Re: [PATCH] xfs: fix buffer use after free on IO error

From: Eric Sandeen <sandeen@sandeen.net>
To: Christoph Hellwig <hch@infradead.org>, Brian Foster <bfoster@redhat.com>
Cc: Eric Sandeen <sandeen@redhat.com>, xfs-oss <xfs@oss.sgi.com>
Subject: Re: [PATCH] xfs: fix buffer use after free on IO error
Date: Tue, 25 Mar 2014 09:05:04 -0700	[thread overview]
Message-ID: <5331A930.9030402@sandeen.net> (raw)
In-Reply-To: <20140325131705.GB25392@infradead.org>

On 3/25/14, 6:17 AM, Christoph Hellwig wrote:
>> On Fri, Mar 21, 2014 at 09:48:50PM -0500, Eric Sandeen wrote:
>>> +	/*
>>> +	 * If _xfs_buf_ioapply failed, we'll get back here with
>>> +	 * only the reference we took above.  _xfs_buf_ioend will
>>> +	 * drop it to zero, so we'd better not queue it for later,
>>> +	 * or we'll free it before it's done.
>>> +	 */
>>> +	_xfs_buf_ioend(bp, bp->b_error ? 0 : 1);
>>>  
>>
>> Out of curiosity, is there any major reason we don't use 0 here
>> unconditionally? Are we worried about I/O completing before we have a
>> chance to decrement the reference?
> 
> I think this should unconditionally avoid the schedule, and while we're
> at it we should kill _xfs_buf_ioend and opencode it here and at the
> other callsite.

And then remove the flag from xfs_buf_ioend which is always 0 at that
point ...

> Also atomic_dec_and_test really just returns true/false - there should
> ne no need for the explicit == 1 in the conditional.

Yeah I have a patch to do that as well; I wanted to separate the
bugfix from the more invasive cleanup, though - and I wanted to
get the fix out for review sooner.

But yeah, I was unsure about whether or not to schedule at all here.
We come here from a lot of callsites and I'm honestly not sure what
the implications are yet.

-Eric

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