[PATCH] ext4: do not accept rec_len 0 for block sizes below 64k

[hengyul]

From: Hengyu Liang <hengyul@xxxxxxxxxx>

Commit afa6d5a16bf2 ("ext4: remove PAGE_SIZE checks for rec_len
conversion") made ext4_rec_len_from_disk() read a rec_len of 0 as "this
entry covers the whole block" for every block size.

However, this special value only exists for block sizes of 64k and
more. With smaller blocks, a rec_len of 0 means that the directory
block is corrupted. As of now, a directory block that has been
overwritten with zeroes is treated as an empty block. The kernel no
longer reports the corruption and can store new entries in that block,
while e2fsck still reports the block as corrupted.

The issue can be reproduced on a file system without metadata_csum:

    mke2fs -q -t ext4 -b 1024 -O ^metadata_csum,^dir_index img 8M
    mount -o loop img /mnt
    mkdir /mnt/d
    for i in $(seq 100); do touch /mnt/d/file_with_a_long_name_$i; done
    umount /mnt
    dd if=/dev/zero of=img bs=1024 count=1 conv=notrunc \
        seek=$(debugfs -R "bmap d 1" img)
    mount -o loop img /mnt
    touch /mnt/d/new

Before commit afa6d5a16bf2 ("ext4: remove PAGE_SIZE checks for rec_len
conversion"), the last command fails with "Structure needs cleaning".
After that commit, it succeeds.

This patch makes ext4_rec_len_from_disk() return the on-disk value
unchanged when the block size is below 64k, as e2fsprogs does.

Fixes: afa6d5a16bf2 ("ext4: remove PAGE_SIZE checks for rec_len conversion")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Hengyu Liang <hengyul@xxxxxxxxxx>
---
 fs/ext4/ext4.h | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 724a27e8be61..ab716a5ad6da 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -2611,6 +2611,13 @@ ext4_rec_len_from_disk(__le16 dlen, unsigned blocksize)
 {
 	unsigned len = le16_to_cpu(dlen);
 
+	/*
+	 * Only blocks of 64k and more need the special encoding of rec_len.
+	 * For smaller blocks 0 and EXT4_MAX_REC_LEN are not valid lengths
+	 * and must not be taken for an entry that spans the whole block.
+	 */
+	if (blocksize < 65536)
+		return len;
 	if (len == EXT4_MAX_REC_LEN || len == 0)
 		return blocksize;
 	return (len & 65532) | ((len & 3) << 16);
-- 
2.53.0