Add some metainfo validation

misc.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"time"
 
+	"github.com/anacrolix/torrent/metainfo"
 	pp "github.com/anacrolix/torrent/peer_protocol"
 )
 
@@ -107,3 +108,13 @@ func torrentRequestOffset(torrentLength, pieceSize int64, r request) (off int64)
 	}
 	return
 }
+
+func validateInfo(info *metainfo.Info) error {
+	if len(info.Pieces)%20 != 0 {
+		return errors.New("pieces has invalid length")
+	}
+	if int((info.TotalLength()+info.PieceLength-1)/info.PieceLength) != info.NumPieces() {
+		return errors.New("piece count and file lengths are at odds")
+	}
+	return nil
+}

torrent.go

@@ -208,6 +208,11 @@ func infoPieceHashes(info *metainfo.Info) (ret []string) {
 
 // Called when metadata for a torrent becomes available.
 func (t *torrent) setMetadata(md *metainfo.Info, infoBytes []byte, eventLocker sync.Locker) (err error) {
+	err = validateInfo(md)
+	if err != nil {
+		err = fmt.Errorf("bad info: %s", err)
+		return
+	}
 	t.Info = md
 	t.length = 0
 	for _, f := range t.Info.UpvertedFiles() {