fs: add the ability for statmount() to report the fs_subtype

/proc/self/mountinfo prints out the sb->s_subtype after the type. This
is particularly useful for disambiguating FUSE mounts (at least when the
userland driver bothers to set it). Add STATMOUNT_FS_SUBTYPE and claim
one of the __spare2 fields to point to the offset into the str[] array.

Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ian Kent <raven@themaw.net>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/20241111-statmount-v4-2-2eaf35d07a80@kernel.org
Acked-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/namespace.c

@@ -5004,6 +5004,14 @@ static int statmount_fs_type(struct kstatmount *s, struct seq_file *seq)
 	return 0;
 }
 
+static void statmount_fs_subtype(struct kstatmount *s, struct seq_file *seq)
+{
+	struct super_block *sb = s->mnt->mnt_sb;
+
+	if (sb->s_subtype)
+		seq_puts(seq, sb->s_subtype);
+}
+
 static void statmount_mnt_ns_id(struct kstatmount *s, struct mnt_namespace *ns)
 {
 	s->sm.mask |= STATMOUNT_MNT_NS_ID;
@@ -5040,7 +5048,7 @@ static int statmount_mnt_opts(struct kstatmount *s, struct seq_file *seq)
 
 static int statmount_string(struct kstatmount *s, u64 flag)
 {
-	int ret;
+	int ret = 0;
 	size_t kbufsize;
 	struct seq_file *seq = &s->seq;
 	struct statmount *sm = &s->sm;
@@ -5063,6 +5071,10 @@ static int statmount_string(struct kstatmount *s, u64 flag)
 		sm->mnt_opts = start;
 		ret = statmount_mnt_opts(s, seq);
 		break;
+	case STATMOUNT_FS_SUBTYPE:
+		sm->fs_subtype = start;
+		statmount_fs_subtype(s, seq);
+		break;
 	default:
 		WARN_ON_ONCE(true);
 		return -EINVAL;
@@ -5208,6 +5220,9 @@ static int do_statmount(struct kstatmount *s, u64 mnt_id, u64 mnt_ns_id,
 	if (!err && s->mask & STATMOUNT_MNT_OPTS)
 		err = statmount_string(s, STATMOUNT_MNT_OPTS);
 
+	if (!err && s->mask & STATMOUNT_FS_SUBTYPE)
+		err = statmount_string(s, STATMOUNT_FS_SUBTYPE);
+
 	if (!err && s->mask & STATMOUNT_MNT_NS_ID)
 		statmount_mnt_ns_id(s, ns);
 
@@ -5229,7 +5244,7 @@ static inline bool retry_statmount(const long ret, size_t *seq_size)
 }
 
 #define STATMOUNT_STRING_REQ (STATMOUNT_MNT_ROOT | STATMOUNT_MNT_POINT | \
-			      STATMOUNT_FS_TYPE | STATMOUNT_MNT_OPTS)
+			      STATMOUNT_FS_TYPE | STATMOUNT_MNT_OPTS | STATMOUNT_FS_SUBTYPE)
 
 static int prepare_kstatmount(struct kstatmount *ks, struct mnt_id_req *kreq,
 			      struct statmount __user *buf, size_t bufsize,

include/uapi/linux/mount.h

@@ -173,7 +173,9 @@ struct statmount {
 	__u32 mnt_root;		/* [str] Root of mount relative to root of fs */
 	__u32 mnt_point;	/* [str] Mountpoint relative to current root */
 	__u64 mnt_ns_id;	/* ID of the mount namespace */
-	__u64 __spare2[49];
+	__u32 fs_subtype;	/* [str] Subtype of fs_type (if any) */
+	__u32 __spare1[1];
+	__u64 __spare2[48];
 	char str[];		/* Variable size part containing strings */
 };
 
@@ -207,6 +209,7 @@ struct mnt_id_req {
 #define STATMOUNT_FS_TYPE		0x00000020U	/* Want/got fs_type */
 #define STATMOUNT_MNT_NS_ID		0x00000040U	/* Want/got mnt_ns_id */
 #define STATMOUNT_MNT_OPTS		0x00000080U	/* Want/got mnt_opts */
+#define STATMOUNT_FS_SUBTYPE		0x00000100U	/* Want/got fs_subtype */
 
 /*
  * Special @mnt_id values that can be passed to listmount