struct_ops_test: Fix TestCreateStructOpsMapSpecSimple

On newer kernels fields have been added to the value type of the
`bpf_testmod_ops` struct. The test was assuming a static size of
448 bytes, but that is no longer valid for all kernels.

Instead of hardcoding the size, we now look up the value type in the BTF
of the module and use that size to create the map contents.

Signed-off-by: Dylan Reimerink <dylan.reimerink@isovalent.com>

Author: dylandreimerink

struct_ops_test.go

@@ -3,6 +3,8 @@ package ebpf
 import (
 	"testing"
 
+	"github.com/go-quicktest/qt"
+
 	"github.com/cilium/ebpf/btf"
 	"github.com/cilium/ebpf/internal/sys"
 	"github.com/cilium/ebpf/internal/testutils"
@@ -11,6 +13,12 @@ import (
 func TestCreateStructOpsMapSpecSimple(t *testing.T) {
 	requireTestmodOps(t)
 
+	btfSpec, err := btf.LoadKernelModuleSpec("bpf_testmod")
+	qt.Assert(t, qt.IsNil(err))
+
+	var outerValueType *btf.Struct
+	qt.Assert(t, qt.IsNil(btfSpec.TypeByName(structOpsValuePrefix+"bpf_testmod_ops", &outerValueType)))
+
 	ms := &MapSpec{
 		Name:       "testmod_ops",
 		Type:       StructOpsMap,
@@ -22,15 +30,13 @@ func TestCreateStructOpsMapSpecSimple(t *testing.T) {
 		Contents: []MapKV{
 			{
 				Key:   uint32(0),
-				Value: make([]byte, 448),
+				Value: make([]byte, outerValueType.Size),
 			},
 		},
 	}
 
 	m, err := NewMap(ms)
 	testutils.SkipIfNotSupported(t, err)
-	if err != nil {
-		t.Fatalf("creating struct_ops map failed: %v", err)
-	}
+	qt.Assert(t, qt.IsNil(err))
 	t.Cleanup(func() { _ = m.Close() })
 }