Re: [PATCH] ext4: No need to continue when the number of entries is 1

[Baokun Li <libaokun1@huawei.com>]

On 2024/8/22 23:00, Theodore Ts'o wrote:
> On Mon, 01 Jul 2024 22:25:03 +0800, Edward Adam Davis wrote:
>> When the number of entries mapped is 1, there is no need to split it.
>>
>>
> Applied, thanks!
>
> [1/1] ext4: No need to continue when the number of entries is 1
>        commit: b2b81e122b5616890ba6657adeb8aa5ca1f05fe2
>
> Best regards,

Hi Ted,

I think this patch is wrong and it will hide the real problem.

The maximum length of a filename is 255 and the minimum block size is 1024,
so it is always guaranteed that the number of entries is greater than or
equal to 2 when do_split() is called.

The problem reported by syzbot was actually caused by a missing check in
make_indexed_dir(). The issue has been fixed:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=50ea741def58

So unless ext4_dx_add_entry() and make_indexed_dir(), or some other function
has a bug, 'split == 0' will not occur.

If we want to defend against future changes that introduce bugs, I think
it's better to add a WARN_ON_ONCE to make sure that the problem isn't hidden
and that it doesn't trigger serious bugs like out-of-bounds access.

continued = WARN_ON_ONCE(split == 0) ? 0 : hash2 == map[split - 1].hash;

-- 
With Best Regards,
Baokun Li