ext4: fix reserved gdt blocks handling in fsmap

JIRA: https://issues.redhat.com/browse/RHEL-109218

commit 3ffbdd1
Author: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Date:   Tue Aug 5 14:00:31 2025 +0530

    ext4: fix reserved gdt blocks handling in fsmap

    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.

    Cc: stable@kernel.org
    Fixes: 0c9ec4b ("ext4: support GETFSMAP ioctls")
    Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
    Reviewed-by: Darrick J. Wong <djwong@kernel.org>
    Link: https://patch.msgid.link/08781b796453a5770112aa96ad14c864fbf31935.1754377641.git.ojaswin@linux.ibm.com
    Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Signed-off-by: Brian Foster <bfoster@redhat.com>

Author: Brian Foster

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)