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

1 files changed, 2 insertions, 2 deletions

diff --git a/pkg/tcpip/checker/checker.go b/pkg/tcpip/checker/checker.go
index 530f2ae2f..8868cf4e3 100644
--- a/pkg/tcpip/checker/checker.go
+++ b/pkg/tcpip/checker/checker.go
@@ -197,7 +197,7 @@ func IPPayload(payload []byte) NetworkChecker {
 }
 
 // IPv4Options returns a checker that checks the options in an IPv4 packet.
-func IPv4Options(want []byte) NetworkChecker {
+func IPv4Options(want header.IPv4Options) NetworkChecker {
 	return func(t *testing.T, h []header.Network) {
 		t.Helper()
 
@@ -205,7 +205,7 @@ func IPv4Options(want []byte) NetworkChecker {
 		if !ok {
 			t.Fatalf("unexpected network header passed to checker, got = %T, want = header.IPv4", h[0])
 		}
-		options := []byte(ip.Options())
+		options := ip.Options()
 		// cmp.Diff does not consider nil slices equal to empty slices, but we do.
 		if len(want) == 0 && len(options) == 0 {
 			return