From 24f956c7392c4d45f9cad95d3ef69014307a3408 Mon Sep 17 00:00:00 2001
From: Tim Schumacher <timschumi@gmx.de>
Date: Mon, 12 Dec 2022 17:52:00 +0100
Subject: [PATCH] Kernel: Convert TCP pseudo-headers through a union

This keeps us from tripping strict aliasing, which previously made TCP
connections inoperable when building without `-fsanitize=undefined` or
`-fno-strict-aliasing`.
---
 Kernel/Net/TCPSocket.cpp | 24 ++++++++++++++----------
 1 file changed, 14 insertions(+), 10 deletions(-)

diff --git a/Kernel/Net/TCPSocket.cpp b/Kernel/Net/TCPSocket.cpp
index 2cd19f5137..d94a8976ae 100644
--- a/Kernel/Net/TCPSocket.cpp
+++ b/Kernel/Net/TCPSocket.cpp
@@ -360,31 +360,35 @@ bool TCPSocket::should_delay_next_ack() const
 
 NetworkOrdered<u16> TCPSocket::compute_tcp_checksum(IPv4Address const& source, IPv4Address const& destination, TCPPacket const& packet, u16 payload_size)
 {
-    struct [[gnu::packed]] PseudoHeader {
-        IPv4Address source;
-        IPv4Address destination;
-        u8 zero;
-        u8 protocol;
-        NetworkOrdered<u16> payload_size;
+    union PseudoHeader {
+        struct [[gnu::packed]] {
+            IPv4Address source;
+            IPv4Address destination;
+            u8 zero;
+            u8 protocol;
+            NetworkOrdered<u16> payload_size;
+        } header;
+        u16 raw[6];
     };
+    static_assert(sizeof(PseudoHeader) == 12);
 
-    PseudoHeader pseudo_header { source, destination, 0, (u8)IPv4Protocol::TCP, packet.header_size() + payload_size };
+    PseudoHeader pseudo_header { .header = { source, destination, 0, (u8)IPv4Protocol::TCP, packet.header_size() + payload_size } };
 
     u32 checksum = 0;
-    auto raw_pseudo_header = bit_cast<u16*>(&pseudo_header);
+    auto* raw_pseudo_header = pseudo_header.raw;
     for (size_t i = 0; i < sizeof(pseudo_header) / sizeof(u16); ++i) {
         checksum += AK::convert_between_host_and_network_endian(raw_pseudo_header[i]);
         if (checksum > 0xffff)
             checksum = (checksum >> 16) + (checksum & 0xffff);
     }
-    auto raw_packet = bit_cast<u16*>(&packet);
+    auto* raw_packet = bit_cast<u16*>(&packet);
     for (size_t i = 0; i < packet.header_size() / sizeof(u16); ++i) {
         checksum += AK::convert_between_host_and_network_endian(raw_packet[i]);
         if (checksum > 0xffff)
             checksum = (checksum >> 16) + (checksum & 0xffff);
     }
     VERIFY(packet.data_offset() * 4 == packet.header_size());
-    auto raw_payload = bit_cast<u16*>(packet.payload());
+    auto* raw_payload = bit_cast<u16*>(packet.payload());
     for (size_t i = 0; i < payload_size / sizeof(u16); ++i) {
         checksum += AK::convert_between_host_and_network_endian(raw_payload[i]);
         if (checksum > 0xffff)