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

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Subject: Re: [PATCH] xfs: fix buffer use after free on IO error
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Wed, 26 Mar 2014 05:08:14 +1100
Cc: Brian Foster <bfoster@xxxxxxxxxx>, Eric Sandeen <sandeen@xxxxxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20140325131705.GB25392@xxxxxxxxxxxxx>
References: <532CFA12.4040104@xxxxxxxxxx> <20140325125754.GA18691@xxxxxxxxxxxxxxx> <20140325131705.GB25392@xxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Tue, Mar 25, 2014 at 06:17:05AM -0700, 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.

I thought we schduled here because we can issue IO from IO
completion and so we need to requeue the IO completion rather than
run it inline in the current IO completion that hasn't fully
completed it's processing yet..

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

Right, there's several other cleanups around this code that can be
done as Eric has mentioned...

Cheers,

Dave.

-- 
Dave Chinner
david@xxxxxxxxxxxxx

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