From 387501219e87701a6adeb8417542927ee39b9950 Mon Sep 17 00:00:00 2001
From: Rahat Mahmood <rahat@google.com>
Date: Tue, 29 Sep 2020 18:06:19 -0700
Subject: Replace remaining uses of reflection-based marshalling.

- Rewrite arch.Stack.{Push,Pop}. For the most part, stack now
  implements marshal.CopyContext and can be used as the target of
  marshal operations. Stack.Push had some extra logic for
  automatically null-terminating slices. This was only used for two
  specific types of slices, and is now handled explicitly.

- Delete usermem.CopyObject{In,Out}.

- Replace most remaining uses of the encoding/binary package with
  go-marshal. Most of these were using the binary package to compute
  the size of a struct, which go-marshal can directly replace. ~3 uses
  of the binary package remain. These aren't reasonably replaceable by
  go-marshal: for example one use is to construct the syscall
  trampoline for systrap.

- Fill out remaining convenience wrappers in the primitive package.

PiperOrigin-RevId: 334502375
---
 pkg/sentry/arch/stack_unsafe.go | 69 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 69 insertions(+)
 create mode 100644 pkg/sentry/arch/stack_unsafe.go

(limited to 'pkg/sentry/arch/stack_unsafe.go')

diff --git a/pkg/sentry/arch/stack_unsafe.go b/pkg/sentry/arch/stack_unsafe.go
new file mode 100644
index 000000000..a90d297ee
--- /dev/null
+++ b/pkg/sentry/arch/stack_unsafe.go
@@ -0,0 +1,69 @@
+// Copyright 2020 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 arch
+
+import (
+	"reflect"
+	"runtime"
+	"unsafe"
+
+	"gvisor.dev/gvisor/pkg/marshal/primitive"
+	"gvisor.dev/gvisor/pkg/usermem"
+)
+
+// pushAddrSliceAndTerminator copies a slices of addresses to the stack, and
+// also pushes an extra null address element at the end of the slice.
+//
+// Internally, we unsafely transmute the slice type from the arch-dependent
+// []usermem.Addr type, to a slice of fixed-sized ints so that we can pass it to
+// go-marshal.
+//
+// On error, the contents of the stack and the bottom cursor are undefined.
+func (s *Stack) pushAddrSliceAndTerminator(src []usermem.Addr) (int, error) {
+	// Note: Stack grows upwards, so push the terminator first.
+	srcHdr := (*reflect.SliceHeader)(unsafe.Pointer(&src))
+	switch s.Arch.Width() {
+	case 8:
+		nNull, err := primitive.CopyUint64Out(s, StackBottomMagic, 0)
+		if err != nil {
+			return 0, err
+		}
+		var dst []uint64
+		dstHdr := (*reflect.SliceHeader)(unsafe.Pointer(&dst))
+		dstHdr.Data = srcHdr.Data
+		dstHdr.Len = srcHdr.Len
+		dstHdr.Cap = srcHdr.Cap
+		n, err := primitive.CopyUint64SliceOut(s, StackBottomMagic, dst)
+		// Ensures src doesn't get GCed until we're done using it through dst.
+		runtime.KeepAlive(src)
+		return n + nNull, err
+	case 4:
+		nNull, err := primitive.CopyUint32Out(s, StackBottomMagic, 0)
+		if err != nil {
+			return 0, err
+		}
+		var dst []uint32
+		dstHdr := (*reflect.SliceHeader)(unsafe.Pointer(&dst))
+		dstHdr.Data = srcHdr.Data
+		dstHdr.Len = srcHdr.Len
+		dstHdr.Cap = srcHdr.Cap
+		n, err := primitive.CopyUint32SliceOut(s, StackBottomMagic, dst)
+		// Ensure src doesn't get GCed until we're done using it through dst.
+		runtime.KeepAlive(src)
+		return n + nNull, err
+	default:
+		panic("Unsupported arch width")
+	}
+}
-- 
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