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btrfs: derive f_fsid from on-disk fsid and dev_t

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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.

Link: https://lore.kernel.org/linux-btrfs/cover.1772095546.git.asj@kernel.org/
Link: https://lore.kernel.org/linux-btrfs/cover.1774092915.git.asj@kernel.org/
Signed-off-by: Anand Jain <asj@kernel.org>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Anand Jain 2026-04-27 18:18:04 +08:00 committed by Johannes Thumshirn
parent df84f6c773
commit c2a74ed049
1 changed files with 33 additions and 8 deletions
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41
fs/btrfs/super.c
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@ -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 &&
@ -1819,14 +1820,38 @@ 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;
}