Kernel: Send Fragmented IPv4 packets if payload size > mtu

This adds IPv4 fragmentation, so now we can send huuuuuuge packets
properly.

Kernel/Net/IPv4.h

@@ -40,6 +40,11 @@ enum class IPv4Protocol : u16 {
     UDP = 17,
 };
 
+enum class IPv4PacketFlags : u16 {
+    DontFragment = 0x4000,
+    MoreFragments = 0x2000,
+};
+
 NetworkOrdered<u16> internet_checksum(const void*, size_t);
 
 class [[gnu::packed]] IPv4Packet
@@ -75,6 +80,28 @@ public:
     void* payload() { return this + 1; }
     const void* payload() const { return this + 1; }
 
+    u16 flags_and_fragment() const { return m_flags_and_fragment; }
+    u16 fragment_offset() const { return ((u16)m_flags_and_fragment & 0x2fff); }
+    u16 flags() const { return (((u16)m_flags_and_fragment) & (((u16)IPv4PacketFlags::MoreFragments) | ((u16)IPv4PacketFlags::DontFragment))); }
+
+    void set_has_more_fragments(bool more_fragments)
+    {
+        if (more_fragments)
+            m_flags_and_fragment = (u16)m_flags_and_fragment | ((u16)IPv4PacketFlags::MoreFragments);
+        else
+            m_flags_and_fragment = (u16)m_flags_and_fragment & ((u16)IPv4PacketFlags::MoreFragments);
+    }
+    void set_fragment_offset(u16 offset)
+    {
+        m_flags_and_fragment = flags() | (offset & 0x2fff);
+    }
+
+    bool is_a_fragment() const
+    {
+        // either has More-Fragments set, or has a fragment offset
+        return (((u16)m_flags_and_fragment) & ((u16)IPv4PacketFlags::MoreFragments)) || ((u16)m_flags_and_fragment & 0x2fff);
+    }
+
     u16 payload_size() const { return m_length - sizeof(IPv4Packet); }
 
     NetworkOrdered<u16> compute_checksum() const
@@ -97,6 +124,7 @@ private:
 };
 
 static_assert(sizeof(IPv4Packet) == 20);
+const LogStream& operator<<(const LogStream& stream, const IPv4Packet& packet);
 
 inline NetworkOrdered<u16> internet_checksum(const void* ptr, size_t count)
 {

Kernel/Net/NetworkAdapter.cpp

@@ -32,6 +32,7 @@
 #include <Kernel/Net/EthernetFrameHeader.h>
 #include <Kernel/Net/LoopbackAdapter.h>
 #include <Kernel/Net/NetworkAdapter.h>
+#include <Kernel/Random.h>
 #include <LibBareMetal/StdLib.h>
 
 namespace Kernel {
@@ -103,8 +104,8 @@ void NetworkAdapter::send_ipv4(const MACAddress& destination_mac, const IPv4Addr
 {
     size_t ipv4_packet_size = sizeof(IPv4Packet) + payload_size;
     if (ipv4_packet_size > mtu()) {
-        // FIXME: Implement IP fragmentation.
+        send_ipv4_fragmented(destination_mac, destination_ipv4, protocol, payload, payload_size, ttl);
-        ASSERT_NOT_REACHED();
+        return;
     }
 
     size_t ethernet_frame_size = sizeof(EthernetFrameHeader) + sizeof(IPv4Packet) + payload_size;
@@ -129,6 +130,44 @@ void NetworkAdapter::send_ipv4(const MACAddress& destination_mac, const IPv4Addr
     send_raw((const u8*)&eth, ethernet_frame_size);
 }
 
+void NetworkAdapter::send_ipv4_fragmented(const MACAddress& destination_mac, const IPv4Address& destination_ipv4, IPv4Protocol protocol, const u8* payload, size_t payload_size, u8 ttl)
+{
+    // packets must be split on the 64-bit boundary
+    auto packet_boundary_size = (mtu() - sizeof(IPv4Packet) - sizeof(EthernetFrameHeader)) & 0xfffffff8;
+    auto fragment_block_count = (payload_size + packet_boundary_size) / packet_boundary_size;
+    auto last_block_size = payload_size - packet_boundary_size * (fragment_block_count - 1);
+    auto number_of_blocks_in_fragment = packet_boundary_size / 8;
+
+    auto identification = get_good_random<u16>();
+
+    size_t ethernet_frame_size = mtu();
+    for (size_t packet_index = 0; packet_index < fragment_block_count; ++packet_index) {
+        auto is_last_block = packet_index + 1 == fragment_block_count;
+        auto packet_payload_size = is_last_block ? last_block_size : packet_boundary_size;
+        auto buffer = ByteBuffer::create_zeroed(ethernet_frame_size);
+        auto& eth = *(EthernetFrameHeader*)buffer.data();
+        eth.set_source(mac_address());
+        eth.set_destination(destination_mac);
+        eth.set_ether_type(EtherType::IPv4);
+        auto& ipv4 = *(IPv4Packet*)eth.payload();
+        ipv4.set_version(4);
+        ipv4.set_internet_header_length(5);
+        ipv4.set_source(ipv4_address());
+        ipv4.set_destination(destination_ipv4);
+        ipv4.set_protocol((u8)protocol);
+        ipv4.set_length(sizeof(IPv4Packet) + packet_payload_size);
+        ipv4.set_has_more_fragments(!is_last_block);
+        ipv4.set_ident(identification);
+        ipv4.set_ttl(ttl);
+        ipv4.set_fragment_offset(packet_index * number_of_blocks_in_fragment);
+        ipv4.set_checksum(ipv4.compute_checksum());
+        m_packets_out++;
+        m_bytes_out += ethernet_frame_size;
+        memcpy(ipv4.payload(), payload + packet_index * packet_boundary_size, packet_payload_size);
+        send_raw((const u8*)&eth, ethernet_frame_size);
+    }
+}
+
 void NetworkAdapter::did_receive(const u8* data, size_t length)
 {
     InterruptDisabler disabler;

Kernel/Net/NetworkAdapter.h

@@ -64,6 +64,7 @@ public:
 
     void send(const MACAddress&, const ARPPacket&);
     void send_ipv4(const MACAddress&, const IPv4Address&, IPv4Protocol, const u8* payload, size_t payload_size, u8 ttl);
+    void send_ipv4_fragmented(const MACAddress&, const IPv4Address&, IPv4Protocol, const u8* payload, size_t payload_size, u8 ttl);
 
     size_t dequeue_packet(u8* buffer, size_t buffer_size);