linux-kernel - Re: [PATCH v2 4/4] proc: avoid ->f_pos overflows in proc_task

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Date:	Tue, 4 Jun 2013 01:58:53 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Michal Hocko <mhocko@...e.cz>,
	Sergey Dyasly <dserrg@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/4] proc: avoid ->f_pos overflows in
 proc_task_readdir() paths

On Mon, Jun 03, 2013 at 09:07:05PM +0200, Oleg Nesterov wrote:
> 1. proc_task_readdir() truncates f_pos to long, this can lead
>    to wrong result on 32bit.
> 
> 2. first_tid() truncates f_pos to int, this is wrong even on
>    64bit.
> 
>    We could check that f_pos < PID_MAX or even INT_MAX in
>    proc_task_readdir(), but this patch simply checks the
>    potential overflow in first_tid(), this check is nop on
>    64bit. We do not care if it was negative and the new
>    unsigned value is huge, all we need to ensure is that we
>    never wrongly return !NULL.
> 
> 3. Remove the 2nd "nr != 0" check before get_nr_threads(),
>    nr_threads == 0 is not distinguishable from !pid_task()
>    above.

Oleg, please take a look at the series in vfs.git#experimental; at the very
least, we don't want to access file->f_pos in any foo_readdir() - it's too
messy and race-prone.  It's pretty much independent from the issues you
are dealing with, but let's avoid creating pointless conflicts...
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