Re: [PATCH v2] btrfs: tree-checker: introduce checks for FREE_SPACE_BITMAP

[David Sterba <dsterba@suse.cz>]

On Tue, Mar 10, 2026 at 06:56:06PM +0800, ZhengYuan Huang wrote:
> Introduce checks for FREE_SPACE_BITMAP item, which include:
> 
> - Key alignment check
>   Same as FREE_SPACE_EXTENT, the objectid is the logical bytenr of the
>   free space, and offset is the length of the free space, so both
>   should be aligned to the fs block size. 
> 
> - Non-zero range check
>   A zero key->offset would describe an empty bitmap, which is invalid.
> 
> - Item size check
>   The item must hold exactly DIV_ROUND_UP(key->offset >> sectorsize_bits,
>   BITS_PER_BYTE) bytes.  A mismatch indicates a truncated or otherwise
>   corrupt bitmap item; without this check, the bitmap loading path would
>   walk past the end of the leaf and trigger a NULL dereference in
>   assert_eb_folio_uptodate().
> 
> Signed-off-by: ZhengYuan Huang <gality369@gmail.com>
> ---
> [CHANGELOG]
> v2:
> - Move the FREE_SPACE_BITMAP item size validation from
>   load_free_space_bitmaps() in free-space-tree.c into tree-checker, so
>   corrupt bitmap items are rejected when the leaf is read from disk.
> - Drop the extent_buffer_test_bit() range check added in v1.
> - Rework the fix to follow Qu Wenruo's suggested tree-checker based
>   validation.
> ---
>  fs/btrfs/tree-checker.c | 39 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 39 insertions(+)
> 
> diff --git a/fs/btrfs/tree-checker.c b/fs/btrfs/tree-checker.c
> index c10b4c242acf..0f12fe462b6c 100644
> --- a/fs/btrfs/tree-checker.c
> +++ b/fs/btrfs/tree-checker.c
> @@ -1901,6 +1901,42 @@ static int check_dev_extent_item(const struct extent_buffer *leaf,
>  	return 0;
>  }
>  
> +static int check_free_space_bitmap(struct extent_buffer *leaf,
> +				   struct btrfs_key *key, int slot)
> +{
> +	struct btrfs_fs_info *fs_info = leaf->fs_info;
> +	const u32 blocksize = fs_info->sectorsize;
> +	u32 expected_item_size;
> +
> +	if (unlikely(!IS_ALIGNED(key->objectid, blocksize) ||
> +		     !IS_ALIGNED(key->offset, blocksize))) {
> +		generic_err(leaf, slot,
> +			    "free space bitmap key range is not aligned to %u, has (%llu %u %llu)",

We have a format specifier macro for keys, KEY_FMT, I'll fix it in git.

> +			    blocksize, key->objectid, key->type, key->offset);