netstack: parse incoming packet headers up-front

Netstack has traditionally parsed headers on-demand as a packet moves up the
stack. This is conceptually simple and convenient, but incompatible with
iptables, where headers can be inspected and mangled before even a routing
decision is made.

This changes header parsing to happen early in the incoming packet path, as soon
as the NIC gets the packet from a link endpoint. Even if an invalid packet is
found (e.g. a TCP header of insufficient length), the packet is passed up the
stack for proper stats bookkeeping.

PiperOrigin-RevId: 315179302

1 files changed, 13 insertions, 0 deletions

diff --git a/pkg/tcpip/stack/registration.go b/pkg/tcpip/stack/registration.go
index 94f177841..5cbc946b6 100644
--- a/pkg/tcpip/stack/registration.go
+++ b/pkg/tcpip/stack/registration.go
@@ -168,6 +168,11 @@ type TransportProtocol interface {
 
 	// Wait waits for any worker goroutines owned by the protocol to stop.
 	Wait()
+
+	// Parse sets pkt.TransportHeader and trims pkt.Data appropriately. It does
+	// neither and returns false if pkt.Data is too small, i.e. pkt.Data.Size() <
+	// MinimumPacketSize()
+	Parse(pkt *PacketBuffer) (ok bool)
 }
 
 // TransportDispatcher contains the methods used by the network stack to deliver
@@ -313,6 +318,14 @@ type NetworkProtocol interface {
 
 	// Wait waits for any worker goroutines owned by the protocol to stop.
 	Wait()
+
+	// Parse sets pkt.NetworkHeader and trims pkt.Data appropriately. It
+	// returns:
+	// - The encapsulated protocol, if present.
+	// - Whether there is an encapsulated transport protocol payload (e.g. ARP
+	//   does not encapsulate anything).
+	// - Whether pkt.Data was large enough to parse and set pkt.NetworkHeader.
+	Parse(pkt *PacketBuffer) (proto tcpip.TransportProtocolNumber, hasTransportHdr bool, ok bool)
 }
 
 // NetworkDispatcher contains the methods used by the network stack to deliver