1 // Copyright 2020 The gVisor Authors.
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
6 //
7 // http://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
14 15 package state
16 17 import (
18 "fmt"
19 "reflect"
20 "runtime"
21 "unsafe"
22 )
23 24 // reflectValueRWAddr is equivalent to obj.Addr(), except that the returned
25 // reflect.Value is usable in assignments even if obj was obtained by the use
26 // of unexported struct fields.
27 //
28 // Preconditions: obj.CanAddr().
29 func reflectValueRWAddr(obj reflect.Value) reflect.Value {
30 return reflect.NewAt(obj.Type(), unsafe.Pointer(obj.UnsafeAddr()))
31 }
32 33 // reflectValueRWSlice3 is equivalent to arr.Slice3(i, j, k), except that the
34 // returned reflect.Value is usable in assignments even if obj was obtained by
35 // the use of unexported struct fields.
36 //
37 // Preconditions:
38 // - arr.Kind() == reflect.Array.
39 // - i, j, k >= 0.
40 // - i <= j <= k <= arr.Len().
41 func reflectValueRWSlice3(arr reflect.Value, i, j, k int) reflect.Value {
42 if arr.Kind() != reflect.Array {
43 panic(fmt.Sprintf("arr has kind %v, wanted %v", arr.Kind(), reflect.Array))
44 }
45 if i < 0 || j < 0 || k < 0 {
46 panic(fmt.Sprintf("negative subscripts (%d, %d, %d)", i, j, k))
47 }
48 if i > j {
49 panic(fmt.Sprintf("subscript i (%d) > j (%d)", i, j))
50 }
51 if j > k {
52 panic(fmt.Sprintf("subscript j (%d) > k (%d)", j, k))
53 }
54 if k > arr.Len() {
55 panic(fmt.Sprintf("subscript k (%d) > array length (%d)", k, arr.Len()))
56 }
57 58 sliceTyp := reflect.SliceOf(arr.Type().Elem())
59 if i == arr.Len() {
60 // By precondition, i == j == k == arr.Len().
61 return reflect.MakeSlice(sliceTyp, 0, 0)
62 }
63 slh := reflect.SliceHeader{
64 // reflect.Value.CanAddr() == false for arrays, so we need to get the
65 // address from the first element of the array.
66 Data: arr.Index(i).UnsafeAddr(),
67 Len: j - i,
68 Cap: k - i,
69 }
70 slobj := reflect.NewAt(sliceTyp, unsafe.Pointer(&slh)).Elem()
71 // Before slobj is constructed, arr holds the only pointer-typed pointer to
72 // the array since reflect.SliceHeader.Data is a uintptr, so arr must be
73 // kept alive.
74 runtime.KeepAlive(arr)
75 return slobj
76 }
77