Kernel/Net: Introduce proper pointers for payload calls

Unifying with other structures, this introduces m_payload for all
relevant structures, except the TCP header (which has a different
concept of payload than the rest).

This allows us to return a pointer directly instead of doing pointer
arithmetic to point to the end of the structure.

Kernel/Net/EthernetFrameHeader.h

@@ -32,7 +32,7 @@ class [[gnu::packed]] EthernetFrameHeader {
     MACAddress m_destination;
     MACAddress m_source;
     NetworkOrdered<u16> m_ether_type;
-    u32 m_payload[0];
+    u8 m_payload[0];
 };
 
 static_assert(sizeof(EthernetFrameHeader) == 14);

Kernel/Net/ICMP.h

@@ -30,14 +30,14 @@ class [[gnu::packed]] ICMPHeader {
     u16 checksum() const { return m_checksum; }
     void set_checksum(u16 w) { m_checksum = w; }
 
-    void const* payload() const { return this + 1; }
-    void* payload() { return this + 1; }
+    void const* payload() const { return &m_payload[0]; }
+    void* payload() { return &m_payload[0]; }
 
 private:
     u8 m_type { 0 };
     u8 m_code { 0 };
     NetworkOrdered<u16> m_checksum { 0 };
-    // NOTE: The rest of the header is 4 bytes
+    u8 m_payload[0];
 };
 
 static_assert(AssertSize<ICMPHeader, 4>());
@@ -46,6 +46,8 @@ struct [[gnu::packed]] ICMPEchoPacket {
     ICMPHeader header;
     NetworkOrdered<u16> identifier;
     NetworkOrdered<u16> sequence_number;
-    void* payload() { return this + 1; }
-    void const* payload() const { return this + 1; }
+    u8 m_payload[0];
+
+    void* payload() { return &m_payload[0]; }
+    void const* payload() const { return &m_payload[0]; }
 };

Kernel/Net/IP/IPv4.h

@@ -51,8 +51,8 @@ class [[gnu::packed]] IPv4Packet {
     IPv4Address const& destination() const { return m_destination; }
     void set_destination(IPv4Address const& address) { m_destination = address; }
 
-    void* payload() { return this + 1; }
-    void const* payload() const { return this + 1; }
+    void* payload() { return &m_payload[0]; }
+    void const* payload() const { return &m_payload[0]; }
 
     u16 flags_and_fragment() const { return m_flags_and_fragment; }
     u16 fragment_offset() const { return ((u16)m_flags_and_fragment & 0x1fff); }
@@ -97,6 +97,7 @@ class [[gnu::packed]] IPv4Packet {
     NetworkOrdered<u16> m_checksum;
     IPv4Address m_source;
     IPv4Address m_destination;
+    u8 m_payload[0];
 };
 
 static_assert(AssertSize<IPv4Packet, 20>());

Kernel/Net/UDP.h

@@ -27,14 +27,15 @@ class [[gnu::packed]] UDPPacket {
     u16 checksum() const { return m_checksum; }
     void set_checksum(u16 checksum) { m_checksum = checksum; }
 
-    void const* payload() const { return this + 1; }
-    void* payload() { return this + 1; }
+    void const* payload() const { return &m_payload[0]; }
+    void* payload() { return &m_payload[0]; }
 
 private:
     NetworkOrdered<u16> m_source_port;
     NetworkOrdered<u16> m_destination_port;
     NetworkOrdered<u16> m_length;
     NetworkOrdered<u16> m_checksum;
+    u8 m_payload[0];
 };
 
 static_assert(sizeof(UDPPacket) == 8);