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ext4: fix reserved gdt blocks handling in fsmap

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commit 3ffbdd1f11 upstream.

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: 0c9ec4beec ("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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Ojaswin Mujoo 2025-08-05 14:00:31 +05:30 committed by Greg Kroah-Hartman
parent 052cd20d8d
commit d387cee49c
1 changed files with 8 additions and 0 deletions
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8
fs/ext4/fsmap.c
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@ -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)