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add unique mount ID

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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@redhat.com>
Link: https://lore.kernel.org/r/20231025140205.3586473-2-mszeredi@redhat.com
Reviewed-by: Ian Kent <raven@themaw.net>
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Miklos Szeredi 2023-10-25 16:01:59 +02:00 committed by Christian Brauner
parent b85ea95d08
commit 98d2b43081
No known key found for this signature in database
GPG Key ID: 91C61BC06578DCA2
4 changed files with 14 additions and 3 deletions
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3
fs/mount.h
View File

@ -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;

4
fs/namespace.c
View File

@ -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 __ro_after_init;
static struct hlist_head *mountpoint_hashtable __ro_after_init;
static struct kmem_cache *mnt_cache __ro_after_init;
@ -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;
}

5
fs/stat.c
View File

@ -243,8 +243,13 @@ retry:
error = vfs_getattr(&path, stat, request_mask, flags);
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;

1
include/uapi/linux/stat.h
View File

@ -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 */