Repository: go
Issue: runtime: libunwind is unable to unwind CGo to Go's stack
Labels: NeedsInvestigation,compiler/runtime

### What version of Go are you using (`go version`)?

`master` as of the build
<pre>
$ go version
devel +dd150176c3 Fri Jul 3 03:31:29 2020 +0000
</pre>

### Does this issue reproduce with the latest release?

Yes

### What operating system and processor architecture are you using (`go env`)?

<details><summary><code>go env</code> Output</summary><br><pre>
$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/steeve/Library/Caches/go-build"
GOENV="/Users/steeve/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/steeve/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/steeve/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/Users/steeve/code/github.com/znly/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/Users/steeve/code/github.com/znly/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/steeve/code/github.com/znly/go/src/go.mod"
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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/bs/51dlb_nn5k35xq9qfsxv9wc00000gr/T/go-build842228435=/tmp/go-build -gno-record-gcc-switches -fno-common"
</pre></details>

### What did you do?

Following @cherrymui's comment on #39524, I figured I tried to check why lots of our backtraces on iOS stop at `runtime.asmcgocall`.
Since I wanted to reproduce it on my computer and `lldb` manges to properly backtrace, I figured I'd give `libunwind` a try, since this is was iOS uses when a program crashes.

Unfortunately `libunwind` didn't manage to walk the stack past CGo generated `_Cfunc_` functions.

Given this program:
```go
package main

/*
#cgo CFLAGS: -O0

#include <libunwind.h>
#include <stdio.h>

void backtrace() {
	unw_cursor_t cursor;
	unw_context_t context;

	// Initialize cursor to current frame for local unwinding.
	unw_getcontext(&context);
	unw_init_local(&cursor, &context);

	// Unwind frames one by one, going up the frame stack.
	while (unw_step(&cursor) > 0) {
		unw_word_t offset, pc;
		unw_get_reg(&cursor, UNW_REG_IP, &pc);
		if (pc == 0) {
			break;
		}
	    printf("0x%llx:", pc);
		char sym[256];
		if (unw_get_proc_name(&cursor, sym, sizeof(sym), &offset) == 0) {
			printf(" (%s+0x%llx)\n", sym, offset);
		} else {
			printf(" -- error: unable to obtain symbol name for this frame\n");
		}
  	}
}

void two() {
	printf("two\n");
	backtrace();
}

void one() {
	printf("one\n");
	two();
}
*/
import "C"

//go:noinline
func goone() {
	C.one()
}

func main() {
	goone()
}
```

It prints:
```
one1
two2
0x40617fe: (two+0x1e)
0x406182e: (one+0x1e)
0x406168b: (_cgo_7c45d1c2feef_Cfunc_one+0x1b)
```

I tried doing Go(1) -> C(1) -> Go(2) -> C(2) and backtrace, and it only unwinds C(2).

Also, I tried to make set `asmcgocall` to have a 16 bytes stack, hoping that the generated frame pointer would help, but it didn't.

### What did you expect to see?

The complete backtrace.

### What did you see instead?

A backtrace for C functions only.