linux-kernel - Re: + remove-the-likelypid-check-in-copy

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Date:	Sat, 17 Mar 2007 11:01:50 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Sukadev Bhattiprolu <sukadev@...ibm.com>,
	Cedric Le Goater <clg@...ibm.com>,
	Dave Hansen <haveblue@...ibm.com>,
	Serge Hallyn <serue@...ibm.com>, containers@...ts.osdl.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: + remove-the-likelypid-check-in-copy_process.patch added to -mm tree

Oleg Nesterov <oleg@...sign.ru> writes:

> On 03/17, Oleg Nesterov wrote:
>>
>> > Well the initial kernel process does not have a struct pid so when
>> > it's children start doing:
>> > 	attach_pid(p, PIDTYPE_PGID, task_group(p));
>> > 	attach_pid(p, PIDTYPE_SID, task_session(p));
>> > We will get an oops.
>> 
>> So far this is the only reason to have init_struct_pid. Because the
>> boot CPU (swapper) forks, right?
>
> Damn. I am afraid I was not clear again :) Not init_struct_pid, but
>
> 	+ .pids = { \
> 	+ [PIDTYPE_PID] = INIT_PID_LINK(PIDTYPE_PID), \
> 	+ [PIDTYPE_PGID] = INIT_PID_LINK(PIDTYPE_PGID), \
> 	+ [PIDTYPE_SID] = INIT_PID_LINK(PIDTYPE_SID), \
> 	+ }, \
>
> for INIT_TASK().
>
>> > So a dummy unhashed struct pid was added for the idle threads.
>> > Allowing several special cases in the code to be removed.
>> > 
>> > With that chance the previous special case to force the idle thread
>> > init session 1 pgrp 1 no longer works because attach_pid no longer
>> > looks at the pid value but instead at the struct pid pointers.
>> > 
>> > So we had to add the __set_special_pids() to continue to keep init
>> > in session 1 pgrp 1.  Since /sbin/init calls setsid() that our setting
>> > the sid and the pgrp may not be strictly necessary.  Still is better
>> > to not take any chances.
>> 
>> Yes, yes, I see. But my (very unclear, sorry) question was: shouldn't we
>> change INIT_SIGNALS then? /sbin/init inherits ->pgrp == ->_session == 1,
>> in that case __set_special_pids(1,1) does nothing.
>
> ... and thus /sbin/init remains attached to the .pids above, no?

The problem is that we dynamically allocate the struct pid for 
pid_t == 1 when we fork init.

Which means we don't have access to it at compile time so we can
no longer make INIT_SIGNALS set ->gprp == ->session == 1.

Eric
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