.  Fair enough.

yes.

>> --- 2.6.18-rc7-mm1.orig/drivers/char/nwbutton.c
>> +++ 2.6.18-rc7-mm1/drivers/char/nwbutton.c
> 
> This driver can be compiled as a module.  I shall add the missing export...

oops thanks,

> (And I'll fix the parisc build too)

the only parisc i have still runs its native hpux 11. 

do we still need /proc/sys/kernel/cad_pid ? If it is obsolete, i'd send a 
different patch to disable this feature with some warning in dmesg and the
kill_proc(1) would stay as they are.

but may be some small distros for embedded linux use it.  

thanks,

C.
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