limit up-front allocation when reading bytes

Fixes #73

sum_test.go

@@ -0,0 +1,40 @@
+package tag
+
+import (
+	"bytes"
+	"testing"
+)
+
+func TestFuzz(t *testing.T) {
+	fuzzIssue73(dataIssue73)
+}
+
+var dataIssue73 = []byte{0x49, 0x44, 0x33, 0x03, 0x00, 0x40, 0x00, 0x00, 0x00, 0x0E, 0xDB, 0xDB, 0xDB, 0xDB, 0xDB, 0xDB,
+	0xDB, 0x06, 0xFF, 0x54, 0x58, 0x58, 0x00}
+
+func fuzzIssue73(in []byte) {
+	r := bytes.NewReader(in)
+
+	Identify(r)
+
+	m, err := ReadFrom(r)
+	if err != nil {
+		return
+	}
+
+	m.Format()
+	m.FileType()
+	m.Title()
+	m.Album()
+	m.Artist()
+	m.AlbumArtist()
+	m.Composer()
+	m.Year()
+	m.Genre()
+	m.Track()
+	m.Disc()
+	m.Picture()
+	m.Lyrics()
+
+	Sum(r)
+}

util.go

@@ -5,6 +5,7 @@
 package tag
 
 import (
+	"bytes"
 	"encoding/binary"
 	"io"
 )
@@ -40,7 +41,18 @@ func readUint64LittleEndian(r io.Reader) (uint64, error) {
 	return binary.LittleEndian.Uint64(b), nil
 }
 
+// readBytesMaxUpfront is the max up-front allocation allowed
+const readBytesMaxUpfront = 10 << 20 // 10MB
+
 func readBytes(r io.Reader, n uint) ([]byte, error) {
+	if n > readBytesMaxUpfront {
+		b := &bytes.Buffer{}
+		if _, err := io.CopyN(b, r, int64(n)); err != nil {
+			return nil, err
+		}
+		return b.Bytes(), nil
+	}
+
 	b := make([]byte, n)
 	_, err := io.ReadFull(r, b)
 	if err != nil {