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cmd/cgo: pointer to incomplete C type is mangled when passed through interface type and generic type assert #51733

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kbolino opened this issue Mar 16, 2022 · 4 comments
Closed

cmd/cgo: pointer to incomplete C type is mangled when passed through interface type and generic type assert #51733

kbolino opened this issue Mar 16, 2022 · 4 comments
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FrozenDueToAge NeedsFix The path to resolution is known, but the work has not been done.
Milestone
Go1.19

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@kbolino
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kbolino commented Mar 16, 2022

What version of Go are you using (go version)?

$ go version
go version go1.18 darwin/arm64

Does this issue reproduce with the latest release?

This is the latest release.

What operating system and processor architecture are you using (go env)?

go env Output
$ go env
GO111MODULE=""
GOARCH="arm64"
GOBIN=""
GOCACHE="/Users/kbolino/Library/Caches/go-build"
GOENV="/Users/kbolino/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="arm64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/kbolino/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/kbolino/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_arm64"
GOVCS=""
GOVERSION="go1.18"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/kbolino/Documents/GoModules/cgo-generics/go.mod"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/59/hjg6jvkd71s8djph6yqbdn0h0000gn/T/go-build78106070=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

package main

// #include <stdlib.h>
// struct incomplete_type;
import "C"

import (
	"fmt"
	"unsafe"
)

func main() {
	someMem := C.malloc(16)
	defer C.free(someMem)
	ptrIn := (*C.struct_incomplete_type)(someMem)
	fmt.Printf("ptrIn  = %016x\n", uintptr(unsafe.Pointer(ptrIn)))
	var iface interface{} = ptrIn
	ptrOut := recoverPointer[C.struct_incomplete_type](iface)
	fmt.Printf("ptrOut = %016x\n", uintptr(unsafe.Pointer(ptrOut)))	
}

func recoverPointer[T any](val interface{}) *T {
	return val.(*T)
}

This is a minimal example; the original use case involved cgo.Handle and passing pointers across the Go/C boundary.

What did you expect to see?

ptrIn  = <some_val>
ptrOut = <some_val>

A compilation error or panic would also have sufficed.

What did you see instead?

ptrIn  = <some_val>
ptrOut = <some_other_val>

for example,

ptrIn  = 0000600000dbc010
ptrOut = 0000014000059f30
@kbolino kbolino changed the title cmd/cgo: Pointer to incomplete C type is mangled when passed through interface type and generic type assert cmd/cgo: pointer to incomplete C type is mangled when passed through interface type and generic type assert Mar 16, 2022
@ianlancetaylor
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Here is a standalone example that doesn't use cgo. This appears to be a bug in the way that generic code handles type assertions, or notinheap pointers, or something. CC @randall77 .

package main

import (
	"log"
	"unsafe"
)

//go:notinheap
type S struct{}

func main() {
	p := (*S)(unsafe.Pointer(uintptr(0x8000)))
	var v any = p
	p2 := v.(*S)
	if p != p2 {
		log.Fatalf("%p != %p", unsafe.Pointer(p), unsafe.Pointer(p2))
	}
	p2 = typeAssert[*S](v)
	if p != p2 {
		log.Fatalf("%p != %p from typeAssert", unsafe.Pointer(p), unsafe.Pointer(p2))
	}
}

func typeAssert[T any](v any) T {
	return v.(T)
}

@ianlancetaylor
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@gopherbot Please open backport to 1.18

This appears to be invalid code generation, so we should backport a fix when we have one.

@gopherbot
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Backport issue(s) opened: #51741 (for 1.18).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.

@gopherbot gopherbot mentioned this issue Mar 17, 2022
Closed
@heschi heschi added the NeedsFix The path to resolution is known, but the work has not been done. label Mar 17, 2022
@heschi heschi added this to the Go1.19 milestone Mar 17, 2022
@gopherbot
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Change https://go.dev/cl/393895 mentions this issue: cmd/compile: pointers to notinheap types need their own shape

@golang golang locked and limited conversation to collaborators Mar 18, 2023
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FrozenDueToAge NeedsFix The path to resolution is known, but the work has not been done.
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Milestone
Go1.19
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@kbolino @ianlancetaylor @gopherbot @heschi