[PATCH v2 3/3] ext4: derive f_fsid from block device to avoid collisions

[Anand Jain <asj@kernel.org>]

statfs() currently reports f_fsid derived from the on-disk UUID.
Cloned block devices share the same UUID, so distinct ext4 instances
can return identical f_fsid values. This leads to collisions in
fanotify.

Encode sb->s_dev into f_fsid instead of using the superblock UUID.
This provides a per-device identifier and avoids conflicts when
filesystem is cloned, matching the behavior with xfs.

Place this change behind the new mount option "-o nouuid" for ABI
compatibility.

Signed-off-by: Anand Jain <asj@kernel.org>
---
 fs/ext4/ext4.h  |  1 +
 fs/ext4/super.c | 12 ++++++++++--
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 293f698b7042..64d98ab64c11 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1282,6 +1282,7 @@ struct ext4_inode_info {
 						    * scanning in mballoc
 						    */
 #define EXT4_MOUNT2_ABORT		0x00000100 /* Abort filesystem */
+#define EXT4_MOUNT2_NOUUID		0x00000200 /* No duplicate f_fsid for cloned filesystem */
 
 #define clear_opt(sb, opt)		EXT4_SB(sb)->s_mount_opt &= \
 						~EXT4_MOUNT_##opt
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 43f680c750ae..65b712af4ad7 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1664,7 +1664,7 @@ static const struct export_operations ext4_export_ops = {
 enum {
 	Opt_bsd_df, Opt_minix_df, Opt_grpid, Opt_nogrpid,
 	Opt_resgid, Opt_resuid, Opt_sb,
-	Opt_nouid32, Opt_debug, Opt_removed,
+	Opt_nouid32, Opt_debug, Opt_removed, Opt_nouuid,
 	Opt_user_xattr, Opt_acl,
 	Opt_auto_da_alloc, Opt_noauto_da_alloc, Opt_noload,
 	Opt_commit, Opt_min_batch_time, Opt_max_batch_time, Opt_journal_dev,
@@ -1743,6 +1743,7 @@ static const struct fs_parameter_spec ext4_param_specs[] = {
 	fsparam_u32	("sb",			Opt_sb),
 	fsparam_enum	("errors",		Opt_errors, ext4_param_errors),
 	fsparam_flag	("nouid32",		Opt_nouid32),
+	fsparam_flag	("nouuid",		Opt_nouuid),
 	fsparam_flag	("debug",		Opt_debug),
 	fsparam_flag	("oldalloc",		Opt_removed),
 	fsparam_flag	("orlov",		Opt_removed),
@@ -1898,6 +1899,7 @@ static const struct mount_opts {
 	{Opt_acl, 0, MOPT_NOSUPPORT},
 #endif
 	{Opt_nouid32, EXT4_MOUNT_NO_UID32, MOPT_SET},
+	{Opt_nouuid, EXT4_MOUNT2_NOUUID, MOPT_SET | MOPT_2},
 	{Opt_debug, EXT4_MOUNT_DEBUG, MOPT_SET},
 	{Opt_quota, EXT4_MOUNT_QUOTA | EXT4_MOUNT_USRQUOTA, MOPT_SET | MOPT_Q},
 	{Opt_usrquota, EXT4_MOUNT_QUOTA | EXT4_MOUNT_USRQUOTA,
@@ -2389,6 +2391,9 @@ static int ext4_parse_param(struct fs_context *fc, struct fs_parameter *param)
 			return -EINVAL;
 		}
 		return 0;
+	case Opt_nouuid:
+		ctx_set_mount_opt2(ctx, EXT4_MOUNT2_NOUUID);
+		return 0;
 	}
 
 	/*
@@ -6941,7 +6946,10 @@ static int ext4_statfs(struct dentry *dentry, struct kstatfs *buf)
 	buf->f_files = le32_to_cpu(es->s_inodes_count);
 	buf->f_ffree = percpu_counter_sum_positive(&sbi->s_freeinodes_counter);
 	buf->f_namelen = EXT4_NAME_LEN;
-	buf->f_fsid = uuid_to_fsid(es->s_uuid);
+	if (test_opt2(sb, NOUUID))
+		buf->f_fsid = u64_to_fsid(huge_encode_dev(sb->s_bdev->bd_dev));
+	else
+		buf->f_fsid = uuid_to_fsid(es->s_uuid);
 
 #ifdef CONFIG_QUOTA
 	if (ext4_test_inode_flag(dentry->d_inode, EXT4_INODE_PROJINHERIT) &&
-- 
2.43.0