btrfs.c: check first two superblocks, for additional robustness

function                                             old     new   delta
volume_id_probe_btrfs                                 94     142     +48

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 15 insertions, 8 deletions

diff --git a/util-linux/volume_id/btrfs.c b/util-linux/volume_id/btrfs.c
index 53bac7438..777b80923 100644
--- a/util-linux/volume_id/btrfs.c
+++ b/util-linux/volume_id/btrfs.c
@@ -80,19 +80,26 @@ struct btrfs_super_block {
 
 int FAST_FUNC volume_id_probe_btrfs(struct volume_id *id /*,uint64_t off*/)
 {
-#define off ((uint64_t) (64 * 1024))
+        // btrfs has superblocks at 64K, 64M and 256G
+        // minimum btrfs size is 256M
+        // so we never step out the device if we analyze
+        // the first and the second superblocks
         struct btrfs_super_block *sb;
+        unsigned off = 64;
 
-        dbg("btrfs: probing at offset 0x%llx", (unsigned long long) off);
+        while (off < 64*1024*1024) {
+                off *= 1024;
+                dbg("btrfs: probing at offset 0x%x", off);
 
-        sb = volume_id_get_buffer(id, off, sizeof(*sb));
-        if (sb == NULL)
-                return -1;
+                sb = volume_id_get_buffer(id, off, sizeof(*sb));
+                if (sb == NULL)
+                        return -1;
 
-        if (memcmp(sb->magic, BTRFS_MAGIC, 8) != 0)
-                return -1;
+                if (memcmp(sb->magic, BTRFS_MAGIC, 8) != 0)
+                        return -1;
+        }
 
-        // N.B.: btrfs supports 256-byte labels
+        // N.B.: btrfs natively supports 256 (>VOLUME_ID_LABEL_SIZE) size labels
         volume_id_set_label_string(id, sb->label, VOLUME_ID_LABEL_SIZE);
         volume_id_set_uuid(id, sb->fsid, UUID_DCE);