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

[Eric Sandeen <sandeen@xxxxxxxxxxx>]

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@xxxxxxxxxx>
Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxx>
---

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 (;;) {