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.

5 files changed, 35 insertions, 21 deletions

diff --git a/pkg/tcpip/transport/icmp/endpoint.go b/pkg/tcpip/transport/icmp/endpoint.go
index feef8dca0..b1d820372 100644
--- a/pkg/tcpip/transport/icmp/endpoint.go
+++ b/pkg/tcpip/transport/icmp/endpoint.go
@@ -747,15 +747,15 @@ func (e *endpoint) HandlePacket(r *stack.Route, id stack.TransportEndpointID, pk
 	// Only accept echo replies.
 	switch e.NetProto {
 	case header.IPv4ProtocolNumber:
-		h := header.ICMPv4(pkt.Data.First())
-		if h.Type() != header.ICMPv4EchoReply {
+		h, ok := pkt.Data.PullUp(header.ICMPv4MinimumSize)
+		if !ok || header.ICMPv4(h).Type() != header.ICMPv4EchoReply {
 			e.stack.Stats().DroppedPackets.Increment()
 			e.stats.ReceiveErrors.MalformedPacketsReceived.Increment()
 			return
 		}
 	case header.IPv6ProtocolNumber:
-		h := header.ICMPv6(pkt.Data.First())
-		if h.Type() != header.ICMPv6EchoReply {
+		h, ok := pkt.Data.PullUp(header.ICMPv6MinimumSize)
+		if !ok || header.ICMPv6(h).Type() != header.ICMPv6EchoReply {
 			e.stack.Stats().DroppedPackets.Increment()
 			e.stats.ReceiveErrors.MalformedPacketsReceived.Increment()
 			return
diff --git a/pkg/tcpip/transport/tcp/segment.go b/pkg/tcpip/transport/tcp/segment.go
index 40461fd31..7712ce652 100644
--- a/pkg/tcpip/transport/tcp/segment.go
+++ b/pkg/tcpip/transport/tcp/segment.go
@@ -144,7 +144,11 @@ func (s *segment) logicalLen() seqnum.Size {
 // TCP checksum and stores the checksum and result of checksum verification in
 // the csum and csumValid fields of the segment.
 func (s *segment) parse() bool {
-	h := header.TCP(s.data.First())
+	h, ok := s.data.PullUp(header.TCPMinimumSize)
+	if !ok {
+		return false
+	}
+	hdr := header.TCP(h)
 
 	// h is the header followed by the payload. We check that the offset to
 	// the data respects the following constraints:
@@ -156,12 +160,16 @@ func (s *segment) parse() bool {
 	// N.B. The segment has already been validated as having at least the
 	//      minimum TCP size before reaching here, so it's safe to read the
 	//      fields.
-	offset := int(h.DataOffset())
-	if offset < header.TCPMinimumSize || offset > len(h) {
+	offset := int(hdr.DataOffset())
+	if offset < header.TCPMinimumSize {
+		return false
+	}
+	hdrWithOpts, ok := s.data.PullUp(offset)
+	if !ok {
 		return false
 	}
 
-	s.options = []byte(h[header.TCPMinimumSize:offset])
+	s.options = []byte(hdrWithOpts[header.TCPMinimumSize:])
 	s.parsedOptions = header.ParseTCPOptions(s.options)
 
 	// Query the link capabilities to decide if checksum validation is
@@ -173,18 +181,19 @@ func (s *segment) parse() bool {
 		s.data.TrimFront(offset)
 	}
 	if verifyChecksum {
-		s.csum = h.Checksum()
+		hdr = header.TCP(hdrWithOpts)
+		s.csum = hdr.Checksum()
 		xsum := s.route.PseudoHeaderChecksum(ProtocolNumber, uint16(s.data.Size()))
-		xsum = h.CalculateChecksum(xsum)
+		xsum = hdr.CalculateChecksum(xsum)
 		s.data.TrimFront(offset)
 		xsum = header.ChecksumVV(s.data, xsum)
 		s.csumValid = xsum == 0xffff
 	}
 
-	s.sequenceNumber = seqnum.Value(h.SequenceNumber())
-	s.ackNumber = seqnum.Value(h.AckNumber())
-	s.flags = h.Flags()
-	s.window = seqnum.Size(h.WindowSize())
+	s.sequenceNumber = seqnum.Value(hdr.SequenceNumber())
+	s.ackNumber = seqnum.Value(hdr.AckNumber())
+	s.flags = hdr.Flags()
+	s.window = seqnum.Size(hdr.WindowSize())
 	return true
 }
 
diff --git a/pkg/tcpip/transport/tcp/tcp_test.go b/pkg/tcpip/transport/tcp/tcp_test.go
index ab1014c7f..286c66cf5 100644
--- a/pkg/tcpip/transport/tcp/tcp_test.go
+++ b/pkg/tcpip/transport/tcp/tcp_test.go
@@ -3548,7 +3548,7 @@ func TestReceivedInvalidSegmentCountIncrement(t *testing.T) {
 		AckNum:  c.IRS.Add(1),
 		RcvWnd:  30000,
 	})
-	tcpbuf := vv.First()[header.IPv4MinimumSize:]
+	tcpbuf := vv.ToView()[header.IPv4MinimumSize:]
 	tcpbuf[header.TCPDataOffset] = ((header.TCPMinimumSize - 1) / 4) << 4
 
 	c.SendSegment(vv)
@@ -3575,7 +3575,7 @@ func TestReceivedIncorrectChecksumIncrement(t *testing.T) {
 		AckNum:  c.IRS.Add(1),
 		RcvWnd:  30000,
 	})
-	tcpbuf := vv.First()[header.IPv4MinimumSize:]
+	tcpbuf := vv.ToView()[header.IPv4MinimumSize:]
 	// Overwrite a byte in the payload which should cause checksum
 	// verification to fail.
 	tcpbuf[(tcpbuf[header.TCPDataOffset]>>4)*4] = 0x4
diff --git a/pkg/tcpip/transport/udp/endpoint.go b/pkg/tcpip/transport/udp/endpoint.go
index edb54f0be..756ab913a 100644
--- a/pkg/tcpip/transport/udp/endpoint.go
+++ b/pkg/tcpip/transport/udp/endpoint.go
@@ -1250,8 +1250,8 @@ func (e *endpoint) Readiness(mask waiter.EventMask) waiter.EventMask {
 // endpoint.
 func (e *endpoint) HandlePacket(r *stack.Route, id stack.TransportEndpointID, pkt stack.PacketBuffer) {
 	// Get the header then trim it from the view.
-	hdr := header.UDP(pkt.Data.First())
-	if int(hdr.Length()) > pkt.Data.Size() {
+	hdr, ok := pkt.Data.PullUp(header.UDPMinimumSize)
+	if !ok || int(header.UDP(hdr).Length()) > pkt.Data.Size() {
 		// Malformed packet.
 		e.stack.Stats().UDP.MalformedPacketsReceived.Increment()
 		e.stats.ReceiveErrors.MalformedPacketsReceived.Increment()
@@ -1286,7 +1286,7 @@ func (e *endpoint) HandlePacket(r *stack.Route, id stack.TransportEndpointID, pk
 		senderAddress: tcpip.FullAddress{
 			NIC:  r.NICID(),
 			Addr: id.RemoteAddress,
-			Port: hdr.SourcePort(),
+			Port: header.UDP(hdr).SourcePort(),
 		},
 	}
 	packet.data = pkt.Data
diff --git a/pkg/tcpip/transport/udp/protocol.go b/pkg/tcpip/transport/udp/protocol.go
index 6e31a9bac..52af6de22 100644
--- a/pkg/tcpip/transport/udp/protocol.go
+++ b/pkg/tcpip/transport/udp/protocol.go
@@ -68,8 +68,13 @@ func (*protocol) ParsePorts(v buffer.View) (src, dst uint16, err *tcpip.Error) {
 // that don't match any existing endpoint.
 func (p *protocol) HandleUnknownDestinationPacket(r *stack.Route, id stack.TransportEndpointID, pkt stack.PacketBuffer) bool {
 	// Get the header then trim it from the view.
-	hdr := header.UDP(pkt.Data.First())
-	if int(hdr.Length()) > pkt.Data.Size() {
+	h, ok := pkt.Data.PullUp(header.UDPMinimumSize)
+	if !ok {
+		// Malformed packet.
+		r.Stack().Stats().UDP.MalformedPacketsReceived.Increment()
+		return true
+	}
+	if int(header.UDP(h).Length()) > pkt.Data.Size() {
 		// Malformed packet.
 		r.Stack().Stats().UDP.MalformedPacketsReceived.Increment()
 		return true