cmd/link: prefix Go syms with _ on darwin

RELNOTE=This change adds an underscore to all Go symbols in darwin, and
the behavior might be confusing to users of tools like "nm", etc.

Fixes #33808

Change-Id: I1849e6618c81215cb9bfa62b678f6f389cd009d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/196217
Run-TryBot: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/link/internal/ld/issue33808_test.go

+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ld
+
+import (
+	"internal/testenv"
+	"io/ioutil"
+	"os"
+	"runtime"
+	"strings"
+	"testing"
+)
+
+const prog = `
+package main
+
+import "log"
+
+func main() {
+	log.Fatalf("HERE")
+}
+`
+
+func TestIssue33808(t *testing.T) {
+	if runtime.GOOS != "darwin" {
+		return
+	}
+	testenv.MustHaveGoBuild(t)
+	testenv.MustHaveCGO(t)
+
+	dir, err := ioutil.TempDir("", "TestIssue33808")
+	if err != nil {
+		t.Fatalf("could not create directory: %v", err)
+	}
+	defer os.RemoveAll(dir)
+
+	f := gobuild(t, dir, prog, "-ldflags=-linkmode=external")
+	f.Close()
+
+	syms, err := f.Symbols()
+	if err != nil {
+		t.Fatalf("Error reading symbols: %v", err)
+	}
+
+	name := "log.Fatalf"
+	for _, sym := range syms {
+		if strings.Contains(sym.Name, name) {
+			return
+		}
+	}
+	t.Fatalf("Didn't find %v", name)
+}

src/cmd/link/internal/ld/macho.go

@@ -869,6 +869,7 @@ func machosymtab(ctxt *Link) {
 		symtab.AddUint32(ctxt.Arch, uint32(symstr.Size))
 
 		export := machoShouldExport(ctxt, s)
+		isGoSymbol := strings.Contains(s.Extname(), ".")
 
 		// In normal buildmodes, only add _ to C symbols, as
 		// Go symbols have dot in the name.
@@ -877,8 +878,8 @@ func machosymtab(ctxt *Link) {
 		// symbols like crosscall2 are in pclntab and end up
 		// pointing at the host binary, breaking unwinding.
 		// See Issue #18190.
-		cexport := !strings.Contains(s.Extname(), ".") && (ctxt.BuildMode != BuildModePlugin || onlycsymbol(s))
-		if cexport || export {
+		cexport := !isGoSymbol && (ctxt.BuildMode != BuildModePlugin || onlycsymbol(s))
+		if cexport || export || isGoSymbol {
 			symstr.AddUint8('_')
 		}
 

src/debug/macho/file.go

@@ -473,7 +473,12 @@ func (f *File) parseSymtab(symdat, strtab, cmddat []byte, hdr *SymtabCmd, offset
 		if n.Name >= uint32(len(strtab)) {
 			return nil, &FormatError{offset, "invalid name in symbol table", n.Name}
 		}
-		sym.Name = cstring(strtab[n.Name:])
+		// We add "_" to Go symbols. Strip it here. See issue 33808.
+		name := cstring(strtab[n.Name:])
+		if strings.Contains(name, ".") && name[0] == '_' {
+			name = name[1:]
+		}
+		sym.Name = name
 		sym.Type = n.Type
 		sym.Sect = n.Sect
 		sym.Desc = n.Desc