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

[Dave Chinner <david@xxxxxxxxxxxxx>]

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.

> +
>       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....

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx