Re: [PATCH RFC 10/17] ext4: use fast incremental CRC update in ext4_free_inode()

On 2026/6/3 22:17, Theodore Tso wrote:
> On Fri, May 08, 2026 at 11:33:05PM +0000, [email protected] wrote:
>> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
>> - [High] ext4_inode_bitmap_csum_set_fast() corrupts the inode bitmap checksum if EXT4_INODES_PER_GROUP(sb) is not a multiple of 8.
>>
>>> -	ext4_inode_bitmap_csum_set(sb, gdp, bitmap_bh);
>>> +	ext4_inode_bitmap_csum_set_fast(sb, gdp, bit);
>> Can this corrupt the inode bitmap checksum if EXT4_INODES_PER_GROUP(sb) is not
>> a multiple of 8?
>>
>> In ext4_inode_bitmap_csum_set(), the CRC32c checksum is computed over exactly
>> EXT4_INODES_PER_GROUP(sb) >> 3 bytes. If s_inodes_per_group is not a multiple
>> of 8, any trailing fractional bits are strictly excluded from the checksum
>> calculation.
>>
>> Because ext4_fill_super() does not enforce that s_inodes_per_group is a
>> multiple of 8, a crafted filesystem can have an unaligned s_inodes_per_group.
> The reason why ext4_fill_super() doesn't enforce that
> s_inodes_per_group is a multiple of 8 was that a long time ago, back
> when Android was allergic to GPLv2 in userspace, they implemented
> their own version of mke2fs (and didn't run fsck on the file system,
> sigh).  Their MIT licensed version of make_ext4fs would occasionally
> make file systems that were not a multiple of 8, and this ran afoul of
> e2fsck[1] if someone actually tried to repair a corrupted Android user
> data file system (as opposed to just wiping the flash and starting
> from scratch).
>
> [1] https://sourceforge.net/p/e2fsprogs/bugs/292/
>
> This was fixed long ago (over a decade ago), and so at this point, I'm
> pretty sure any such mobile handsets are in the landfill, so we
> probably should fix this by adding a check in ext4_fill_super() and a
> corresponding check in e2fsck.
>
> 					- Ted

Hi Ted,

Thank you for your information and suggestions.

I will send two fix patches to synchronize the checks in mke2fs
with ext4_fill_super and e2fsck.


Thanks,
Baokun