[PATCH 1/3] btrfs: derive f_fsid from on-disk fsuuid and dev_t

[Anand Jain <asj@kernel.org>]

Currently, f_fsid depends on fs_devices->fsid. For cloned devices, this
value is dynamic and fluctuates across mount cycles. This inconsistency
breaks persistence for subsystems like IMA.

Switch to a stable derivation using the persistent on-disk fsuuid +
root id + devt of the block device for the single device filesystem.
This is consistent as long as the device remains unchanged/replace
(excludes btrfs device replace secnario for now).

Signed-off-by: Anand Jain <asj@kernel.org>
---
 fs/btrfs/super.c | 27 +++++++++++++++++++++++----
 1 file changed, 23 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 125fca57c164..68473663fe1e 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1733,7 +1733,7 @@ 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;
@@ -1819,15 +1819,34 @@ 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 */
+	/*
+	 * fs_devices->fsid is dynamically generated when temp_fsid is active
+	 * to support cloned devices. Use the original on-disk fsid instead,
+	 * as it remains consistent across mount cycles.
+	 */
+	fsid = (__be32 *)fs_info->super_copy->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.
+	 */
 	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]);
 	/* 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);
 
+	/*
+	 * dev_t provides way to differentiate mounted cloned devices keeps
+	 * the statfs fid is consistent and unique.
+	 */
+	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));
+		buf->f_fsid.val[0] ^= dev_fsid.val[1];
+		buf->f_fsid.val[1] ^= dev_fsid.val[0];
+	}
+
 	return 0;
 }
 
-- 
2.43.0