[PATCH AUTOSEL 6.6 12/14] add unique mount ID

From: Sasha Levin
Date: Mon Jan 15 2024 - 18:31:41 EST


From: Miklos Szeredi <mszeredi@xxxxxxxxxx>

[ Upstream commit 98d2b43081972abeb5bb5a087bc3e3197531c46e ]

If a mount is released then its mnt_id can immediately be reused. This is
bad news for user interfaces that want to uniquely identify a mount.

Implementing a unique mount ID is trivial (use a 64bit counter).
Unfortunately userspace assumes 32bit size and would overflow after the
counter reaches 2^32.

Introduce a new 64bit ID alongside the old one. Initialize the counter to
2^32, this guarantees that the old and new IDs are never mixed up.

Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20231025140205.3586473-2-mszeredi@xxxxxxxxxx
Reviewed-by: Ian Kent <raven@xxxxxxxxxx>
Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
fs/mount.h | 3 ++-
fs/namespace.c | 4 ++++
fs/stat.c | 9 +++++++--
include/uapi/linux/stat.h | 1 +
4 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/fs/mount.h b/fs/mount.h
index 130c07c2f8d2..a14f762b3f29 100644
--- a/fs/mount.h
+++ b/fs/mount.h
@@ -72,7 +72,8 @@ struct mount {
struct fsnotify_mark_connector __rcu *mnt_fsnotify_marks;
__u32 mnt_fsnotify_mask;
#endif
- int mnt_id; /* mount identifier */
+ int mnt_id; /* mount identifier, reused */
+ u64 mnt_id_unique; /* mount ID unique until reboot */
int mnt_group_id; /* peer group identifier */
int mnt_expiry_mark; /* true if marked for expiry */
struct hlist_head mnt_pins;
diff --git a/fs/namespace.c b/fs/namespace.c
index e157efc54023..e02bc5f41c7b 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -68,6 +68,9 @@ static u64 event;
static DEFINE_IDA(mnt_id_ida);
static DEFINE_IDA(mnt_group_ida);

+/* Don't allow confusion with old 32bit mount ID */
+static atomic64_t mnt_id_ctr = ATOMIC64_INIT(1ULL << 32);
+
static struct hlist_head *mount_hashtable __read_mostly;
static struct hlist_head *mountpoint_hashtable __read_mostly;
static struct kmem_cache *mnt_cache __read_mostly;
@@ -131,6 +134,7 @@ static int mnt_alloc_id(struct mount *mnt)
if (res < 0)
return res;
mnt->mnt_id = res;
+ mnt->mnt_id_unique = atomic64_inc_return(&mnt_id_ctr);
return 0;
}

diff --git a/fs/stat.c b/fs/stat.c
index 5375be5f97cc..e2a124a2d5f3 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -247,8 +247,13 @@ static int vfs_statx(int dfd, struct filename *filename, int flags,

error = vfs_getattr(&path, stat, request_mask, flags);

- stat->mnt_id = real_mount(path.mnt)->mnt_id;
- stat->result_mask |= STATX_MNT_ID;
+ if (request_mask & STATX_MNT_ID_UNIQUE) {
+ stat->mnt_id = real_mount(path.mnt)->mnt_id_unique;
+ stat->result_mask |= STATX_MNT_ID_UNIQUE;
+ } else {
+ stat->mnt_id = real_mount(path.mnt)->mnt_id;
+ stat->result_mask |= STATX_MNT_ID;
+ }

if (path.mnt->mnt_root == path.dentry)
stat->attributes |= STATX_ATTR_MOUNT_ROOT;
diff --git a/include/uapi/linux/stat.h b/include/uapi/linux/stat.h
index 7cab2c65d3d7..2f2ee82d5517 100644
--- a/include/uapi/linux/stat.h
+++ b/include/uapi/linux/stat.h
@@ -154,6 +154,7 @@ struct statx {
#define STATX_BTIME 0x00000800U /* Want/got stx_btime */
#define STATX_MNT_ID 0x00001000U /* Got stx_mnt_id */
#define STATX_DIOALIGN 0x00002000U /* Want/got direct I/O alignment info */
+#define STATX_MNT_ID_UNIQUE 0x00004000U /* Want/got extended stx_mount_id */

#define STATX__RESERVED 0x80000000U /* Reserved for future struct statx expansion */

--
2.43.0