fs: export mount options via statmount()

statmount() can export arbitrary strings, so utilize the __spare1 slot
for a mnt_opts string pointer, and then support asking for and setting
the mount options during statmount().  This calls into the helper for
showing mount options, which already uses a seq_file, so fits in nicely
with our existing mechanism for exporting strings via statmount().

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/3aa6bf8bd5d0a21df9ebd63813af8ab532c18276.1719257716.git.josef@toxicpanda.com
Reviewed-by: Jeff Layton <jlayton@kernel.org>
[brauner: only call sb->s_op->show_options()]
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/namespace.c

@@ -4980,6 +4980,34 @@ static void statmount_mnt_ns_id(struct kstatmount *s, struct mnt_namespace *ns)
 	s->sm.mnt_ns_id = ns->seq;
 }
 
+static int statmount_mnt_opts(struct kstatmount *s, struct seq_file *seq)
+{
+	struct vfsmount *mnt = s->mnt;
+	struct super_block *sb = mnt->mnt_sb;
+	int err;
+
+	if (sb->s_op->show_options) {
+		size_t start = seq->count;
+
+		err = sb->s_op->show_options(seq, mnt->mnt_root);
+		if (err)
+			return err;
+
+		if (unlikely(seq_has_overflowed(seq)))
+			return -EAGAIN;
+
+		if (seq->count == start)
+			return 0;
+
+		/* skip leading comma */
+		memmove(seq->buf + start, seq->buf + start + 1,
+			seq->count - start - 1);
+		seq->count--;
+	}
+
+	return 0;
+}
+
 static int statmount_string(struct kstatmount *s, u64 flag)
 {
 	int ret;
@@ -5000,6 +5028,10 @@ static int statmount_string(struct kstatmount *s, u64 flag)
 		sm->mnt_point = seq->count;
 		ret = statmount_mnt_point(s, seq);
 		break;
+	case STATMOUNT_MNT_OPTS:
+		sm->mnt_opts = seq->count;
+		ret = statmount_mnt_opts(s, seq);
+		break;
 	default:
 		WARN_ON_ONCE(true);
 		return -EINVAL;
@@ -5130,6 +5162,9 @@ static int do_statmount(struct kstatmount *s, u64 mnt_id, u64 mnt_ns_id,
 	if (!err && s->mask & STATMOUNT_MNT_POINT)
 		err = statmount_string(s, STATMOUNT_MNT_POINT);
 
+	if (!err && s->mask & STATMOUNT_MNT_OPTS)
+		err = statmount_string(s, STATMOUNT_MNT_OPTS);
+
 	if (!err && s->mask & STATMOUNT_MNT_NS_ID)
 		statmount_mnt_ns_id(s, ns);
 
@@ -5151,7 +5186,7 @@ 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_FS_TYPE | STATMOUNT_MNT_OPTS)
 
 static int prepare_kstatmount(struct kstatmount *ks, struct mnt_id_req *kreq,
 			      struct statmount __user *buf, size_t bufsize,

include/uapi/linux/mount.h

@@ -154,7 +154,7 @@ struct mount_attr {
  */
 struct statmount {
 	__u32 size;		/* Total size, including strings */
-	__u32 __spare1;
+	__u32 mnt_opts;		/* [str] Mount options of the mount */
 	__u64 mask;		/* What results were written */
 	__u32 sb_dev_major;	/* Device ID */
 	__u32 sb_dev_minor;
@@ -206,6 +206,7 @@ struct mnt_id_req {
 #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 */
+#define STATMOUNT_MNT_OPTS		0x00000080U	/* Want/got mnt_opts */
 
 /*
  * Special @mnt_id values that can be passed to listmount