statmount: add flag to retrieve unescaped options

Filesystem options can be retrieved with STATMOUNT_MNT_OPTS, which
returns a string of comma separated options, where some characters are
escaped using the \OOO notation.

Add a new flag, STATMOUNT_OPT_ARRAY, which instead returns the raw
option values separated with '\0' charaters.

Since escaped charaters are rare, this inteface is preferable for
non-libmount users which likley don't want to deal with option
de-escaping.

Example code:

	if (st->mask & STATMOUNT_OPT_ARRAY) {
		const char *opt = st->str + st->opt_array;

		for (unsigned int i = 0; i < st->opt_num; i++) {
			printf("opt_array[%i]: <%s>\n", i, opt);
			opt += strlen(opt) + 1;
		}
	}

Example ouput:

(1) mnt_opts: <lowerdir+=/l\054w\054r,lowerdir+=/l\054w\054r1,upperdir=/upp\054r,workdir=/w\054rk,redirect_dir=nofollow,uuid=null>

(2) opt_array[0]: <lowerdir+=/l,w,r>
    opt_array[1]: <lowerdir+=/l,w,r1>
    opt_array[2]: <upperdir=/upp,r>
    opt_array[3]: <workdir=/w,rk>
    opt_array[4]: <redirect_dir=nofollow>
    opt_array[5]: <uuid=null>

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Link: https://lore.kernel.org/r/20241112101006.30715-1-mszeredi@redhat.com
Acked-by: Jeff Layton <jlayton@kernel.org>
[brauner: tweak variable naming and parsing add example output]
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/namespace.c

@@ -5072,6 +5072,43 @@ static int statmount_mnt_opts(struct kstatmount *s, struct seq_file *seq)
 	return 0;
 }
 
+static int statmount_opt_array(struct kstatmount *s, struct seq_file *seq)
+{
+	struct vfsmount *mnt = s->mnt;
+	struct super_block *sb = mnt->mnt_sb;
+	size_t start = seq->count;
+	char *buf_start, *buf_end, *opt_start, *opt_end;
+	u32 count = 0;
+	int err;
+
+	if (!sb->s_op->show_options)
+		return 0;
+
+	buf_start = seq->buf + start;
+	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;
+
+	buf_end = seq->buf + seq->count;
+	*buf_end = '\0';
+	for (opt_start = buf_start + 1; opt_start < buf_end; opt_start = opt_end + 1) {
+		opt_end = strchrnul(opt_start, ',');
+		*opt_end = '\0';
+		buf_start += string_unescape(opt_start, buf_start, 0, UNESCAPE_OCTAL) + 1;
+		if (WARN_ON_ONCE(++count == 0))
+			return -EOVERFLOW;
+	}
+	seq->count = buf_start - 1 - seq->buf;
+	s->sm.opt_num = count;
+	return 0;
+}
+
 static int statmount_string(struct kstatmount *s, u64 flag)
 {
 	int ret = 0;
@@ -5097,6 +5134,10 @@ static int statmount_string(struct kstatmount *s, u64 flag)
 		sm->mnt_opts = start;
 		ret = statmount_mnt_opts(s, seq);
 		break;
+	case STATMOUNT_OPT_ARRAY:
+		sm->opt_array = start;
+		ret = statmount_opt_array(s, seq);
+		break;
 	case STATMOUNT_FS_SUBTYPE:
 		sm->fs_subtype = start;
 		statmount_fs_subtype(s, seq);
@@ -5250,6 +5291,9 @@ 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_OPT_ARRAY)
+		err = statmount_string(s, STATMOUNT_OPT_ARRAY);
+
 	if (!err && s->mask & STATMOUNT_FS_SUBTYPE)
 		err = statmount_string(s, STATMOUNT_FS_SUBTYPE);
 
@@ -5278,7 +5322,8 @@ 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_SB_SOURCE)
+			      STATMOUNT_FS_SUBTYPE | STATMOUNT_SB_SOURCE | \
+			      STATMOUNT_OPT_ARRAY)
 
 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 mnt_opts;		/* [str] Mount options of the mount */
+	__u32 mnt_opts;		/* [str] Options (comma separated, escaped) */
 	__u64 mask;		/* What results were written */
 	__u32 sb_dev_major;	/* Device ID */
 	__u32 sb_dev_minor;
@@ -175,7 +175,9 @@ struct statmount {
 	__u64 mnt_ns_id;	/* ID of the mount namespace */
 	__u32 fs_subtype;	/* [str] Subtype of fs_type (if any) */
 	__u32 sb_source;	/* [str] Source string of the mount */
-	__u64 __spare2[48];
+	__u32 opt_num;		/* Number of fs options */
+	__u32 opt_array;	/* [str] Array of nul terminated fs options */
+	__u64 __spare2[47];
 	char str[];		/* Variable size part containing strings */
 };
 
@@ -211,6 +213,7 @@ struct mnt_id_req {
 #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 */
+#define STATMOUNT_OPT_ARRAY		0x00000400U	/* Want/got opt_... */
 
 /*
  * Special @mnt_id values that can be passed to listmount