[PATCH v8 1/5] xfs_db: sanitize agcount on load

[Eric Sandeen <sandeen@sandeen.net>]

Before we get into libxfs_initialize_perag and try to blindly
allocate a perag struct for every (possibly corrupted number of)
AGs, see if we can read the last one.  If not, assume it's corrupt,
and load only the first AG.

Do this only for an arbitrarily high-ish agcount, so that normal-ish
geometry on a possibly truncated file or device will still do
its best to make all readable AGs available.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
---

v7: blow it all up
v8: use bare libxfs_readbuf so verifiers don't matter,
    "ours goes to 1 million!"

diff --git a/libxfs/init.c b/libxfs/init.c
index a08575a..a14aa17 100644
--- a/libxfs/init.c
+++ b/libxfs/init.c
@@ -817,6 +817,29 @@ libxfs_mount(
 			return NULL;
 	}
 
+	/*
+	 * libxfs_initialize_perag will allocate a perag structure for each ag.
+	 * If agcount is corrupted and insanely high, this will OOM the box.
+	 * If the agount seems (arbitrarily) high, try to read what would be
+	 * the last AG, and if that fails for a relatively high agcount, just
+	 * read the first one and let the user know to check the geometry.
+	 */
+	if (sbp->sb_agcount > 1000000) {
+		bp = libxfs_readbuf(mp->m_dev,
+				XFS_AG_DADDR(mp, sbp->sb_agcount - 1, 0), 1,
+				!(flags & LIBXFS_MOUNT_DEBUGGER), NULL);
+		if (bp->b_error) {
+			fprintf(stderr, _("%s: read of AG %d failed\n"),
+						progname, sbp->sb_agcount);
+			if (!(flags & LIBXFS_MOUNT_DEBUGGER))
+				return NULL;
+			fprintf(stderr, _("%s: limiting reads to AG 0\n"),
+								progname);
+			sbp->sb_agcount = 1;
+		}
+		libxfs_putbuf(bp);
+	}
+
 	error = libxfs_initialize_perag(mp, sbp->sb_agcount, &mp->m_maxagi);
 	if (error) {
 		fprintf(stderr, _("%s: perag init failed\n"),