DEV9: Handle adapters that provide FCS in capture

pcsx2/DEV9/PacketReader/ARP/ARP_PacketEditor.cpp

@@ -62,4 +62,9 @@ namespace PacketReader::ARP
 		int offset = 8 + 2 * GetHardwareAddressLength() + GetProtocolAddressLength();
 		return &basePkt->data[offset];
 	}
+
+	int ARP_PacketEditor::GetLength()
+	{
+		return 8 + 2 * GetHardwareAddressLength() + 2 * GetProtocolAddressLength();
+	}
 } // namespace PacketReader::ARP

pcsx2/DEV9/PacketReader/ARP/ARP_PacketEditor.h

@@ -13,7 +13,6 @@ namespace PacketReader::ARP
 		PayloadPtr* basePkt;
 
 	public:
-
 		ARP_PacketEditor(PayloadPtr* pkt);
 
 		u16 GetHardwareType();
@@ -26,5 +25,7 @@ namespace PacketReader::ARP
 		u8* SenderProtocolAddress();
 		u8* TargetHardwareAddress();
 		u8* TargetProtocolAddress();
+
+		int GetLength();
 	};
 } // namespace PacketReader::ARP

pcsx2/DEV9/pcap_io.cpp

@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: LGPL-3.0+
 
 #include "common/Assertions.h"
+#include <algorithm>
 #include <memory>
 
 #ifdef _WIN32
@@ -124,9 +125,9 @@ bool PCAPAdapter::recv(NetPacket* pkt)
 	// This delays getting packets we need, so instead loop untill a valid packet, or no packet, is returned from pcap_next_ex.
 	while (pcap_next_ex(hpcap, &header, &pkt_data) > 0)
 	{
-		// 1514 is the largest etherframe we can get with an MTU of 1500 (assuming no VLAN tagging).
-		// We don't (typically?) get the FCS, and we might need to strip it if we do.
-		if (header->len > 1514)
+		// 1518 is the largest Ethernet frame we can get using an MTU of 1500 (assuming no VLAN tagging).
+		// This includes the FCS, which should be trimmed (PS2 SDK dosn't allow extra space for this).
+		if (header->len > 1518)
 		{
 			Console.Error("DEV9: Dropped jumbo frame of size: %u", header->len);
 			continue;
@@ -142,6 +143,15 @@ bool PCAPAdapter::recv(NetPacket* pkt)
 
 		if (VerifyPkt(pkt, header->len))
 		{
+			HandleFrameCheckSequence(pkt);
+
+			// FCS (if present) has been removed, apply correct limit
+			if (pkt->size > 1514)
+			{
+				Console.Error("DEV9: Dropped jumbo frame of size: %u", pkt->size);
+				continue;
+			}
+
 			InspectRecv(pkt);
 			return true;
 		}
@@ -366,3 +376,76 @@ void PCAPAdapter::SetMACBridgedSend(NetPacket* pkt)
 	}
 	frame.SetSourceMAC(hostMAC);
 }
+
+/*
+ * Strips the Frame Check Sequence if we manage to capture it.
+ * 
+ * On Windows, (some?) Intel NICs can be configured to capture FCS.
+ * 
+ * Linux can be configure to capture FCS, using `ethtool -K <interface> rx-fcs on` on supported devices.
+ * Support for capturing FCS can be checked with `ethtool -k <interface> | grep rx-fcs`.
+ * if it's `off [Fixed]`, then the interface/driver dosn't support capturing FCS.
+ * 
+ * BSD based systems might capture FCS by default.
+ * 
+ * Packets sent by host won't have FCS, We identify these packets by checking the source MAC address.
+ * Packets sent by another application via packet injection also won't have FCS and may not match the adapter MAC.
+ */
+void PCAPAdapter::HandleFrameCheckSequence(NetPacket* pkt)
+{
+	EthernetFrameEditor frame(pkt);
+	if (frame.GetSourceMAC() == hostMAC)
+		return;
+
+	// There is a (very) low chance of the last 4 bytes of payload somehow acting as a valid checksum for the whole Ethernet frame.
+	// For EtherTypes we already can parse, trim the Ethernet frame based on the payload length.
+
+	int payloadSize = -1;
+	if (frame.GetProtocol() == static_cast<u16>(EtherType::IPv4)) // IP
+	{
+		PayloadPtr* payload = frame.GetPayload();
+		IP_Packet ippkt(payload->data, payload->GetLength());
+		payloadSize = ippkt.GetLength();
+	}
+	if (frame.GetProtocol() == static_cast<u16>(EtherType::ARP)) // ARP
+	{
+		ARP_PacketEditor arpPkt(frame.GetPayload());
+		payloadSize = arpPkt.GetLength();
+	}
+
+	if (payloadSize != -1)
+	{
+		// Minumum frame size is 60 + 4 byte FCS.
+		// Virtual NICs may omit this padding, so check we arn't increasing pkt size.
+		payloadSize = std::min(std::max(payloadSize, 60 - frame.headerLength), pkt->size);
+
+		pkt->size = payloadSize + frame.headerLength;
+		return;
+	}
+
+	// Ethertype unknown, rely on checking for a FCS.
+	if (ValidateEtherFrame(pkt))
+		pkt->size -= 4;
+}
+
+bool PCAPAdapter::ValidateEtherFrame(NetPacket* pkt)
+{
+	u32 crc = 0xFFFFFFFF;
+
+	for (int i = 0; i < pkt->size; i++)
+	{
+		// Neads unsigned value
+		crc = crc ^ static_cast<u8>(pkt->buffer[i]);
+		for (int bit = 0; bit < 8; bit++)
+		{
+			if ((crc & 1) != 0)
+				crc = (crc >> 1) ^ 0xEDB88320;
+			else
+				crc = (crc >> 1);
+		}
+	}
+
+	crc = ~crc;
+
+	return crc == 0x2144DF1C;
+}

pcsx2/DEV9/pcap_io.h

@@ -42,4 +42,7 @@ private:
 
 	void SetMACBridgedRecv(NetPacket* pkt);
 	void SetMACBridgedSend(NetPacket* pkt);
+
+	void HandleFrameCheckSequence(NetPacket* pkt);
+	bool ValidateEtherFrame(NetPacket* pkt);
 };