cmd/cgo: false cgo "Go pointer to Go pointer"

[Cubixmeister]

Cgo panics if C function is called with pointer to []byte assigned to interface{} variable.
I've tested it with simple function returning []byte in same package, but error does not occur.

package main

/*
static void _test_ptr(void *p) {}
*/
import "C"

import (
    "encoding/json"
    "fmt"
    "unsafe"
)

func main() {
    var err error
    var val interface{}
    val, err = json.Marshal(struct{ A, B int }{1, 2})
    if err != nil {
        fmt.Println("marshal error:", err)
        return
    }

    // crashes
    var b = val.([]byte)
    var p *byte
    p = &b[0]
    C._test_ptr(unsafe.Pointer(p))

    // works
    var copied = append([]byte(nil), b...)
    p = &copied[0]
    C._test_ptr(unsafe.Pointer(p))
}

run:

$ go run crashtest.go 
panic: runtime error: cgo argument has Go pointer to Go pointer

goroutine 1 [running]:
panic(0x40af460, 0xc4200782c0)
    /usr/local/Cellar/go/1.7.4/libexec/src/runtime/panic.go:500 +0x1a1
main._cgoCheckPointer0(0xc42009e024, 0x0, 0x0, 0x0, 0xc42009c120)
    command-line-arguments/_obj/_cgo_gotypes.go:35 +0x59
main.main()
    /tmp/crashtest.go:28 +0x280
exit status 2

env:

$ go version
go version go1.7.4 darwin/amd64

$ go env
GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/cubix/Developing/godaemon/"
GORACE=""
GOROOT="/usr/local/Cellar/go/1.7.4/libexec"
GOTOOLDIR="/usr/local/Cellar/go/1.7.4/libexec/pkg/tool/darwin_amd64"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/5w/7t9m60w12390mjlr63qvxg1h0000gn/T/go-build441330328=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"

[ianlancetaylor]

This is essentially a duplicate of #14210. The problem is that json.Marshal returns a slice that points into scratch space stored in a larger struct (json.encodeState). When you set p = &b[0] in a statement separate from the call to the cgo function, the cgo command is not smart enough to see that you are passing the address of a slice element to the C function. Since it doesn't see that, it assumes that the argument points to the entire heap block, and the entire heap block does contain a pointer.

A simple workaround until cgo is fixed is to change

C._test_ptr(unsafe.Pointer(p))

to

C._test_ptr(unsafe.Pointer(&b[0]))

That will let cgo see that you are passing the address of a slice element and that it should only check the slice, not the entire heap block.

Closing as dup of #14210.