[PATCH 3.16 247/366] ext4: check for allocation block validity with block group locked

[Ben Hutchings]

3.16.61-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Theodore Ts'o <tytso@xxxxxxx>

commit 8d5a803c6a6ce4ec258e31f76059ea5153ba46ef upstream.

With commit 044e6e3d74a3: "ext4: don't update checksum of new
initialized bitmaps" the buffer valid bit will get set without
actually setting up the checksum for the allocation bitmap, since the
checksum will get calculated once we actually allocate an inode or
block.

If we are doing this, then we need to (re-)check the verified bit
after we take the block group lock.  Otherwise, we could race with
another process reading and verifying the bitmap, which would then
complain about the checksum being invalid.

https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1780137

Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/fs/ext4/balloc.c
+++ b/fs/ext4/balloc.c
@@ -377,6 +377,8 @@ static void ext4_validate_block_bitmap(s
 		return;
 
 	ext4_lock_group(sb, block_group);
+	if (buffer_verified(bh))
+		goto verified;
 	blk = ext4_valid_block_bitmap(sb, desc, block_group, bh);
 	if (unlikely(blk != 0)) {
 		ext4_unlock_group(sb, block_group);
@@ -399,6 +401,7 @@ static void ext4_validate_block_bitmap(s
 		return;
 	}
 	set_buffer_verified(bh);
+verified:
 	ext4_unlock_group(sb, block_group);
 }
 
--- a/fs/ext4/ialloc.c
+++ b/fs/ext4/ialloc.c
@@ -166,6 +166,8 @@ ext4_read_inode_bitmap(struct super_bloc
 
 verify:
 	ext4_lock_group(sb, block_group);
+	if (buffer_verified(bh))
+		goto verified;
 	if (!buffer_verified(bh) &&
 	    !ext4_inode_bitmap_csum_verify(sb, block_group, desc, bh,
 					   EXT4_INODES_PER_GROUP(sb) / 8)) {
@@ -183,8 +185,9 @@ verify:
 		set_bit(EXT4_GROUP_INFO_IBITMAP_CORRUPT_BIT, &grp->bb_state);
 		return NULL;
 	}
-	ext4_unlock_group(sb, block_group);
 	set_buffer_verified(bh);
+verified:
+	ext4_unlock_group(sb, block_group);
 	return bh;
 }