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

[Jeff Layton <jlayton@kernel.org>]

/proc/self/mountinfo prints out the sb->s_subtype after the type. In
particular, FUSE makes use of this to display the fstype as
fuse.<subtype>.

Add STATMOUNT_FS_SUBTYPE and claim one of the __spare2 fields to point
to the offset into the str[] array. The STATMOUNT_FS_SUBTYPE will only
be set in the return mask if there is a subtype associated with the
mount.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/namespace.c             | 20 +++++++++++++++++++-
 include/uapi/linux/mount.h |  5 ++++-
 2 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index ba77ce1c6788dfe461814b5826fcbb3aab68fad4..5f2fb692449a9c0a15b60549fb9f7bedd10f1f3d 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -5006,6 +5006,14 @@ static int statmount_fs_type(struct kstatmount *s, struct seq_file *seq)
 	return 0;
 }
 
+static int statmount_fs_subtype(struct kstatmount *s, struct seq_file *seq)
+{
+	struct super_block *sb = s->mnt->mnt_sb;
+
+	seq_puts(seq, sb->s_subtype);
+	return 0;
+}
+
 static void statmount_mnt_ns_id(struct kstatmount *s, struct mnt_namespace *ns)
 {
 	s->sm.mask |= STATMOUNT_MNT_NS_ID;
@@ -5064,6 +5072,13 @@ static int statmount_string(struct kstatmount *s, u64 flag)
 		sm->mnt_opts = seq->count;
 		ret = statmount_mnt_opts(s, seq);
 		break;
+	case STATMOUNT_FS_SUBTYPE:
+		/* ignore if no s_subtype */
+		if (!s->mnt->mnt_sb->s_subtype)
+			return 0;
+		sm->fs_subtype = seq->count;
+		ret = statmount_fs_subtype(s, seq);
+		break;
 	default:
 		WARN_ON_ONCE(true);
 		return -EINVAL;
@@ -5203,6 +5218,9 @@ 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);
 
@@ -5224,7 +5242,7 @@ 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,
diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h
index 225bc366ffcbf0319929e2f55f1fbea88e4d7b81..fa206fb56b3b25cf80f7d430e1b6bab19c3220e4 100644
--- a/include/uapi/linux/mount.h
+++ b/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 subtype */
 
 /*
  * Special @mnt_id values that can be passed to listmount

---
base-commit: 26213e1a6caa5a7f508b919059b0122b451f4dfe
change-id: 20241106-statmount-3f91a7ed75fa

Best regards,
-- 
Jeff Layton <jlayton@kernel.org>