go_marshal: Implement automatic generation of ABI marshalling code.

This CL implements go_marshal, a code generation utility for
automatically serializing and deserializing ABI structs.

The go_marshal tool automatically generates implementations of the new
marshal interface. Unlike binary.Marshal/Unmarshal, the generated
interface implementations use no runtime reflection, and translates to
a single memcpy for most structs. See go_marshal/README.md for
details.

PiperOrigin-RevId: 268065475

2 files changed, 188 insertions, 0 deletions

diff --git a/tools/go_marshal/analysis/BUILD b/tools/go_marshal/analysis/BUILD
new file mode 100644
index 000000000..c859ced77
--- /dev/null
+++ b/tools/go_marshal/analysis/BUILD
@@ -0,0 +1,13 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
+
+package(licenses = ["notice"])
+
+go_library(
+    name = "analysis",
+    testonly = 1,
+    srcs = ["analysis_unsafe.go"],
+    importpath = "gvisor.dev/gvisor/tools/go_marshal/analysis",
+    visibility = [
+        "//:sandbox",
+    ],
+)
diff --git a/tools/go_marshal/analysis/analysis_unsafe.go b/tools/go_marshal/analysis/analysis_unsafe.go
new file mode 100644
index 000000000..9a9a4f298
--- /dev/null
+++ b/tools/go_marshal/analysis/analysis_unsafe.go
@@ -0,0 +1,175 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package analysis implements common functionality used by generated
+// go_marshal tests.
+package analysis
+
+// All functions in this package are unsafe and are not intended for general
+// consumption. They contain sharp edge cases and the caller is responsible for
+// ensuring none of them are hit. Callers must be carefully to pass in only sane
+// arguments. Failure to do so may cause panics at best and arbitrary memory
+// corruption at worst.
+//
+// Never use outside of tests.
+
+import (
+	"fmt"
+	"math/rand"
+	"reflect"
+	"testing"
+	"unsafe"
+)
+
+// RandomizeValue assigns random value(s) to an abitrary type. This is intended
+// for used with ABI structs from go_marshal, meaning the typical restrictions
+// apply (fixed-size types, no pointers, maps, channels, etc), and should only
+// be used on zeroed values to avoid overwriting pointers to active go objects.
+//
+// Internally, we populate the type with random data by doing an unsafe cast to
+// access the underlying memory of the type and filling it as if it were a byte
+// slice. This almost gets us what we want, but padding fields named "_" are
+// normally not accessible, so we walk the type and recursively zero all "_"
+// fields.
+//
+// Precondition: x must be a pointer. x must not contain any valid
+// pointers to active go objects (pointer fields aren't allowed in ABI
+// structs anyways), or we'd be violating the go runtime contract and
+// the GC may malfunction.
+func RandomizeValue(x interface{}) {
+	v := reflect.Indirect(reflect.ValueOf(x))
+	if !v.CanSet() {
+		panic("RandomizeType() called with an unaddressable value. You probably need to pass a pointer to the argument")
+	}
+
+	// Cast the underlying memory for the type into a byte slice.
+	var b []byte
+	hdr := (*reflect.SliceHeader)(unsafe.Pointer(&b))
+	// Note: v.UnsafeAddr panics if x is passed by value. x should be a pointer.
+	hdr.Data = v.UnsafeAddr()
+	hdr.Len = int(v.Type().Size())
+	hdr.Cap = hdr.Len
+
+	// Fill the byte slice with random data, which in effect fills the type with
+	// random values.
+	n, err := rand.Read(b)
+	if err != nil || n != len(b) {
+		panic("unreachable")
+	}
+
+	// Normally, padding fields are not accessible, so zero them out.
+	reflectZeroPaddingFields(v.Type(), b, false)
+}
+
+// reflectZeroPaddingFields assigns zero values to padding fields for the value
+// of type r, represented by the memory in data. Padding fields are defined as
+// fields with the name "_". If zero is true, the immediate value itself is
+// zeroed. In addition, the type is recursively scanned for padding fields in
+// inner types.
+//
+// This is used for zeroing padding fields after calling RandomizeValue.
+func reflectZeroPaddingFields(r reflect.Type, data []byte, zero bool) {
+	if zero {
+		for i, _ := range data {
+			data[i] = 0
+		}
+	}
+	switch r.Kind() {
+	case reflect.Int8, reflect.Uint8, reflect.Int16, reflect.Uint16, reflect.Int32, reflect.Uint32, reflect.Int64, reflect.Uint64:
+		// These types are explicitly allowed in an ABI type, but we don't need
+		// to recurse further as they're scalar types.
+	case reflect.Struct:
+		for i, numFields := 0, r.NumField(); i < numFields; i++ {
+			f := r.Field(i)
+			off := f.Offset
+			len := f.Type.Size()
+			window := data[off : off+len]
+			reflectZeroPaddingFields(f.Type, window, f.Name == "_")
+		}
+	case reflect.Array:
+		eLen := int(r.Elem().Size())
+		if int(r.Size()) != eLen*r.Len() {
+			panic("Array has unexpected size?")
+		}
+		for i, n := 0, r.Len(); i < n; i++ {
+			reflectZeroPaddingFields(r.Elem(), data[i*eLen:(i+1)*eLen], false)
+		}
+	default:
+		panic(fmt.Sprintf("Type %v not allowed in ABI struct", r.Kind()))
+
+	}
+}
+
+// AlignmentCheck ensures the definition of the type represented by typ doesn't
+// cause the go compiler to emit implicit padding between elements of the type
+// (i.e. fields in a struct).
+//
+// AlignmentCheck doesn't explicitly recurse for embedded structs because any
+// struct present in an ABI struct must also be Marshallable, and therefore
+// they're aligned by definition (or their alignment check would have failed).
+func AlignmentCheck(t *testing.T, typ reflect.Type) (ok bool, delta uint64) {
+	switch typ.Kind() {
+	case reflect.Int8, reflect.Uint8, reflect.Int16, reflect.Uint16, reflect.Int32, reflect.Uint32, reflect.Int64, reflect.Uint64:
+		// Primitive types are always considered well aligned. Primitive types
+		// that are fields in structs are checked independently, this branch
+		// exists to handle recursive calls to alignmentCheck.
+	case reflect.Struct:
+		xOff := 0
+		nextXOff := 0
+		skipNext := false
+		for i, numFields := 0, typ.NumField(); i < numFields; i++ {
+			xOff = nextXOff
+			f := typ.Field(i)
+			fmt.Printf("Checking alignment of %s.%s @ %d [+%d]...\n", typ.Name(), f.Name, f.Offset, f.Type.Size())
+			nextXOff = int(f.Offset + f.Type.Size())
+
+			if f.Name == "_" {
+				// Padding fields need not be aligned.
+				fmt.Printf("Padding field of type %v\n", f.Type)
+				continue
+			}
+
+			if tag, ok := f.Tag.Lookup("marshal"); ok && tag == "unaligned" {
+				skipNext = true
+				continue
+			}
+
+			if skipNext {
+				skipNext = false
+				fmt.Printf("Skipping alignment check for field %s.%s explicitly marked as unaligned.\n", typ.Name(), f.Name)
+				continue
+			}
+
+			if xOff != int(f.Offset) {
+				implicitPad := int(f.Offset) - xOff
+				t.Fatalf("Suspect offset for field %s.%s, detected an implicit %d byte padding from offset %d to %d; either add %d bytes of explicit padding before this field or tag it as `marshal:\"unaligned\"`.", typ.Name(), f.Name, implicitPad, xOff, f.Offset, implicitPad)
+			}
+		}
+
+		// Ensure structs end on a byte explicitly defined by the type.
+		if typ.NumField() > 0 && nextXOff != int(typ.Size()) {
+			implicitPad := int(typ.Size()) - nextXOff
+			f := typ.Field(typ.NumField() - 1) // Final field
+			t.Fatalf("Suspect offset for field %s.%s at the end of %s, detected an implicit %d byte padding from offset %d to %d at the end of the struct; either add %d bytes of explict padding at end of the struct or tag the final field %s as `marshal:\"unaligned\"`.",
+				typ.Name(), f.Name, typ.Name(), implicitPad, nextXOff, typ.Size(), implicitPad, f.Name)
+		}
+	case reflect.Array:
+		// Independent arrays are also always considered well aligned. We only
+		// need to worry about their alignment when they're embedded in structs,
+		// which we handle above.
+	default:
+		t.Fatalf("Unsupported type in ABI struct while checking for field alignment for type: %v", typ.Kind())
+	}
+	return true, uint64(typ.Size())
+}