linux-kernel - Re: [PATCH try #4] proc: fix PAGE_SIZE limit of /proc/$PID/cmdline

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Message-Id: <7B725C19-A41C-4F65-923F-9990FED8EE1E@redhat.com>
Date:	Wed, 27 May 2015 01:27:13 -0400 (EDT)
From:	Jarod Wilson <jwilson@...hat.com>
To:	Alexey Dobriyan <adobriyan@...il.com>
Cc:	Jarod Wilson <jarod@...hat.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH try #4] proc: fix PAGE_SIZE limit of /proc/$PID/cmdline

On May 26, 2015, at 5:24 PM, Alexey Dobriyan <adobriyan@...il.com> wrote:
> 
>> On Tue, May 26, 2015 at 04:42:36PM -0400, Jarod Wilson wrote:
>>> On 5/8/2015 8:28 AM, Alexey Dobriyan wrote:
>>> /proc/$PID/cmdline truncates output at PAGE_SIZE. It is easy to see with
>>> 
>>>    $ cat /proc/self/cmdline $(seq 1037) 2>/dev/null
>>> 
>>> However, command line size was never limited to PAGE_SIZE but to 128 KB and
>>> relatively recently limitation was removed altogether.
>>> 
>>> People noticed and ask questions:
>>> http://stackoverflow.com/questions/199130/how-do-i-increase-the-proc-pid-cmdline-4096-byte-limit
>>> 
>>> seq file interface is not OK, because it kmalloc's for whole output and
>>> open + read(, 1) + sleep will pin arbitrary amounts of kernel memory.
>>> To not do that, limit must be imposed which is incompatible with
>>> arbitrary sized command lines.
>>> 
>>> I apologize for hairy code, but this it direct consequence of command line
>>> layout in memory and hacks to support things like "init [3]".
>>> 
>>> The loops are "unrolled" otherwise it is either macros which hide
>>> control flow or functions with 7-8 arguments with equal line count.
>>> 
>>> There should be real setproctitle(2) or something.
>>> 
>>> Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
>>> Tested-by: Jarod Wilson <jarod@...hat.com>
>>> Acked-by: Jarod Wilson <jarod@...hat.com>
>> 
>> Should have tested on more than just x86, it appears. We've started 
>> hammering on this internally across all arches, and its exploded 
>> multiple times on ppc64 now:
>> 
>> [ 2717.074699] ------------[ cut here ]------------
>> [ 2717.074787] kernel BUG at fs/proc/base.c:244!
> 
>> OE--------------   3.10.0-255.el7.ppc64.debug #1
> 
> Which BUG_ON is this?
> 
>    BUG_ON(*pos < 0);
>    BUG_ON(arg_start > arg_end);
>    BUG_ON(env_start > env_end);

Ah, sorry, right, might not be exactly the same with the back-up ported version... It was the env_start > env_end one.

-- 
Jarod Wilson--
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