fs: don't let statmount return empty strings

When one of the statmount_string() handlers doesn't emit anything to
seq, the kernel currently sets the corresponding flag and emits an empty
string.

Given that statmount() returns a mask of accessible fields, just leave
the bit unset in this case, and skip any NULL termination. If nothing
was emitted to the seq, then the EOVERFLOW and EAGAIN cases aren't
applicable and the function can just return immediately.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/20241111-statmount-v4-1-2eaf35d07a80@kernel.org
Acked-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/namespace.c

@@ -5044,29 +5044,36 @@ static int statmount_string(struct kstatmount *s, u64 flag)
 	size_t kbufsize;
 	struct seq_file *seq = &s->seq;
 	struct statmount *sm = &s->sm;
+	u32 start = seq->count;
 
 	switch (flag) {
 	case STATMOUNT_FS_TYPE:
-		sm->fs_type = seq->count;
+		sm->fs_type = start;
 		ret = statmount_fs_type(s, seq);
 		break;
 	case STATMOUNT_MNT_ROOT:
-		sm->mnt_root = seq->count;
+		sm->mnt_root = start;
 		ret = statmount_mnt_root(s, seq);
 		break;
 	case STATMOUNT_MNT_POINT:
-		sm->mnt_point = seq->count;
+		sm->mnt_point = start;
 		ret = statmount_mnt_point(s, seq);
 		break;
 	case STATMOUNT_MNT_OPTS:
-		sm->mnt_opts = seq->count;
+		sm->mnt_opts = start;
 		ret = statmount_mnt_opts(s, seq);
 		break;
 	default:
 		WARN_ON_ONCE(true);
 		return -EINVAL;
 	}
 
+	/*
+	 * If nothing was emitted, return to avoid setting the flag
+	 * and terminating the buffer.
+	 */
+	if (seq->count == start)
+		return ret;
 	if (unlikely(check_add_overflow(sizeof(*sm), seq->count, &kbufsize)))
 		return -EOVERFLOW;
 	if (kbufsize >= s->bufsize)