cmd/cgo: ptr type casts of struct fields cause the whole struct to be checked #32970
Description
What version of Go are you using (go version)?
$ go version go version go1.12.6 linux/amd64
$ go version go version devel +13327f219e Sat Jul 6 13:25:59 2019 +0000 linux/amd64
Does this issue reproduce with the latest release?
Yes. Tried also several older versions back to 1.10
What operating system and processor architecture are you using (go env)?
go env Output
$ go env GOARCH="amd64" GOBIN="" GOCACHE="/home/hidden/.cache/go-build" GOEXE="" GOFLAGS="" GOHOSTARCH="amd64" GOHOSTOS="linux" GOOS="linux" GOPATH="/home/hidden/go" GOPROXY="" GORACE="" GOROOT="/usr/lib/go" GOTMPDIR="" GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64" GCCGO="gccgo" CC="gcc" CXX="g++" CGO_ENABLED="1" GOMOD="" CGO_CFLAGS="-g -O2" CGO_CPPFLAGS="" CGO_CXXFLAGS="-g -O2" CGO_FFLAGS="-g -O2" CGO_LDFLAGS="-g -O2" PKG_CONFIG="pkg-config" GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build181091171=/tmp/go-build -gno-record-gcc-switches"
What did you do?
package main
/*
struct I {
int i;
int *p;
};
int f(struct I *x) {
return x->i;
}
*/
import "C"
import (
"fmt"
)
type x struct {
p *int
i C.struct_I
}
func main() {
t1 := new(x)
t1.p = new(int)
t1.i.i = 2
fmt.Println("1:", C.f(&t1.i)) // <- this works
fmt.Println("2:", C.f((*C.struct_I)(&t1.i))) // <- this doesn't work
}What did you expect to see?
1: 2
2: 2
What did you see instead?
1: 2
panic: runtime error: cgo argument has Go pointer to Go pointer
goroutine 1 [running]:
main.main.func2(0xc000090020, 0xc00009a008)
/home/notti/projects/cgocheck/main.go:30 +0x53
main.main()
/home/notti/projects/cgocheck/main.go:30 +0x114
Why does this happen?
checkAddr in src/cmd/cgo/gcc.go doesn't recognize type conversions containing parenthesis, which are required for pointers (see https://golang.org/ref/spec#Conversions), and, therefore emits a _cgoCheckPointer without true as second argument, causing the check to look at the whole struct instead of just the element. Looks like pointer type conversions should be supported since ast.StarExpr is handled in isType.
This is the cause for google/gopacket#664
Possible fix: https://go-review.googlesource.com/c/go/+/185098
(updated with better demonstration program)