Re: [PATCH] xfs_repair: skip freelist scan of dodgy agf in no-modify mode

[Eric Sandeen <sandeen@xxxxxxxxxxx>]

On Mar 1, 2013, at 7:18 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:

> On Fri, Mar 01, 2013 at 05:46:48PM -0600, Eric Sandeen wrote:
>> In no-modify mode (-n), verify_set_agf doesn't fix up bad
>> freelist blocks that it finds.  When we get to scan_freelist,
>> this can wreak havoc if, for example, first > last and the loop
>> never exits; we index agfl->agfl_bno[i] off into the weeds.
>> 
>> To fix this, re-check the values in no-modify mode, and if
>> they're off, warn about it and skip the scan.
>> 
>> In addition, add a check to verify_set_agf() to ensure that
>> first <= last.
>> 
>> Reported-by: Ole Tange <tange@xxxxxxxxxx>
>> Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxx>
>> ---
>> 
>> diff --git a/repair/agheader.c b/repair/agheader.c
>> index 769022d..68789fe 100644
>> --- a/repair/agheader.c
>> +++ b/repair/agheader.c
>> @@ -86,6 +86,14 @@ verify_set_agf(xfs_mount_t *mp, xfs_agf_t *agf, 
>> xfs_agnumber_t i)
>>     * check first/last AGF fields.  if need be, lose the free
>>     * space in the AGFL, we'll reclaim it later.
>>     */
>> +    if (be32_to_cpu(agf->agf_flfirst) > be32_to_cpu(agf->agf_fllast)) {
>> +        do_warn(_("flfirst %d in agf %d > fllast %d\n"),
>> +            be32_to_cpu(agf->agf_flfirst),
>> +            i, be32_to_cpu(agf->agf_fllast));
>> +        if (!no_modify)
>> +            agf->agf_fllast = agf->agf_flfirst = cpu_to_be32(0);
>> +    }
> 
> I don't think that test is correct. The free list is circular and is
> indexed as a pair of head/tail pointers. Hence flfirst > fllast can
> be actually valid. e.g. flcount = 4, flfirst=126, fllast = 2 is a
> valid free list.
> 
Doh, ok.  Will fix.
>> +
>>    if (be32_to_cpu(agf->agf_flfirst) >= XFS_AGFL_SIZE(mp))  {
>>        do_warn(_("flfirst %d in agf %d too large (max = %zu)\n"),
>>            be32_to_cpu(agf->agf_flfirst),
>> diff --git a/repair/scan.c b/repair/scan.c
>> index 5345094..0f83fb4 100644
>> --- a/repair/scan.c
>> +++ b/repair/scan.c
>> @@ -1067,6 +1067,17 @@ scan_freelist(
>>    }
>>    agfl = XFS_BUF_TO_AGFL(agflbuf);
>>    i = be32_to_cpu(agf->agf_flfirst);
>> +    if (no_modify) {
>> +        /* agf values not sanitized, so double check */
>> +        if (i >= XFS_AGFL_SIZE(mp) ||
>> +            be32_to_cpu(agf->agf_fllast) >= XFS_AGFL_SIZE(mp) ||
>> +            i > be32_to_cpu(agf->agf_fllast))
>> +            do_warn(_("agf %d freelist blocks bad, skipping scan\n"),
> 
> "skipping freelist scan"
> 
>> +                i);
>> +            return;
> 
> Also, you might be missing a set of {} there - that will always
> return immediately if no_modify is set....
> 
Cripes.  1 demerit for me, sorry.  Will fix and resend.

Thanks for the embarrassing review.  :)

-Eric

> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@xxxxxxxxxxxxx
>