From d15cbb2d6a9f116e47f71458f075529910bcf27f Mon Sep 17 00:00:00 2001
From: Bananymous <oskari.alaranta@bananymous.com>
Date: Mon, 5 Feb 2024 17:31:15 +0200
Subject: [PATCH] Kernel: Fix IPv4 header checksum calculation

---
 BAN/include/BAN/IPv4.h                  |  1 +
 kernel/include/kernel/Networking/IPv4.h | 13 +++++++------
 kernel/kernel/Networking/IPv4.cpp       |  4 ++--
 3 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/BAN/include/BAN/IPv4.h b/BAN/include/BAN/IPv4.h
index 31a6bafc..b72949d6 100644
--- a/BAN/include/BAN/IPv4.h
+++ b/BAN/include/BAN/IPv4.h
@@ -1,5 +1,6 @@
 #pragma once
 
+#include <BAN/Endianness.h>
 #include <BAN/Formatter.h>
 #include <BAN/Hash.h>
 
diff --git a/kernel/include/kernel/Networking/IPv4.h b/kernel/include/kernel/Networking/IPv4.h
index f893f53b..610e64d4 100644
--- a/kernel/include/kernel/Networking/IPv4.h
+++ b/kernel/include/kernel/Networking/IPv4.h
@@ -23,12 +23,13 @@ namespace Kernel
 
 		constexpr uint16_t calculate_checksum() const
 		{
-			return 0xFFFF
-				- (((uint16_t)version_IHL << 8) | DSCP_ECN)
-				- total_length
-				- identification
-				- flags_frament
-				- (((uint16_t)time_to_live << 8) | protocol);
+			uint32_t total_sum = 0;
+			for (size_t i = 0; i < sizeof(IPv4Header) / sizeof(uint16_t); i++)
+				total_sum += reinterpret_cast<const BAN::NetworkEndian<uint16_t>*>(this)[i];
+			total_sum -= checksum;
+			while (total_sum >> 16)
+				total_sum = (total_sum >> 16) + (total_sum & 0xFFFF);
+			return ~(uint16_t)total_sum;
 		}
 	};
 	static_assert(sizeof(IPv4Header) == 20);
diff --git a/kernel/kernel/Networking/IPv4.cpp b/kernel/kernel/Networking/IPv4.cpp
index e9d76a0b..33f9044d 100644
--- a/kernel/kernel/Networking/IPv4.cpp
+++ b/kernel/kernel/Networking/IPv4.cpp
@@ -8,15 +8,15 @@ namespace Kernel
 	{
 		auto& header = packet.as<IPv4Header>();
 		header.version_IHL		= 0x45;
-		header.DSCP_ECN			= 0x10;
+		header.DSCP_ECN			= 0x00;
 		header.total_length		= packet.size();
 		header.identification	= 1;
 		header.flags_frament	= 0x00;
 		header.time_to_live		= 0x40;
 		header.protocol			= protocol;
-		header.checksum			= header.calculate_checksum();
 		header.src_address		= src_ipv4;
 		header.dst_address		= dst_ipv4;
+		header.checksum			= header.calculate_checksum();
 	}
 
 }