cmd/cgo: cgo pointer checking issue with struct fields

[egonelbre]

Issue: go 1.10.2 and current go tip (b7d9e6e).
Machine: darwin/amd64

When you use a pointer inside to another struct with pointers when calling C then CGO check fails with "panic: runtime error: cgo argument has Go pointer to Go pointer".

Possibly related to #14210

Code to reproduce:

package main

// void exampleCall(void *p) {}
import "C"
import "unsafe"

type Data struct {
	Map          map[int32]int32
	InternalData InternalData
}
type InternalData struct{ X, Y, Z float32 }

func main() {
	data := &Data{}
	data.Map = make(map[int32]int32)
	C.exampleCall(unsafe.Pointer(&data.InternalData))
}

[andybons]

/cc @ianlancetaylor

[ianlancetaylor]

This is a bug that should be fixed some day. The problem is that the conversion to unsafe.Pointer breaks the cgo pointer check. It no longer knows the type of the value, so it examines the entire memory area.

[egonelbre]

I was examining / thinking how to fix it. What if it were to unwrap unsafe.Pointer part for the cgoCheckPointer?

So, instead of generating:

func (_cgo0 unsafe.Pointer) {
    _cgoCheckPointer(_cgo0)
    C.f(_cgo0)
}(unsafe.Pointer(p))

It would generate:

func (_cgo0 ptype) {
    _cgoCheckPointer(_cgo0)
    C.f(unsafe.Pointer(_cgo0))
}(p)

The same goes for the needsPointerCheck - i.e. ignore the outermost cast.

Thoughts?

[egonelbre]

Probably only in cases when ptype itself is a pointer. Because it still would need to check unsafe.Pointer(uintptr(p)) and similar.

[gopherbot]

Change https://golang.org/cl/119976 mentions this issue: cmd/cgo: avoid losing some typeinfo with cgoCheckPointer

[gopherbot]

Change https://golang.org/cl/120615 mentions this issue: cmd/cgo: rewrite pointer checking

[gopherbot]

Change https://golang.org/cl/142884 mentions this issue: cmd/cgo: rewrite pointer checking to use more function literals