linux-kernel - Re: [PATCH 2/4] pids: init_struct_pid.tasks should never see the swapper process

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Date:	Tue, 11 May 2010 04:54:10 -0500
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Cedric Le Goater <clg@...t.ibm.com>,
	Dave Hansen <haveblue@...ibm.com>,
	Eric Biederman <ebiederm@...ssion.com>,
	Herbert Poetzl <herbert@...hfloor.at>,
	Ingo Molnar <mingo@...e.hu>,
	Mathias Krause <Mathias.Krause@...unet.com>,
	Roland McGrath <roland@...hat.com>,
	Sukadev Bhattiprolu <sukadev@...ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] pids: init_struct_pid.tasks should never see the
 swapper process

Quoting Oleg Nesterov (oleg@...hat.com):
> "statically initialize struct pid for swapper" commit 820e45db says:
> 
> 	Statically initialize a struct pid for the swapper process (pid_t == 0)
> 	and attach it to init_task.  This is needed so task_pid(), task_pgrp()
> 	and task_session() interfaces work on the swapper process also.
> 
> OK, but:
> 
> 	- it doesn't make sense to add init_task.pids[].node into
> 	  init_struct_pid.tasks[], and in fact this just wrong.
> 
> 	  idle threads are special, they shouldn't be visible on any
> 	  global list. In particular do_each_pid_task(init_struct_pid)
> 	  shouldn't see swapper.
> 
> 	  This is the actual reason why kill(0, SIGKILL) from /sbin/init
> 	  (which starts with 0,0 special pids) crashes the kernel. The
> 	  signal sent to pgid/sid == 0 must never see idle threads, even
> 	  if the previous patch fixed the crash itself.
> 
> 	- we have other idle threads running on the non-boot CPUs, see
> 	  the next patch.
> Change INIT_STRUCT_PID/INIT_PID_LINK to create the empty/unhashed
> hlist_head/hlist_node. Like any other idle thread swapper can never exit,
> so detach_pid()->__hlist_del() is not possible, but we could change
> INIT_PID_LINK() to set pprev = &next if needed.
> 
> All we need is the valid swapper->pids[].pid == &init_struct_pid.
> 
> Reported-by: Mathias Krause <mathias.krause@...unet.com>

Crimey, trying to find some way this could get dereferenced, finding
myself impressed with the likes of set_ftrace_swapper().

Anyway, not finding anything, so

> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

Acked-by: Serge E. Hallyn <serue@...ibm.com>

thanks,
-serge
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