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

/proc/self/mountinfo displays the source for the mount, but statmount()
doesn't yet have a way to return it. Add a new STATMOUNT_SB_SOURCE flag,
claim the 32-bit __spare1 field to hold the offset into the str[] array.

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/20241111-statmount-v4-3-2eaf35d07a80@kernel.org
Acked-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/namespace.c

@@ -5012,6 +5012,32 @@ static void statmount_fs_subtype(struct kstatmount *s, struct seq_file *seq)
 		seq_puts(seq, sb->s_subtype);
 }
 
+static int statmount_sb_source(struct kstatmount *s, struct seq_file *seq)
+{
+	struct super_block *sb = s->mnt->mnt_sb;
+	struct mount *r = real_mount(s->mnt);
+
+	if (sb->s_op->show_devname) {
+		size_t start = seq->count;
+		int ret;
+
+		ret = sb->s_op->show_devname(seq, s->mnt->mnt_root);
+		if (ret)
+			return ret;
+
+		if (unlikely(seq_has_overflowed(seq)))
+			return -EAGAIN;
+
+		/* Unescape the result */
+		seq->buf[seq->count] = '\0';
+		seq->count = start;
+		seq_commit(seq, string_unescape_inplace(seq->buf + start, UNESCAPE_OCTAL));
+	} else if (r->mnt_devname) {
+		seq_puts(seq, r->mnt_devname);
+	}
+	return 0;
+}
+
 static void statmount_mnt_ns_id(struct kstatmount *s, struct mnt_namespace *ns)
 {
 	s->sm.mask |= STATMOUNT_MNT_NS_ID;
@@ -5075,6 +5101,10 @@ static int statmount_string(struct kstatmount *s, u64 flag)
 		sm->fs_subtype = start;
 		statmount_fs_subtype(s, seq);
 		break;
+	case STATMOUNT_SB_SOURCE:
+		sm->sb_source = start;
+		ret = statmount_sb_source(s, seq);
+		break;
 	default:
 		WARN_ON_ONCE(true);
 		return -EINVAL;
@@ -5223,6 +5253,9 @@ static int do_statmount(struct kstatmount *s, u64 mnt_id, u64 mnt_ns_id,
 	if (!err && s->mask & STATMOUNT_FS_SUBTYPE)
 		err = statmount_string(s, STATMOUNT_FS_SUBTYPE);
 
+	if (!err && s->mask & STATMOUNT_SB_SOURCE)
+		err = statmount_string(s, STATMOUNT_SB_SOURCE);
+
 	if (!err && s->mask & STATMOUNT_MNT_NS_ID)
 		statmount_mnt_ns_id(s, ns);
 
@@ -5244,7 +5277,8 @@ 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_SUBTYPE)
+			      STATMOUNT_FS_TYPE | STATMOUNT_MNT_OPTS | \
+			      STATMOUNT_FS_SUBTYPE | STATMOUNT_SB_SOURCE)
 
 static int prepare_kstatmount(struct kstatmount *ks, struct mnt_id_req *kreq,
 			      struct statmount __user *buf, size_t bufsize,

include/uapi/linux/mount.h

@@ -174,7 +174,7 @@ struct statmount {
 	__u32 mnt_point;	/* [str] Mountpoint relative to current root */
 	__u64 mnt_ns_id;	/* ID of the mount namespace */
 	__u32 fs_subtype;	/* [str] Subtype of fs_type (if any) */
-	__u32 __spare1[1];
+	__u32 sb_source;	/* [str] Source string of the mount */
 	__u64 __spare2[48];
 	char str[];		/* Variable size part containing strings */
 };
@@ -210,6 +210,7 @@ struct mnt_id_req {
 #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 */
+#define STATMOUNT_SB_SOURCE		0x00000200U	/* Want/got sb_source */
 
 /*
  * Special @mnt_id values that can be passed to listmount