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cmd/cgo: sometimes when passing a struct to C containing a go pointer to a struct field, the entire go struct is checked for go pointers instead of just the field #63460

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AlexRouSg opened this issue Oct 9, 2023 · 5 comments
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cmd/cgo: sometimes when passing a struct to C containing a go pointer to a struct field, the entire go struct is checked for go pointers instead of just the field #63460

AlexRouSg opened this issue Oct 9, 2023 · 5 comments
Labels
compiler/runtime Issues related to the Go compiler and/or runtime. FrozenDueToAge NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one.
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Backlog

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

$ go version
go version go1.21.2 windows/amd64

Does this issue reproduce with the latest release?

yes

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

go env Output
$ go env
set GO111MODULE=
set GOARCH=amd64
set GOBIN=
set GOCACHE=~\AppData\Local\go-build
set GOENV=~\AppData\Roaming\go\env
set GOEXE=.exe
set GOEXPERIMENT=
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=~\go\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=~\go
set GOPRIVATE=
set GOPROXY=https://proxy.golang.org,direct
set GOROOT=C:\Program Files\Go
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLCHAIN=auto
set GOTOOLDIR=C:\Program Files\Go\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=go1.21.2
set GCCGO=gccgo
set GOAMD64=v1
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=
set GOWORK=
set CGO_CFLAGS=-O2 -g
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-O2 -g
set CGO_FFLAGS=-O2 -g
set CGO_LDFLAGS=-O2 -g
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fno-caret-diagnostics -Qunused-arguments -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=C:\Users\alexr\AppData\Local\Temp\go-build886793640=/tmp/go-build -gno-record-gcc-switches

What did you do?

package main

/*
 typedef struct foo_t *foo;

 typedef struct {
	foo* pfoo;
 }info;

 void test(info i) {}
*/
import "C"

type foo struct {
	cFoo C.foo
	bar  []interface{}
}

func main() {
	f := foo{bar: []interface{}{0}}
	info := C.info{&f.cFoo}
	C.test(info)
}

What did you expect to see?

Code compiles and runs with no output

What did you see instead?

panic: runtime error: cgo argument has Go pointer to unpinned Go pointer

Notes:
Not setting the field bar: []interface{}{0} or changing function call to look like C.test(&f.cFoo) does not trigger this
@seankhliao seankhliao changed the title runtime: cgo: sometimes when passing a struct to C containing a go pointer to a struct field, the entire go struct is checked for go pointers instead of just the field runtime/cgo: sometimes when passing a struct to C containing a go pointer to a struct field, the entire go struct is checked for go pointers instead of just the field Oct 9, 2023
@dmitshur dmitshur added NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. compiler/runtime Issues related to the Go compiler and/or runtime. labels Oct 9, 2023
@dmitshur dmitshur added this to the Backlog milestone Oct 9, 2023
@dmitshur
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dmitshur commented Oct 9, 2023

CC @golang/runtime.

@ianlancetaylor
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This is similar to #14210. The cgo tool is not smart enough to pull apart what you are doing in your Go code. It loses the &f.cFoo and just checks the entire allocation to which the pointer points.

It would be nice to fix if somebody can figure out how. But nobody is working on this.

@ianlancetaylor ianlancetaylor changed the title runtime/cgo: sometimes when passing a struct to C containing a go pointer to a struct field, the entire go struct is checked for go pointers instead of just the field cmd/cgo: sometimes when passing a struct to C containing a go pointer to a struct field, the entire go struct is checked for go pointers instead of just the field Oct 9, 2023
@AlexRouSg
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That's fair and I have no expectation/need of this getting fixed anytime soon since the workaround is just to add a c := f.cFoo
However, would it be a good idea to at a minimum point this out in the doc as a known limitation that may or may not get fixed in the future?

@bcmills
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bcmills commented Oct 13, 2023

Is this a duplicate of #28606?

@ianlancetaylor
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Thanks, I thought there was an open issue but I couldn't find it. Marking as a dup since it's essentially the same problem.

@ianlancetaylor ianlancetaylor closed this as not planned Won't fix, can't repro, duplicate, stale Oct 13, 2023
@golang golang locked and limited conversation to collaborators Oct 12, 2024
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compiler/runtime Issues related to the Go compiler and/or runtime. FrozenDueToAge NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one.
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@dmitshur @ianlancetaylor @bcmills @AlexRouSg @gopherbot