Don't use reflection in fpu.alignedBytes.

reflect.ValueOf takes an interface{}, so when passed a slice the compiler emits
a call to runtime.convTslice to heap-allocate a copy of the slice header.

PiperOrigin-RevId: 394310052

pkg/sentry/arch/fpu/BUILD

@@ -9,6 +9,7 @@ go_library(
         "fpu_amd64.go",
         "fpu_amd64.s",
         "fpu_arm64.go",
+        "fpu_unsafe.go",
     ],
     visibility = ["//:sandbox"],
     deps = [

pkg/sentry/arch/fpu/fpu.go

@@ -17,7 +17,6 @@ package fpu
 
 import (
 	"fmt"
-	"reflect"
 )
 
 // State represents floating point state.
@@ -40,15 +39,3 @@ type ErrLoadingState struct {
 func (e ErrLoadingState) Error() string {
 	return fmt.Sprintf("floating point state contains unsupported features; supported: %#x saved: %#x", e.supportedFeatures, e.savedFeatures)
 }
-
-// alignedBytes returns a slice of size bytes, aligned in memory to the given
-// alignment. This is used because we require certain structures to be aligned
-// in a specific way (for example, the X86 floating point data).
-func alignedBytes(size, alignment uint) []byte {
-	data := make([]byte, size+alignment-1)
-	offset := uint(reflect.ValueOf(data).Index(0).Addr().Pointer() % uintptr(alignment))
-	if offset == 0 {
-		return data[:size:size]
-	}
-	return data[alignment-offset:][:size:size]
-}

pkg/sentry/arch/fpu/fpu_unsafe.go

@@ -0,0 +1,31 @@
+// Copyright 2021 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 fpu
+
+import (
+	"unsafe"
+)
+
+// alignedBytes returns a slice of size bytes, aligned in memory to the given
+// alignment. This is used because we require certain structures to be aligned
+// in a specific way (for example, the X86 floating point data).
+func alignedBytes(size, alignment uint) []byte {
+	data := make([]byte, size+alignment-1)
+	offset := uint(uintptr(unsafe.Pointer(&data[0])) % uintptr(alignment))
+	if offset == 0 {
+		return data[:size:size]
+	}
+	return data[alignment-offset:][:size:size]
+}