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

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Date:	Wed, 27 May 2015 00:24:23 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	Jarod Wilson <jarod@...hat.com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH try #4] proc: fix PAGE_SIZE limit of /proc/$PID/cmdline

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);
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