fs: export the mount ns id via statmount

In order to allow users to iterate through children mount namespaces via
listmount we need a way for them to know what the ns id for the mount.
Add a new field to statmount called mnt_ns_id which will carry the ns id
for the given mount entry.

Co-developed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/6dabf437331fb7415d886f7c64b21cb2a50b1c66.1719243756.git.josef@toxicpanda.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/namespace.c

@@ -4974,6 +4974,14 @@ static int statmount_fs_type(struct kstatmount *s, struct seq_file *seq)
 	return 0;
 }
 
+static void statmount_mnt_ns_id(struct kstatmount *s)
+{
+	struct mnt_namespace *ns = current->nsproxy->mnt_ns;
+
+	s->sm.mask |= STATMOUNT_MNT_NS_ID;
+	s->sm.mnt_ns_id = ns->seq;
+}
+
 static int statmount_string(struct kstatmount *s, u64 flag)
 {
 	int ret;
@@ -5070,6 +5078,9 @@ static int do_statmount(struct kstatmount *s)
 	if (!err && s->mask & STATMOUNT_MNT_POINT)
 		err = statmount_string(s, STATMOUNT_MNT_POINT);
 
+	if (!err && s->mask & STATMOUNT_MNT_NS_ID)
+		statmount_mnt_ns_id(s);
+
 	if (err)
 		return err;
 

include/uapi/linux/mount.h

@@ -172,7 +172,8 @@ struct statmount {
 	__u64 propagate_from;	/* Propagation from in current namespace */
 	__u32 mnt_root;		/* [str] Root of mount relative to root of fs */
 	__u32 mnt_point;	/* [str] Mountpoint relative to current root */
-	__u64 __spare2[50];
+	__u64 mnt_ns_id;	/* ID of the mount namespace */
+	__u64 __spare2[49];
 	char str[];		/* Variable size part containing strings */
 };
 
@@ -202,6 +203,7 @@ struct mnt_id_req {
 #define STATMOUNT_MNT_ROOT		0x00000008U	/* Want/got mnt_root  */
 #define STATMOUNT_MNT_POINT		0x00000010U	/* Want/got mnt_point */
 #define STATMOUNT_FS_TYPE		0x00000020U	/* Want/got fs_type */
+#define STATMOUNT_MNT_NS_ID		0x00000040U	/* Want/got mnt_ns_id */
 
 /*
  * Special @mnt_id values that can be passed to listmount