diff --git a/sandbox/vfs/mountinfo.go b/sandbox/vfs/mountinfo.go
index 44cf009..2f609b1 100644
--- a/sandbox/vfs/mountinfo.go
+++ b/sandbox/vfs/mountinfo.go
@@ -52,10 +52,14 @@ type (
 )
 
 // ParseMountInfo parses a mountinfo file according to proc_pid_mountinfo(5).
-func ParseMountInfo(r io.Reader) (*MountInfo, error) {
+func ParseMountInfo(r io.Reader) (*MountInfo, int, error) {
 	var m, cur *MountInfo
 	s := bufio.NewScanner(r)
+
+	var n int
 	for s.Scan() {
+		n++
+
 		if cur == nil {
 			m = new(MountInfo)
 			cur = m
@@ -67,7 +71,7 @@ func ParseMountInfo(r io.Reader) (*MountInfo, error) {
 		// prevent proceeding with misaligned fields due to optional fields
 		f := strings.Split(s.Text(), " ")
 		if len(f) < 10 {
-			return nil, ErrMountInfoFields
+			return nil, -1, ErrMountInfoFields
 		}
 
 		// 36 35 98:0 /mnt1 /mnt2 rw,noatime master:1 - ext3 /dev/root rw,errors=continue
@@ -75,42 +79,42 @@ func ParseMountInfo(r io.Reader) (*MountInfo, error) {
 
 		// (1) id
 		if id, err := strconv.Atoi(f[0]); err != nil { // 0
-			return nil, err
+			return nil, -1, err
 		} else {
 			cur.ID = id
 		}
 
 		// (2) parent
 		if parent, err := strconv.Atoi(f[1]); err != nil { // 1
-			return nil, err
+			return nil, -1, err
 		} else {
 			cur.Parent = parent
 		}
 
 		// (3) maj:min
 		if n, err := fmt.Sscanf(f[2], "%d:%d", &cur.Devno[0], &cur.Devno[1]); err != nil {
-			return nil, err
+			return nil, -1, err
 		} else if n != 2 {
 			// unreachable
-			return nil, ErrMountInfoDevno
+			return nil, -1, ErrMountInfoDevno
 		}
 
 		// (4) mountroot
 		cur.Root = Unmangle(f[3])
 		if cur.Root == "" {
-			return nil, ErrMountInfoEmpty
+			return nil, -1, ErrMountInfoEmpty
 		}
 
 		// (5) target
 		cur.Target = Unmangle(f[4])
 		if cur.Target == "" {
-			return nil, ErrMountInfoEmpty
+			return nil, -1, ErrMountInfoEmpty
 		}
 
 		// (6) vfs options (fs-independent)
 		cur.VfsOptstr = Unmangle(f[5])
 		if cur.VfsOptstr == "" {
-			return nil, ErrMountInfoEmpty
+			return nil, -1, ErrMountInfoEmpty
 		}
 
 		// (7) optional fields, terminated by " - "
@@ -119,14 +123,14 @@ func ParseMountInfo(r io.Reader) (*MountInfo, error) {
 
 		// (8) optional fields end marker
 		if f[i] != "-" {
-			return nil, ErrMountInfoSep
+			return nil, -1, ErrMountInfoSep
 		}
 		i++
 
 		// (9) FS type
 		cur.FsType = Unmangle(f[i])
 		if cur.FsType == "" {
-			return nil, ErrMountInfoEmpty
+			return nil, -1, ErrMountInfoEmpty
 		}
 		i++
 
@@ -137,5 +141,5 @@ func ParseMountInfo(r io.Reader) (*MountInfo, error) {
 		// (11) fs options (fs specific)
 		cur.FsOptstr = Unmangle(f[i])
 	}
-	return m, s.Err()
+	return m, n, s.Err()
 }
diff --git a/sandbox/vfs/mountinfo_test.go b/sandbox/vfs/mountinfo_test.go
index 8d1c2e4..93738a4 100644
--- a/sandbox/vfs/mountinfo_test.go
+++ b/sandbox/vfs/mountinfo_test.go
@@ -120,7 +120,7 @@ id 20 0:53 / /mnt/test rw,relatime shared:212 - tmpfs  rw
 
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
-			got, err := vfs.ParseMountInfo(strings.NewReader(tc.sample))
+			got, n, err := vfs.ParseMountInfo(strings.NewReader(tc.sample))
 			if !errors.Is(err, tc.wantErr) {
 				if tc.wantError == "" {
 					t.Errorf("ParseMountInfo: error = %v, wantErr %v",
@@ -131,6 +131,14 @@ id 20 0:53 / /mnt/test rw,relatime shared:212 - tmpfs  rw
 				}
 			}
 
+			wantCount := len(tc.want)
+			if tc.wantErr != nil || tc.wantError != "" {
+				wantCount = -1
+			}
+			if n != wantCount {
+				t.Errorf("ParseMountInfo: got %d entries, want %d", n, wantCount)
+			}
+
 			i := 0
 			for cur := got; cur != nil; cur = cur.Next {
 				if i == len(tc.want) {