statmount: let unset strings be empty

commit e52e97f upstream.

Just like it's normal for unset values to be zero, unset strings should be
empty instead of containing random values.

It seems to be a typical mistake that the mask returned by statmount is not
checked, which can result in various bugs.

With this fix, these bugs are prevented, since it is highly likely that
userspace would just want to turn the missing mask case into an empty
string anyway (most of the recently found cases are of this type).

Link: https://lore.kernel.org/all/CAJfpegsVCPfCn2DpM8iiYSS5DpMsLB8QBUCHecoj6s0Vxf4jzg@mail.gmail.com/
Fixes: 68385d7 ("statmount: simplify string option retrieval")
Fixes: 46eae99 ("add statmount(2) syscall")
Cc: [email protected] # v6.8
Signed-off-by: Miklos Szeredi <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Reviewed-by: Jeff Layton <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: Miklos Szeredi

fs/namespace.c

@@ -5053,22 +5053,29 @@ 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, *offp;
+
+	/* Reserve an empty string at the beginning for any unset offsets */
+	if (!seq->count)
+		seq_putc(seq, 0);
+
+	start = seq->count;
 
 	switch (flag) {
 	case STATMOUNT_FS_TYPE:
-		sm->fs_type = seq->count;
+		offp = &sm->fs_type;
 		ret = statmount_fs_type(s, seq);
 		break;
 	case STATMOUNT_MNT_ROOT:
-		sm->mnt_root = seq->count;
+		offp = &sm->mnt_root;
 		ret = statmount_mnt_root(s, seq);
 		break;
 	case STATMOUNT_MNT_POINT:
-		sm->mnt_point = seq->count;
+		offp = &sm->mnt_point;
 		ret = statmount_mnt_point(s, seq);
 		break;
 	case STATMOUNT_MNT_OPTS:
-		sm->mnt_opts = seq->count;
+		offp = &sm->mnt_opts;
 		ret = statmount_mnt_opts(s, seq);
 		break;
 	default:
@@ -5090,6 +5097,7 @@ static int statmount_string(struct kstatmount *s, u64 flag)
 
 	seq->buf[seq->count++] = '\0';
 	sm->mask |= flag;
+	*offp = start;
 	return 0;
 }