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

[Ojaswin Mujoo <ojaswin@linux.ibm.com>]

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>
---
 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