Re: [PATCH] ext4: return an error when a journal block is not mapped

[Zhang Yi <yizhang089@gmail.com>]

On 10/5/2026 2:06 AM, Karl Mehltretter wrote:
> If the internal journal inode contains a hole, ext4_map_blocks() returns
> 0. ext4_journal_bmap() logs "journal bmap failed" and aborts the
> journal, but also returns 0 and leaves *block unchanged.
> 
> jbd2_journal_bmap() takes that as success and uses the unchanged logical
> journal block number as a physical filesystem block number. Journal I/O
> can then overwrite filesystem blocks outside the journal despite the
> abort.
> 
> Return -EFSCORRUPTED for a hole, matching the error passed to
> jbd2_journal_abort(). Keep propagating negative mapping errors.
> 
> Fixes: 62913ae96de7 ("ext4, jbd2: add an optimized bmap for the journal inode")
> Cc: stable@vger.kernel.org
> Assisted-by: LLM
> Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>

Looks good to me.

Reviewed-by: Zhang Yi <yi.zhang@huawei.com>

> ---
> Found with an experimental clang that warns when a function returns 0
> right after reporting a failure.
> 
> Recorded QEMU A/B testing used x86_64 kernels and a 1 KiB-block image
> with a hole in the journal inode starting at logical block 40. The
> unpatched kernel overwrote filesystem block 40, a reserved GDT block.
> With the patch, none of the marker blocks changed. There were three
> runs per kernel.
> 
> Both kernels logged the mapping failure and journal abort:
> 
>    EXT4-fs (vda): journal bmap failed: block 40 ret 0
>    Aborting journal on device vda-8.
> 
>   fs/ext4/super.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index bca0dc87d0b7..3e7dabe45153 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -5967,7 +5967,7 @@ static int ext4_journal_bmap(journal_t *journal, sector_t *block)
>   			 "journal bmap failed: block %llu ret %d\n",
>   			 *block, ret);
>   		jbd2_journal_abort(journal, ret ? ret : -EFSCORRUPTED);
> -		return ret;
> +		return ret ? ret : -EFSCORRUPTED;
>   	}
>   	*block = map.m_pblk;
>   	return 0;
> 
> base-commit: e767a4ea70a3992c37ed604157d32f0dfbf9b1e3