Re: [PATCH v2 4/4] proc: avoid ->f_pos overflows inproc_task_readdir() paths

From: Al Viro
Date: Mon Jun 03 2013 - 20:59:20 EST

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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