attempt at unsafe manipulation of chan dir

dirmod.go

package main

import (
	"fmt"
	"reflect"
	"runtime"
	"unsafe"
)

type commonType struct {
	_ uintptr
	_ uint32
	_ uint8
	_ uint8
	_ uint8
	_ uint8
	_ *struct{}
	_ *struct{}
	_ *struct{}
	_ *struct{}
}

func (t *commonType) runtimeType() *runtime.Type {
	// The runtime.Type always precedes the commonType in memory.
	// Adjust pointer to find it.
	var rt struct {
		i  runtime.Type
		ct commonType
	}
	return (*runtime.Type)(unsafe.Pointer(uintptr(unsafe.Pointer(t)) - unsafe.Offsetof(rt.ct)))
}

func dirPointer(c interface{}) *uintptr {
	v := reflect.ValueOf(c)
	t := v.Type()
	f := (*commonType)(unsafe.Pointer(reflect.ValueOf(t).Pointer()))
	type chanType struct {
		_    commonType `reflect:"chan"`
		elem *runtime.Type
		dir  uintptr
	}
	r := (*chanType)(unsafe.Pointer(reflect.ValueOf(*f.runtimeType()).Pointer()))
	return &r.dir
}

func main() {
	var a <-chan int
	a = make(chan int)
	d := dirPointer(a)
	fmt.Printf("chanType.dir: %v\n", reflect.ChanDir(*d))
	*d = uintptr(reflect.BothDir) // throws unexpected fault address
}