Teach netstack how to add options to IPv4 packets

Most packets don't have options but they are an integral part of the
standard. Teaching the ipv4 code how to handle them will simplify future
testing and use.  Because Options are so rare it is worth making sure
that the extra work is kept out of the fast path as much as possible.

Prior to this change, all usages of the IHL field of the IPv4Fields/Encode
system set it to the same constant value except in a couple of tests
for bad values. From this change IHL will not be a constant as it will
depend on the size of any Options. Since ipv4.Encode() now handles the
options it becomes a possible source of errors to let the callers set
this value, so remove it entirely and calculate the value from the size
of the Options if present (or not) therefore guaranteeing a correct value.

Fixes #4709
RELNOTES: n/a
PiperOrigin-RevId: 341864765

2 files changed, 37 insertions, 10 deletions

diff --git a/pkg/tcpip/header/ipv4.go b/pkg/tcpip/header/ipv4.go
index 961b77628..7e32b31b4 100644
--- a/pkg/tcpip/header/ipv4.go
+++ b/pkg/tcpip/header/ipv4.go
@@ -56,12 +56,9 @@ const (
 )
 
 // IPv4Fields contains the fields of an IPv4 packet. It is used to describe the
-// fields of a packet that needs to be encoded.
+// fields of a packet that needs to be encoded. The IHL field is not here as
+// it is totally defined by the size of the options.
 type IPv4Fields struct {
-	// IHL is the "internet header length" field of an IPv4 packet. The value
-	// is in bytes.
-	IHL uint8
-
 	// TOS is the "type of service" field of an IPv4 packet.
 	TOS uint8
 
@@ -91,6 +88,9 @@ type IPv4Fields struct {
 
 	// DstAddr is the "destination ip address" of an IPv4 packet.
 	DstAddr tcpip.Address
+
+	// Options is between 0 and 40 bytes or nil if empty.
+	Options IPv4Options
 }
 
 // IPv4 is an IPv4 header.
@@ -118,7 +118,7 @@ const (
 	// Linux limits this to 65,515 octets (the max IP datagram size - the IPv4
 	// header size). But RFC 791 section 3.2 discusses the design of the IPv4
 	// fragment "allows 2**13 = 8192 fragments of 8 octets each for a total of
-	// 65,536 octets. Note that this is consistent with the the datagram total
+	// 65,536 octets. Note that this is consistent with the datagram total
 	// length field (of course, the header is counted in the total length and not
 	// in the fragments)."
 	IPv4MaximumPayloadSize = 65536
@@ -275,10 +275,22 @@ func (b IPv4) DestinationAddress() tcpip.Address {
 // IPv4Options is a buffer that holds all the raw IP options.
 type IPv4Options []byte
 
-// Options returns a buffer holding the options.
+// AllocationSize implements stack.NetOptions.
+// It reports the size to allocate for the Options. RFC 791 page 23 (end of
+// section 3.1) says of the padding at the end of the options:
+//    The internet header padding is used to ensure that the internet
+//    header ends on a 32 bit boundary.
+func (o IPv4Options) AllocationSize() int {
+	return (len(o) + IPv4IHLStride - 1) & ^(IPv4IHLStride - 1)
+}
+
+// Options returns a buffer holding the options or nil.
 func (b IPv4) Options() IPv4Options {
 	hdrLen := b.HeaderLength()
-	return IPv4Options(b[options:hdrLen:hdrLen])
+	if hdrLen > IPv4MinimumSize {
+		return IPv4Options(b[options:hdrLen:hdrLen])
+	}
+	return nil
 }
 
 // TransportProtocol implements Network.TransportProtocol.
@@ -351,7 +363,22 @@ func (b IPv4) CalculateChecksum() uint16 {
 
 // Encode encodes all the fields of the IPv4 header.
 func (b IPv4) Encode(i *IPv4Fields) {
-	b.SetHeaderLength(i.IHL)
+	// The size of the options defines the size of the whole header and thus the
+	// IHL field. Options are rare and this is a heavily used function so it is
+	// worth a bit of optimisation here to keep the copy out of the fast path.
+	hdrLen := IPv4MinimumSize
+	if len(i.Options) != 0 {
+		// AllocationSize is always >= len(i.Options).
+		aLen := i.Options.AllocationSize()
+		hdrLen += aLen
+		if hdrLen > len(b) {
+			panic(fmt.Sprintf("encode received %d bytes, wanted >= %d", len(b), hdrLen))
+		}
+		if aLen != copy(b[options:], i.Options) {
+			_ = copy(b[options+len(i.Options):options+aLen], []byte{0, 0, 0, 0})
+		}
+	}
+	b.SetHeaderLength(uint8(hdrLen))
 	b[tos] = i.TOS
 	b.SetTotalLength(i.TotalLength)
 	binary.BigEndian.PutUint16(b[id:], i.ID)
diff --git a/pkg/tcpip/header/ipversion_test.go b/pkg/tcpip/header/ipversion_test.go
index 17a49d4fa..b5540bf66 100644
--- a/pkg/tcpip/header/ipversion_test.go
+++ b/pkg/tcpip/header/ipversion_test.go
@@ -22,7 +22,7 @@ import (
 
 func TestIPv4(t *testing.T) {
 	b := header.IPv4(make([]byte, header.IPv4MinimumSize))
-	b.Encode(&header.IPv4Fields{IHL: header.IPv4MinimumSize})
+	b.Encode(&header.IPv4Fields{})
 
 	const want = header.IPv4Version
 	if v := header.IPVersion(b); v != want {