[go-marshal] Add dynamic tag in go_marshal.

This makes it easier to implement dynamically sized types in go-marshal. You
really only need to implement MarshalBytes, UnmarshalBytes and SizeBytes to
implement the entire interface.

By using the `dynamic` tag, the autogenerator will generate the rest of the
methods for us.

This change also simplifies how KernelIPTGetEntries implements Marshallable
using the newly added utility.

PiperOrigin-RevId: 356397114

3 files changed, 59 insertions, 1 deletions

diff --git a/tools/go_marshal/test/BUILD b/tools/go_marshal/test/BUILD
index 4b27773c2..cb2d4e6e3 100644
--- a/tools/go_marshal/test/BUILD
+++ b/tools/go_marshal/test/BUILD
@@ -26,7 +26,10 @@ go_library(
     srcs = ["test.go"],
     marshal = True,
     visibility = ["//tools/go_marshal/test:__subpackages__"],
-    deps = ["//tools/go_marshal/test/external"],
+    deps = [
+        "//pkg/marshal/primitive",
+        "//tools/go_marshal/test/external",
+    ],
 )
 
 go_test(
@@ -36,6 +39,7 @@ go_test(
     deps = [
         ":test",
         "//pkg/marshal",
+        "//pkg/marshal/primitive",
         "//pkg/syserror",
         "//pkg/usermem",
         "//tools/go_marshal/analysis",
diff --git a/tools/go_marshal/test/marshal_test.go b/tools/go_marshal/test/marshal_test.go
index a00f9a684..b0091dc64 100644
--- a/tools/go_marshal/test/marshal_test.go
+++ b/tools/go_marshal/test/marshal_test.go
@@ -28,6 +28,7 @@ import (
 
 	"github.com/google/go-cmp/cmp"
 	"gvisor.dev/gvisor/pkg/marshal"
+	"gvisor.dev/gvisor/pkg/marshal/primitive"
 	"gvisor.dev/gvisor/pkg/syserror"
 	"gvisor.dev/gvisor/pkg/usermem"
 	"gvisor.dev/gvisor/tools/go_marshal/analysis"
@@ -513,3 +514,21 @@ func TestLimitedSliceMarshalling(t *testing.T) {
 		})
 	}
 }
+
+func TestDynamicType(t *testing.T) {
+	t12 := test.Type12Dynamic{
+		X: 32,
+		Y: []primitive.Int64{5, 6, 7},
+	}
+
+	var m marshal.Marshallable
+	m = &t12 // Ensure that all methods were generated.
+	b := make([]byte, m.SizeBytes())
+	m.MarshalBytes(b)
+
+	var res test.Type12Dynamic
+	res.UnmarshalBytes(b)
+	if !reflect.DeepEqual(t12, res) {
+		t.Errorf("dynamic type is not same after marshalling and unmarshalling: before = %+v, after = %+v", t12, res)
+	}
+}
diff --git a/tools/go_marshal/test/test.go b/tools/go_marshal/test/test.go
index e7e3ed74a..b8eb989d9 100644
--- a/tools/go_marshal/test/test.go
+++ b/tools/go_marshal/test/test.go
@@ -16,6 +16,8 @@
 package test
 
 import (
+	"gvisor.dev/gvisor/pkg/marshal/primitive"
+
 	// We're intentionally using a package name alias here even though it's not
 	// necessary to test the code generator's ability to handle package aliases.
 	ex "gvisor.dev/gvisor/tools/go_marshal/test/external"
@@ -198,3 +200,36 @@ type Type11 struct {
 	ex.External
 	y int64
 }
+
+// Type12Dynamic is a dynamically sized struct which depends on the autogenerator
+// to generate some Marshallable methods for it.
+//
+// +marshal dynamic
+type Type12Dynamic struct {
+	X primitive.Int64
+	Y []primitive.Int64
+}
+
+// SizeBytes implements marshal.Marshallable.SizeBytes.
+func (t *Type12Dynamic) SizeBytes() int {
+	return (len(t.Y) * 8) + t.X.SizeBytes()
+}
+
+// MarshalBytes implements marshal.Marshallable.MarshalBytes.
+func (t *Type12Dynamic) MarshalBytes(dst []byte) {
+	t.X.MarshalBytes(dst)
+	dst = dst[t.X.SizeBytes():]
+	for i, x := range t.Y {
+		x.MarshalBytes(dst[i*8 : (i+1)*8])
+	}
+}
+
+// UnmarshalBytes implements marshal.Marshallable.UnmarshalBytes.
+func (t *Type12Dynamic) UnmarshalBytes(src []byte) {
+	t.X.UnmarshalBytes(src)
+	for i := t.X.SizeBytes(); i < len(src); i += 8 {
+		var x primitive.Int64
+		x.UnmarshalBytes(src[i:])
+		t.Y = append(t.Y, x)
+	}
+}