From 7599ebb89d7cbbb7e90587a4eb0e08c5eaa675dc Mon Sep 17 00:00:00 2001
From: Nico Schottelius <nico@nico-notebook.schottelius.org>
Date: Sat, 13 Jul 2019 18:05:50 +0200
Subject: [PATCH] Try to prevent underrun / possible off by one

Checking for bigger value
---
 p4src/actions_delta_checksum.p4 | 29 ++++++++++++++++-------------
 p4src/headers.p4                |  1 +
 p4src/parsers.p4                |  1 +
 3 files changed, 18 insertions(+), 13 deletions(-)

diff --git a/p4src/actions_delta_checksum.p4 b/p4src/actions_delta_checksum.p4
index 7b03af3..0a1b9de 100644
--- a/p4src/actions_delta_checksum.p4
+++ b/p4src/actions_delta_checksum.p4
@@ -37,30 +37,33 @@ action v6sum() {
     meta.v6sum = meta.v6sum + (bit<16>) hdr.ipv6.next_header;  // 8 bit
 }
 
-action delta_udp_from_v4_to_v6()
+action delta_prepare()
 {
     v4sum();
     v6sum();
-    bit<16> diff = meta.v6sum - meta.v4sum;
-    hdr.udp.checksum = hdr.udp.checksum + ~diff +1;
+    if(meta.v6sum > meta.v4sum) {
+        meta.headerdiff = meta.v6sum - meta.v4sum;
+    } else {
+        meta.headerdiff = meta.v4sum - meta.v6sum;
+    }
+}
+
+action delta_udp_from_v4_to_v6()
+{
+    delta_prepare();
+    hdr.udp.checksum = hdr.udp.checksum + ~meta.headerdiff;
 }
 
 action delta_tcp_from_v4_to_v6()
 {
-    v4sum();
-    v6sum();
-    bit<16> diff = meta.v6sum - meta.v4sum;
-    hdr.tcp.checksum = hdr.tcp.checksum + ~diff +1;
+    delta_prepare();
+    hdr.tcp.checksum = hdr.tcp.checksum + ~meta.headerdiff;
 }
 
 action delta_ipv4_from_v6_to_v4()
 {
-    v4sum();
-    v6sum();
-    bit<16> diff = meta.v4sum - meta.v6sum ;
-    hdr.tcp.checksum = hdr.tcp.checksum + ~diff +1;
+    delta_prepare();
+    hdr.tcp.checksum = hdr.tcp.checksum + ~meta.headerdiff;
 }
 
-
-
 #endif
\ No newline at end of file
diff --git a/p4src/headers.p4 b/p4src/headers.p4
index 36ee76d..fc20d51 100644
--- a/p4src/headers.p4
+++ b/p4src/headers.p4
@@ -234,6 +234,7 @@ struct metadata {
     /* for delta checksums */
     bit<16> v4sum;
     bit<16> v6sum;
+    bit<16> headerdiff;
 
     table_t table_id;
 }
diff --git a/p4src/parsers.p4 b/p4src/parsers.p4
index 97887e3..25bada6 100644
--- a/p4src/parsers.p4
+++ b/p4src/parsers.p4
@@ -15,6 +15,7 @@
         meta.chk_tcp_v4      = 0;
         meta.v4sum           = 0;
         meta.v6sum           = 0;
+        meta.headerdiff      = 0;
 
         packet.extract(hdr.ethernet);
         transition select(hdr.ethernet.ethertype){