Re: [PATCH 2/2] ext4: Fix reserved gdt blocks handling in fsmap

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Aug 05, 2025 at 02:00:31PM +0530, Ojaswin Mujoo wrote:
> In some cases like small FSes with no meta_bg and where the resize doesn't
> need extra gdt blocks as it can fit in the current one,
> s_reserved_gdt_blocks is set as 0, which causes fsmap to emit a 0 length
> entry, which is incorrect.
> 
>   $ mkfs.ext4 -b 65536 -O bigalloc /dev/sda 5G
>   $ mount /dev/sda /mnt/scratch
>   $ xfs_io -c "fsmap -d" /mnt/scartch
> 
>         0: 253:48 [0..127]: static fs metadata 128
>         1: 253:48 [128..255]: special 102:1 128
>         2: 253:48 [256..255]: special 102:2 0     <---- 0 len entry
>         3: 253:48 [256..383]: special 102:3 128
> 
> Fix this by adding a check for this case.
> 
> Fixes: 0c9ec4beecac ("ext4: support GETFSMAP ioctls")
> Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>

I had no idea that this could be zero, so....
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

--D

> ---
>  fs/ext4/fsmap.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/fs/ext4/fsmap.c b/fs/ext4/fsmap.c
> index 9d63c39f6077..91185c40f755 100644
> --- a/fs/ext4/fsmap.c
> +++ b/fs/ext4/fsmap.c
> @@ -393,6 +393,14 @@ static unsigned int ext4_getfsmap_find_sb(struct super_block *sb,
>  	/* Reserved GDT blocks */
>  	if (!ext4_has_feature_meta_bg(sb) || metagroup < first_meta_bg) {
>  		len = le16_to_cpu(sbi->s_es->s_reserved_gdt_blocks);
> +
> +		/*
> +		 * mkfs.ext4 can set s_reserved_gdt_blocks as 0 in some cases,
> +		 * check for that.
> +		 */
> +		if (!len)
> +			return 0;
> +
>  		error = ext4_getfsmap_fill(meta_list, fsb, len,
>  					   EXT4_FMR_OWN_RESV_GDT);
>  		if (error)
> -- 
> 2.49.0
> 
>