[PATCH V2] xfs_repair: skip freelist scan of corrupt agf in no-modify mode

[Dave Chinner]

On Sat, Mar 02, 2013 at 03:23:12PM -0600, Eric Sandeen wrote:
> In xfs_repair's 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.
> 
> Reported-by: Ole Tange <tange at binf.ku.dk>
> Signed-off-by: Eric Sandeen <sandeen at redhat.com>
> ---
> 
> V2: Remove dumb mistakes :/
> 
> diff --git a/repair/scan.c b/repair/scan.c
> index 5345094..1d39bdc 100644
> --- a/repair/scan.c
> +++ b/repair/scan.c
> @@ -1066,6 +1066,18 @@ scan_freelist(
>  		return;
>  	}
>  	agfl = XFS_BUF_TO_AGFL(agflbuf);
> +
> +	if (no_modify) {
> +		/* agf values not fixed in verify_set_agf, so recheck */
> +		if (be32_to_cpu(agf->agf_flfirst) >= XFS_AGFL_SIZE(mp) ||
> +		    be32_to_cpu(agf->agf_fllast) >= XFS_AGFL_SIZE(mp)) {
> +			do_warn(_("agf %d freelist blocks bad, skipping "
> +				  "freelist scan\n"), i);
> +			return;
> +		}
> +	} else /* should have been fixed in verify_set_agf() */
> +		ASSERT(0);
> +
>  	i = be32_to_cpu(agf->agf_flfirst);
>  	count = 0;
>  	for (;;) {

Looks ok, but IIRC there are overruns in these functions for the
same reason (i.e. unchecked use of agf->agf_flfirst as an array
index)

db/check.c::scan_freelist()
db/freesp.c::scan_freelist()

I found lots of almost-but-not-quite-exact code duplication like
this recenty when doing CRC updates to the userspace code....

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com