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Date: Sat, 17 Mar 2007 18:24:37 +0300
From: Oleg Nesterov <oleg@tv-sign.ru>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Sukadev Bhattiprolu <sukadev@us.ibm.com>,
       Cedric Le Goater <clg@fr.ibm.com>, Dave Hansen <haveblue@us.ibm.com>,
       Serge Hallyn <serue@us.ibm.com>, containers@lists.osdl.org,
       Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org
Subject: Re: + remove-the-likelypid-check-in-copy_process.patch added to -mm tree
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References: <20070316170255.GA194@tv-sign.ru> <m1y7lxm4cc.fsf@ebiederm.dsl.xmission.com> <20070317130216.GA78@tv-sign.ru> <m1k5xgklun.fsf@ebiederm.dsl.xmission.com> <20070317150949.GA302@tv-sign.ru>
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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?

Oleg.