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

[Eric Sandeen <sandeen@xxxxxxxxxxx>]

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);
+       }
+
        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"),
+                               i);
+                       return;
+       } else /* should have been fixed in verify_set_agf() */
+               ASSERT(0);
+
        count = 0;
        for (;;) {
                bno = be32_to_cpu(agfl->agfl_bno[i]);