linux-ext4 - Re: [PATCH v2 01/11] rbtree: Fix rbtree_postorder_for_each_entry

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Message-ID: <527C0CF7.705@linux.vnet.ibm.com>
Date:	Thu, 07 Nov 2013 13:58:15 -0800
From:	Cody P Schafer <cody@...ux.vnet.ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	EXT4 <linux-ext4@...r.kernel.org>, Jan Kara <jack@...e.cz>,
	rostedt@...dmis.org, Seth Jennings <sjennings@...iantweb.net>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 01/11] rbtree: Fix rbtree_postorder_for_each_entry_safe()
 iterator

On 11/07/2013 01:38 PM, Andrew Morton wrote:
> On Wed,  6 Nov 2013 17:42:30 -0800 Cody P Schafer <cody@...ux.vnet.ibm.com> wrote:
>
>> The iterator rbtree_postorder_for_each_entry_safe() relies on pointer
>> underflow behavior when testing for loop termination. In particular
>> it expects that
>>    &rb_entry(NULL, type, field)->field
>> is NULL. But the result of this expression is not defined by a C standard
>> and some gcc versions (e.g. 4.3.4) assume the above expression can never
>> be equal to NULL. The net result is an oops because the iteration is not
>> properly terminated.
>>
>> Fix the problem by modifying the iterator to avoid pointer underflows.
>
> So the sole caller is in zswap.c.  Is that code actually generating oopses?

I can't reproduce the oopses (at all) with my build/gcc version, but Jan 
has reported seeing them (not in zswap, however).

>
> IOW, is there any need to fix this in 3.12 or earlier?
>

The zswap usage change showed up in 3.12.
In my opinion, it is probably a good idea to apply the fix to 3.12.

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