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

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Date:	Wed, 27 May 2015 16:46:26 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	Jan Stancek <jstancek@...hat.com>
Cc:	Jarod Wilson <jwilson@...hat.com>, 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 Wed, May 27, 2015 at 1:56 PM, Jan Stancek <jstancek@...hat.com> wrote:
> On Wed, May 27, 2015 at 01:27:13AM -0400, Jarod Wilson wrote:
>> On May 26, 2015, at 5:24 PM, Alexey Dobriyan <adobriyan@...il.com> wrote:
>> >> 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);
>>
>
> Is create_elf_tables() taking mm->mmap_sem when it's initialising env_start/end?

No, and it wasn't needed before checkpoint/restart (prctl(PR_SET_MM ...)).
Fields were write-once, now they aren't. C/R is doing down_read() in
prctl_set_mm()
which is an obvious bug.

Check "if (!mm->arg_end)" is safeguard exactly against new process not having
ARGV/ENVP fields completely initialized. I didn't understand what it
does exactly,
now it's clear.
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