Remove View.First() and View.RemoveFirst()

These methods let users eaily break the VectorisedView abstraction, and
allowed netstack to slip into pseudo-enforcement of the "all headers are
in the first View" invariant. Removing them and replacing with PullUp(n)
breaks this reliance and will make it easier to add iptables support and
rework network buffer management.

The new View.PullUp(n) method is low cost in the common case, when when
all the headers fit in the first View.

PiperOrigin-RevId: 308163542

1 files changed, 11 insertions, 23 deletions

diff --git a/pkg/tcpip/stack/nic.go b/pkg/tcpip/stack/nic.go
index 016dbe15e..0c2b1f36a 100644
--- a/pkg/tcpip/stack/nic.go
+++ b/pkg/tcpip/stack/nic.go
@@ -1203,12 +1203,12 @@ func (n *NIC) DeliverNetworkPacket(linkEP LinkEndpoint, remote, local tcpip.Link
 		n.stack.stats.IP.PacketsReceived.Increment()
 	}
 
-	if len(pkt.Data.First()) < netProto.MinimumPacketSize() {
+	netHeader, ok := pkt.Data.PullUp(netProto.MinimumPacketSize())
+	if !ok {
 		n.stack.stats.MalformedRcvdPackets.Increment()
 		return
 	}
-
-	src, dst := netProto.ParseAddresses(pkt.Data.First())
+	src, dst := netProto.ParseAddresses(netHeader)
 
 	if n.stack.handleLocal && !n.isLoopback() && n.getRef(protocol, src) != nil {
 		// The source address is one of our own, so we never should have gotten a
@@ -1289,22 +1289,8 @@ func (n *NIC) DeliverNetworkPacket(linkEP LinkEndpoint, remote, local tcpip.Link
 
 func (n *NIC) forwardPacket(r *Route, protocol tcpip.NetworkProtocolNumber, pkt PacketBuffer) {
 	// TODO(b/143425874) Decrease the TTL field in forwarded packets.
-
-	firstData := pkt.Data.First()
-	pkt.Data.RemoveFirst()
-
-	if linkHeaderLen := int(n.linkEP.MaxHeaderLength()); linkHeaderLen == 0 {
-		pkt.Header = buffer.NewPrependableFromView(firstData)
-	} else {
-		firstDataLen := len(firstData)
-
-		// pkt.Header should have enough capacity to hold n.linkEP's headers.
-		pkt.Header = buffer.NewPrependable(firstDataLen + linkHeaderLen)
-
-		// TODO(b/151227689): avoid copying the packet when forwarding
-		if n := copy(pkt.Header.Prepend(firstDataLen), firstData); n != firstDataLen {
-			panic(fmt.Sprintf("copied %d bytes, expected %d", n, firstDataLen))
-		}
+	if linkHeaderLen := int(n.linkEP.MaxHeaderLength()); linkHeaderLen != 0 {
+		pkt.Header = buffer.NewPrependable(linkHeaderLen)
 	}
 
 	if err := n.linkEP.WritePacket(r, nil /* gso */, protocol, pkt); err != nil {
@@ -1332,12 +1318,13 @@ func (n *NIC) DeliverTransportPacket(r *Route, protocol tcpip.TransportProtocolN
 	// validly formed.
 	n.stack.demux.deliverRawPacket(r, protocol, pkt)
 
-	if len(pkt.Data.First()) < transProto.MinimumPacketSize() {
+	transHeader, ok := pkt.Data.PullUp(transProto.MinimumPacketSize())
+	if !ok {
 		n.stack.stats.MalformedRcvdPackets.Increment()
 		return
 	}
 
-	srcPort, dstPort, err := transProto.ParsePorts(pkt.Data.First())
+	srcPort, dstPort, err := transProto.ParsePorts(transHeader)
 	if err != nil {
 		n.stack.stats.MalformedRcvdPackets.Increment()
 		return
@@ -1375,11 +1362,12 @@ func (n *NIC) DeliverTransportControlPacket(local, remote tcpip.Address, net tcp
 	// ICMPv4 only guarantees that 8 bytes of the transport protocol will
 	// be present in the payload. We know that the ports are within the
 	// first 8 bytes for all known transport protocols.
-	if len(pkt.Data.First()) < 8 {
+	transHeader, ok := pkt.Data.PullUp(8)
+	if !ok {
 		return
 	}
 
-	srcPort, dstPort, err := transProto.ParsePorts(pkt.Data.First())
+	srcPort, dstPort, err := transProto.ParsePorts(transHeader)
 	if err != nil {
 		return
 	}