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

[Andreas Dilger]

On Oct 6, 2026, at 10:16, Jan Kara <jack@xxxxxxx> wrote:
> 
> On Tue 06-10-26 07:15:46, hengyul@xxxxxxxxxx wrote:
>> 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>
> 
> Makes sense. Feel free to add:
> 
> Reviewed-by: Jan Kara <jack@xxxxxxx>
> 
> BTW, the kernel never allowed larger than 64k block size so I'm kind of at
> loss why we have that strange code trying to accommodate upto 256k
> blocksize here. I'd just delete that code (not really related to your
> chnage). Ted?
> 
> Honza
> -- 
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR

I think Fujitsu was using 256KiB blocksize on SPARC servers?  Something
like that, but I never saw any patches for it.

Cheers, Andreas