// Copyright (c) 2015 Dave Collins // // Permission to use, copy, modify, and distribute this software for any // purpose with or without fee is hereby granted, provided that the above // copyright notice and this permission notice appear in all copies. // // THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES // WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF // MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR // ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES // WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN // ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF // OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. // NOTE: Due to the following build constraints, this file will only be compiled // when the code is not running on Google App Engine and "-tags disableunsafe" // is not added to the go build command line. // +build !appengine,!disableunsafe package spew import ( "reflect" "unsafe" ) const ( // UnsafeDisabled is a build-time constant which specifies whether or // not access to the unsafe package is available. UnsafeDisabled = false // ptrSize is the size of a pointer on the current arch. ptrSize = unsafe.Sizeof((*byte)(nil)) ) var ( // offsetPtr, offsetScalar, and offsetFlag are the offsets for the // internal reflect.Value fields. These values are valid before golang // commit ecccf07e7f9d which changed the format. The are also valid // after commit 82f48826c6c7 which changed the format again to mirror // the original format. Code in the init function updates these offsets // as necessary. offsetPtr = uintptr(ptrSize) offsetScalar = uintptr(0) offsetFlag = uintptr(ptrSize * 2) // flagKindWidth and flagKindShift indicate various bits that the // reflect package uses internally to track kind information. // // flagRO indicates whether or not the value field of a reflect.Value is // read-only. // // flagIndir indicates whether the value field of a reflect.Value is // the actual data or a pointer to the data. // // These values are valid before golang commit 90a7c3c86944 which // changed their positions. Code in the init function updates these // flags as necessary. flagKindWidth = uintptr(5) flagKindShift = uintptr(flagKindWidth - 1) flagRO = uintptr(1 << 0) flagIndir = uintptr(1 << 1) ) func init() { // Older versions of reflect.Value stored small integers directly in the // ptr field (which is named val in the older versions). Versions // between commits ecccf07e7f9d and 82f48826c6c7 added a new field named // scalar for this purpose which unfortunately came before the flag // field, so the offset of the flag field is different for those // versions. // // This code constructs a new reflect.Value from a known small integer // and checks if the size of the reflect.Value struct indicates it has // the scalar field. When it does, the offsets are updated accordingly. vv := reflect.ValueOf(0xf00) if unsafe.Sizeof(vv) == (ptrSize * 4) { offsetScalar = ptrSize * 2 offsetFlag = ptrSize * 3 } // Commit 90a7c3c86944 changed the flag positions such that the low // order bits are the kind. This code extracts the kind from the flags // field and ensures it's the correct type. When it's not, the flag // order has been changed to the newer format, so the flags are updated // accordingly. upf := unsafe.Pointer(uintptr(unsafe.Pointer(&vv)) + offsetFlag) upfv := *(*uintptr)(upf) flagKindMask := uintptr((1<>flagKindShift != uintptr(reflect.Int) { flagKindShift = 0 flagRO = 1 << 5 flagIndir = 1 << 6 } } // unsafeReflectValue converts the passed reflect.Value into a one that bypasses // the typical safety restrictions preventing access to unaddressable and // unexported data. It works by digging the raw pointer to the underlying // value out of the protected value and generating a new unprotected (unsafe) // reflect.Value to it. // // This allows us to check for implementations of the Stringer and error // interfaces to be used for pretty printing ordinarily unaddressable and // inaccessible values such as unexported struct fields. func unsafeReflectValue(v reflect.Value) (rv reflect.Value) { indirects := 1 vt := v.Type() upv := unsafe.Pointer(uintptr(unsafe.Pointer(&v)) + offsetPtr) rvf := *(*uintptr)(unsafe.Pointer(uintptr(unsafe.Pointer(&v)) + offsetFlag)) if rvf&flagIndir != 0 { vt = reflect.PtrTo(v.Type()) indirects++ } else if offsetScalar != 0 { // The value is in the scalar field when it's not one of the // reference types. switch vt.Kind() { case reflect.Uintptr: case reflect.Chan: case reflect.Func: case reflect.Map: case reflect.Ptr: case reflect.UnsafePointer: default: upv = unsafe.Pointer(uintptr(unsafe.Pointer(&v)) + offsetScalar) } } pv := reflect.NewAt(vt, upv) rv = pv for i := 0; i < indirects; i++ { rv = rv.Elem() } return rv }