Re: [PATCH v3 2/2] btrfs: derive f_fsid from on-disk fsuuid and dev_t

[Anand Jain <anajain.sg@gmail.com>]

On 27/4/26 18:18, Anand Jain wrote:
> f_fsid depends on fs_devices->fsid and subvol root id.
> 
> For cloned devices either same as the source or dynamical generated
> at mount won't suite because tools like fanotify and ima depends on it.
> 
> Switch to a stable derivation using the persistent on-disk fsuuid +
> root id + devt of the block device for the single device filesystem
> when temp_fsid is active.
> This is consistent as long as the device remains unchanged/replace
> (excludes btrfs device replace secnario for now).
> 

David,

The changelog was inaccurate per Sashiko. I missed updating it to
reflect the final implementation.
I have corrected it below to match the current code changes.
I hope below change log can be used during merge.

-----------
btrfs: derive f_fsid from on-disk fsid and dev_t

The f_fsid was originally derived from fs_devices->fsid and the
subvolume root ID. However, when temp_fsid is active, fs_devices->fsid
is randomized, making the standard derivation inconsistent.

Since metadata_uuid is optional, it is not a reliable alternative.
This patch instead retrieves the on-disk UUID from
fs_info->super_copy->fsid.

To prevent f_fsid collisions between original and cloned filesystems,
this implementation hashes the dev_t for single-device btrfs
filesystems to ensure uniqueness. This is limited to single-device
filesystems as cloned mounts are currently only supported for that
configuration. Note that f_fsid will change if the device is replaced.
Additionally, since the kernel cannot distinguish between the
original and the cloned filesystem, this new f_fsid derivation
is applied to both.
---------------


Thanks
Anand


> Signed-off-by: Anand Jain <asj@kernel.org>
> ---
>  fs/btrfs/super.c | 41 +++++++++++++++++++++++++++++++++--------
>  1 file changed, 33 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index a63ad6bd89f6..00eff951917c 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -1732,12 +1732,13 @@ static int btrfs_statfs(struct dentry *dentry, struct kstatfs *buf)
>  	u64 total_free_data = 0;
>  	u64 total_free_meta = 0;
>  	u32 bits = fs_info->sectorsize_bits;
> -	__be32 *fsid = (__be32 *)fs_info->fs_devices->fsid;
> +	__be32 *fsid;
>  	unsigned factor = 1;
>  	struct btrfs_block_rsv *block_rsv = &fs_info->global_block_rsv;
>  	int ret;
>  	u64 thresh = 0;
>  	int mixed = 0;
> +	__kernel_fsid_t f_fsid;
>  
>  	list_for_each_entry(found, &fs_info->space_info, list) {
>  		if (found->flags & BTRFS_BLOCK_GROUP_DATA) {
> @@ -1818,14 +1819,38 @@ static int btrfs_statfs(struct dentry *dentry, struct kstatfs *buf)
>  	buf->f_bsize = fs_info->sectorsize;
>  	buf->f_namelen = BTRFS_NAME_LEN;
>  
> -	/* We treat it as constant endianness (it doesn't matter _which_)
> -	   because we want the fsid to come out the same whether mounted
> -	   on a big-endian or little-endian host */
> -	buf->f_fsid.val[0] = be32_to_cpu(fsid[0]) ^ be32_to_cpu(fsid[2]);
> -	buf->f_fsid.val[1] = be32_to_cpu(fsid[1]) ^ be32_to_cpu(fsid[3]);
> +	/*
> +	 * fs_devices->fsid is dynamically generated when temp_fsid is active
> +	 * to support cloned filesystems. Use the original on-disk fsid instead,
> +	 * as it remains consistent across mount cycles.
> +	 */
> +	if (fs_info->fs_devices->temp_fsid)
> +		fsid = (__be32 *)fs_info->super_copy->fsid;
> +	else
> +		fsid = (__be32 *)fs_info->fs_devices->fsid;
> +
> +	/*
> +	 * We treat it as constant endianness (it doesn't matter _which_)
> +	 * because we want the fsid to come out the same whether mounted
> +	 * on a big-endian or little-endian host.
> +	 */
> +	f_fsid.val[0] = be32_to_cpu(fsid[0]) ^ be32_to_cpu(fsid[2]);
> +	f_fsid.val[1] = be32_to_cpu(fsid[1]) ^ be32_to_cpu(fsid[3]);
> +
>  	/* Mask in the root object ID too, to disambiguate subvols */
> -	buf->f_fsid.val[0] ^= btrfs_root_id(BTRFS_I(d_inode(dentry))->root) >> 32;
> -	buf->f_fsid.val[1] ^= btrfs_root_id(BTRFS_I(d_inode(dentry))->root);
> +	f_fsid.val[0] ^= btrfs_root_id(BTRFS_I(d_inode(dentry))->root) >> 32;
> +	f_fsid.val[1] ^= btrfs_root_id(BTRFS_I(d_inode(dentry))->root);
> +
> +	/* Hash dev_t to avoid f_fsid collision with cloned filesystems. */
> +	if (fs_info->fs_devices->total_devices == 1) {
> +		__kernel_fsid_t dev_fsid = \
> +	u64_to_fsid(huge_encode_dev(fs_info->fs_devices->latest_dev->bdev->bd_dev));
> +
> +		f_fsid.val[0] ^= dev_fsid.val[1];
> +		f_fsid.val[1] ^= dev_fsid.val[0];
> +	}
> +
> +	memcpy(&buf->f_fsid, &f_fsid, sizeof(f_fsid));
>  
>  	return 0;
>  }