eth2tr: Don't assume proto, it is length!

tr-bridge.c

@@ -176,7 +176,7 @@ static int eth2tr(struct bridge_state *bst)
 	uint8_t buf[2000];
 	const struct ethhdr *ethh = (struct ethhdr *) buf;
 	struct tr_hdr trh;
-	int rc, ethlen;
+	int rc, ethlen, eth_payload_len;
 
 	rc = read(bst->eth.socket, buf, sizeof(buf));
 	if (rc <= 0) {
@@ -193,9 +193,11 @@ static int eth2tr(struct bridge_state *bst)
 	mac2str_buf(eth_src, ethh->h_source);
 	mac2str_buf(eth_dst, ethh->h_dest);
 
-	if (ethh->h_proto != htons(ETH_P_802_2)) {
-		fprintf(stderr, "TR<-ETH: Not bridging unexpected frame for proto %s->%s 0x%04x\n",
-			eth_src, eth_dst, ntohs(ethh->h_proto));
+	/* 802.3 frame: 'proto' is actually the length */
+	eth_payload_len = htons(ethh->h_proto);
+	if (sizeof(*ethh) + eth_payload_len > ethlen) {
+		fprintf(stderr, "TR<-ETH: Not bridging unrealistically long (%lu > %u) frame %s->%s\n",
+			sizeof(*ethh) + eth_payload_len, ethlen, eth_src, eth_dst);
 		return 0;
 	}
 
@@ -211,7 +213,7 @@ static int eth2tr(struct bridge_state *bst)
 
 	if (mac_table_empty(&bst->tr.mac_tbl) || mac_table_contains(&bst->tr.mac_tbl, trh.daddr)) {
 		printf("TR<-ETH: %s/%s <- %s/%s\n", tr_src, eth_src, tr_dst, eth_dst);
-		return write_tr(bst->tr.socket, &trh, buf + sizeof(*ethh), ethlen - sizeof(*ethh));
+		return write_tr(bst->tr.socket, &trh, buf + sizeof(*ethh), eth_payload_len);
 	} else {
 		printf("TR<-ETH: Ignoring frame to unknown MAC %s\n", mac2str(trh.daddr));
 		return 0;