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Date: Mon, 22 Apr 2013 08:45:02 -0500
From: Mark Tinguely <tinguely@sgi.com>
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Subject: Re: [PATCH] xfs: shutdown filesystem if xfs_perag_get fails
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	<20130421174107.007313126@sgi.com> <5174603A.8030208@sandeen.net>
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To: Eric Sandeen <sandeen@sandeen.net>
Cc: xfs@oss.sgi.com

On 04/21/13 16:55, Eric Sandeen wrote:
> On 4/21/13 12:41 PM, Mark Tinguely wrote:
>
>> This problem happened locally with a bad inode number from xfs
>> recovery. xfs_perag_get() can return NULL if given a bad agno.
>> Most callers of xfs_perag_get() do not check for a NULL before
>> using the pointer. This patch forces a shutdown of the filesystem
>> for those callers that do not check the return value rather than
>> crashing on a dereferenced NULL pointer.
>
> Hi Mark -
>
> I'm curious, what was the callchain when this happened?  Was it
> during recovery?  If so, would aborting recovery be more prudent?
>
> I might be missing something, but I'm not sure how shutting
> down avoids a subsequent null ptr deref&  crash.
>
> i.e. if a caller does something like:
>
>          pag = xfs_perag_get(mp, agno);
>          spin_lock(&pag->pagb_lock);
>
> shutting down in xfs_perag_get doesn't save us from a
> null pag pointer, would it?
>
> Thanks,
> -Eric

You are correct, we have to exit the routine(s) to avoid the 
dereference. Let the callers handle the error.

Sorry for the noise.

--Mark.

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