[PATCH v2 0/1] statmount: mountinfo for "unmounted" mounts

[PATCH v2 0/1] statmount: mountinfo for "unmounted" mounts

[Bhavik Sachdev]

By "unmounted" mounts we mean mounts that have been unmounted using
umount2(mnt, MNT_DETACH) but we still have file descriptors to files on
that mount. We want to add the ability to handle such mounts in CRIU
(Checkpoint/Restore in Userspace).

Currently, we have no way to get mount info for these mounts as they do
not appear in /proc/<pid>/mountinfo and statmount does not work on them.

We solve this problem by introducing a file descriptor parameter to
statmount, along with a STATMOUNT_FD flag. Even if this file descriptor
is on a "unmounted" mount we are still able to get mountinfo for the
mount. We report the mountpoint of the mount to be "[detached]" and
mnt_ns_id to be 0.

v1 of this patchset, took a different approach and introduced a new
umount_mnt_ns, to which "unmounted" mounts would be moved to (instead of
their namespace being NULL) thus allowing them to be still available via
statmount:
https://lore.kernel.org/linux-fsdevel/20251002125422.203598-1-b.sachdev1904@gmail.com/

That approach complicated namespace locking and modified performance
sensitive code.
See: https://lore.kernel.org/linux-fsdevel/7e4d9eb5-6dde-4c59-8ee3-358233f082d0@virtuozzo.com/

Christian also talked about a separate approach of tying the _lifetime_
of the mount namespace to the lifetime of the unmounted mounts through
the passive reference count by moving them to a separate rb_root
`unmounted` in the namespace instance.

This approach has a few problems, some of them are:
1. It further extends the scope of the namespace semaphore.
2. Weird to be able to statmount() via mount id if the mount namespace
is dead.

For a more complete description, see:
https://lore.kernel.org/linux-fsdevel/20251006-erlesen-anlagen-9af59899a969@brauner/

Aleska Sarai also pointed out that this fd based approach is similiar to
the fstatfs(2) which returns information about a mounted filesystem when
given a fd open on that filesystem.
https://lore.kernel.org/linux-fsdevel/2025-10-07-lavish-refried-navy-journey-EqHk9K@cyphar.com/

We use this patchset with CRIU to support checkpoint/restore of
"unmounted" mounts in this pull request:
https://github.com/checkpoint-restore/criu/pull/2754.

All these patches are also available in this branch on github:
https://github.com/bsach64/linux/tree/statmount-fd-v2

Bhavik Sachdev (1):
  statmount: accept fd as a parameter

 fs/namespace.c             | 80 ++++++++++++++++++++++++++++----------
 include/uapi/linux/mount.h |  8 ++++
 2 files changed, 67 insertions(+), 21 deletions(-)

-- 
2.51.0

[PATCH v2 1/1] statmount: accept fd as a parameter

[Bhavik Sachdev]

Extend `struct mnt_id_req` to take in a fd and introduce STATMOUNT_FD
flag. When a valid fd is provided and STATMOUNT_FD is set, statmount
will return mountinfo about the mount the fd is on.

This even works for "unmounted" mounts (mounts that have been umounted
using umount2(mnt, MNT_DETACH)), if you have access to a file descriptor
on that mount. These "umounted" mounts will have no mountpoint hence we
return "[detached]" and the mnt_ns_id to be 0.

Co-developed-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Bhavik Sachdev <b.sachdev1904@gmail.com>
---
 fs/namespace.c             | 80 ++++++++++++++++++++++++++++----------
 include/uapi/linux/mount.h |  8 ++++
 2 files changed, 67 insertions(+), 21 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index d82910f33dc4..eb82a22cffd5 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -5207,6 +5207,12 @@ static int statmount_mnt_root(struct kstatmount *s, struct seq_file *seq)
 	return 0;
 }
 
+static int statmount_mnt_point_detached(struct kstatmount *s, struct seq_file *seq)
+{
+	seq_puts(seq, "[detached]");
+	return 0;
+}
+
 static int statmount_mnt_point(struct kstatmount *s, struct seq_file *seq)
 {
 	struct vfsmount *mnt = s->mnt;
@@ -5262,7 +5268,10 @@ static int statmount_sb_source(struct kstatmount *s, struct seq_file *seq)
 static void statmount_mnt_ns_id(struct kstatmount *s, struct mnt_namespace *ns)
 {
 	s->sm.mask |= STATMOUNT_MNT_NS_ID;
-	s->sm.mnt_ns_id = ns->ns.ns_id;
+	if (ns)
+		s->sm.mnt_ns_id = ns->ns.ns_id;
+	else
+		s->sm.mnt_ns_id = 0;
 }
 
 static int statmount_mnt_opts(struct kstatmount *s, struct seq_file *seq)
@@ -5431,7 +5440,10 @@ static int statmount_string(struct kstatmount *s, u64 flag)
 		break;
 	case STATMOUNT_MNT_POINT:
 		offp = &sm->mnt_point;
-		ret = statmount_mnt_point(s, seq);
+		if (!s->root.mnt && !s->root.dentry)
+			ret = statmount_mnt_point_detached(s, seq);
+		else
+			ret = statmount_mnt_point(s, seq);
 		break;
 	case STATMOUNT_MNT_OPTS:
 		offp = &sm->mnt_opts;
@@ -5572,29 +5584,33 @@ static int grab_requested_root(struct mnt_namespace *ns, struct path *root)
 
 /* locks: namespace_shared */
 static int do_statmount(struct kstatmount *s, u64 mnt_id, u64 mnt_ns_id,
-			struct mnt_namespace *ns)
+			struct mnt_namespace *ns, unsigned int flags)
 {
 	struct mount *m;
 	int err;
 
 	/* Has the namespace already been emptied? */
-	if (mnt_ns_id && mnt_ns_empty(ns))
+	if (!(flags & STATMOUNT_FD) && mnt_ns_id && mnt_ns_empty(ns))
 		return -ENOENT;
 
-	s->mnt = lookup_mnt_in_ns(mnt_id, ns);
-	if (!s->mnt)
-		return -ENOENT;
+	if (!(flags & STATMOUNT_FD)) {
+		s->mnt = lookup_mnt_in_ns(mnt_id, ns);
+		if (!s->mnt)
+			return -ENOENT;
+	}
 
-	err = grab_requested_root(ns, &s->root);
-	if (err)
-		return err;
+	if (ns) {
+		err = grab_requested_root(ns, &s->root);
+		if (err)
+			return err;
+	}
 
 	/*
 	 * Don't trigger audit denials. We just want to determine what
 	 * mounts to show users.
 	 */
 	m = real_mount(s->mnt);
-	if (!is_path_reachable(m, m->mnt.mnt_root, &s->root) &&
+	if (ns && !is_path_reachable(m, m->mnt.mnt_root, &s->root) &&
 	    !ns_capable_noaudit(ns->user_ns, CAP_SYS_ADMIN))
 		return -EPERM;
 
@@ -5718,12 +5734,12 @@ static int prepare_kstatmount(struct kstatmount *ks, struct mnt_id_req *kreq,
 }
 
 static int copy_mnt_id_req(const struct mnt_id_req __user *req,
-			   struct mnt_id_req *kreq)
+			   struct mnt_id_req *kreq, unsigned int flags)
 {
 	int ret;
 	size_t usize;
 
-	BUILD_BUG_ON(sizeof(struct mnt_id_req) != MNT_ID_REQ_SIZE_VER1);
+	BUILD_BUG_ON(sizeof(struct mnt_id_req) != MNT_ID_REQ_SIZE_VER2);
 
 	ret = get_user(usize, &req->size);
 	if (ret)
@@ -5738,6 +5754,11 @@ static int copy_mnt_id_req(const struct mnt_id_req __user *req,
 		return ret;
 	if (kreq->spare != 0)
 		return -EINVAL;
+	if (flags & STATMOUNT_FD) {
+		if (kreq->fd < 0)
+			return -EINVAL;
+		return 0;
+	}
 	/* The first valid unique mount id is MNT_UNIQUE_ID_OFFSET + 1. */
 	if (kreq->mnt_id <= MNT_UNIQUE_ID_OFFSET)
 		return -EINVAL;
@@ -5788,23 +5809,37 @@ SYSCALL_DEFINE4(statmount, const struct mnt_id_req __user *, req,
 {
 	struct mnt_namespace *ns __free(mnt_ns_release) = NULL;
 	struct kstatmount *ks __free(kfree) = NULL;
+	struct vfsmount *fd_mnt;
 	struct mnt_id_req kreq;
 	/* We currently support retrieval of 3 strings. */
 	size_t seq_size = 3 * PATH_MAX;
 	int ret;
 
-	if (flags)
+	if (flags & ~STATMOUNT_FD)
 		return -EINVAL;
 
-	ret = copy_mnt_id_req(req, &kreq);
+	ret = copy_mnt_id_req(req, &kreq, flags);
 	if (ret)
 		return ret;
 
-	ns = grab_requested_mnt_ns(&kreq);
-	if (!ns)
-		return -ENOENT;
+	if (flags & STATMOUNT_FD) {
+		CLASS(fd_raw, f)(kreq.fd);
+		if (fd_empty(f))
+			return -EBADF;
+		fd_mnt = fd_file(f)->f_path.mnt;
+		ns = real_mount(fd_mnt)->mnt_ns;
+		if (ns)
+			refcount_inc(&ns->passive);
+		else
+			if (!ns_capable_noaudit(fd_file(f)->f_cred->user_ns, CAP_SYS_ADMIN))
+				return -ENOENT;
+	} else {
+		ns = grab_requested_mnt_ns(&kreq);
+		if (!ns)
+			return -ENOENT;
+	}
 
-	if (kreq.mnt_ns_id && (ns != current->nsproxy->mnt_ns) &&
+	if (ns && (ns != current->nsproxy->mnt_ns) &&
 	    !ns_capable_noaudit(ns->user_ns, CAP_SYS_ADMIN))
 		return -ENOENT;
 
@@ -5817,8 +5852,11 @@ SYSCALL_DEFINE4(statmount, const struct mnt_id_req __user *, req,
 	if (ret)
 		return ret;
 
+	if (flags & STATMOUNT_FD)
+		ks->mnt = fd_mnt;
+
 	scoped_guard(namespace_shared)
-		ret = do_statmount(ks, kreq.mnt_id, kreq.mnt_ns_id, ns);
+		ret = do_statmount(ks, kreq.mnt_id, kreq.mnt_ns_id, ns, flags);
 
 	if (!ret)
 		ret = copy_statmount_to_user(ks);
@@ -5957,7 +5995,7 @@ SYSCALL_DEFINE4(listmount, const struct mnt_id_req __user *, req,
 	if (!access_ok(mnt_ids, nr_mnt_ids * sizeof(*mnt_ids)))
 		return -EFAULT;
 
-	ret = copy_mnt_id_req(req, &kreq);
+	ret = copy_mnt_id_req(req, &kreq, 0);
 	if (ret)
 		return ret;
 
diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h
index 7fa67c2031a5..dfe8b8e7fa8d 100644
--- a/include/uapi/linux/mount.h
+++ b/include/uapi/linux/mount.h
@@ -201,11 +201,14 @@ struct mnt_id_req {
 	__u64 mnt_id;
 	__u64 param;
 	__u64 mnt_ns_id;
+	__s32 fd;
+	__u32 spare2;
 };
 
 /* List of all mnt_id_req versions. */
 #define MNT_ID_REQ_SIZE_VER0	24 /* sizeof first published struct */
 #define MNT_ID_REQ_SIZE_VER1	32 /* sizeof second published struct */
+#define MNT_ID_REQ_SIZE_VER2	40 /* sizeof third published struct */
 
 /*
  * @mask bits for statmount(2)
@@ -232,4 +235,9 @@ struct mnt_id_req {
 #define LSMT_ROOT		0xffffffffffffffff	/* root mount */
 #define LISTMOUNT_REVERSE	(1 << 0) /* List later mounts first */
 
+/*
+ * @flag bits for statmount(2)
+ */
+#define STATMOUNT_FD		0x0000001U /* want mountinfo for given fd */
+
 #endif /* _UAPI_LINUX_MOUNT_H */
-- 
2.51.0