cmd/link, runtime: put hasmain bit in moduledata

Currently we look to see if the main.main symbol address is in the
module data text range. This requires access to the main.main
symbol, which usually the runtime has, but does not when building
a plugin.

To avoid a dynamic relocation to main.main (which I haven't worked
out how to have the linker generate on darwin), stop using the
symbol. Instead record a boolean in the moduledata if the module
has the main function.

Fixes #22175

Change-Id: If313a118f17ab499d0a760bbc2519771ed654530
Reviewed-on: https://go-review.googlesource.com/69370
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/testplugin/src/issue22175/main.go

@@ -0,0 +1,28 @@
+// Copyright 2017 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 main
+
+import (
+	"fmt"
+	"os"
+	"plugin"
+)
+
+func main() {
+	p2, err := plugin.Open("issue22175_plugin1.so")
+	if err != nil {
+		panic(err)
+	}
+	f, err := p2.Lookup("F")
+	if err != nil {
+		panic(err)
+	}
+	got := f.(func() int)()
+	const want = 971
+	if got != want {
+		fmt.Fprintf(os.Stderr, "issue22175: F()=%d, want %d", got, want)
+		os.Exit(1)
+	}
+}

misc/cgo/testplugin/src/issue22175/plugin1.go

@@ -0,0 +1,21 @@
+// Copyright 2017 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 main
+
+import "plugin"
+
+func F() int {
+	p2, err := plugin.Open("issue22175_plugin2.so")
+	if err != nil {
+		panic(err)
+	}
+	g, err := p2.Lookup("G")
+	if err != nil {
+		panic(err)
+	}
+	return g.(func() int)()
+}
+
+func main() {}

misc/cgo/testplugin/src/issue22175/plugin2.go

@@ -0,0 +1,9 @@
+// Copyright 2017 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 main
+
+func G() int { return 971 }
+
+func main() {}

misc/cgo/testplugin/test.bash

@@ -74,3 +74,9 @@ GOPATH=$(pwd) go build -gcflags "$GO_GCFLAGS" -o issue19418 src/issue19418/main.
 
 # Test for issue 19529
 GOPATH=$(pwd) go build -gcflags "$GO_GCFLAGS" -buildmode=plugin -o plugin.so src/issue19529/plugin.go
+
+# Test for issue 22175
+GOPATH=$(pwd) go build -gcflags "$GO_GCFLAGS" -buildmode=plugin -o issue22175_plugin1.so src/issue22175/plugin1.go
+GOPATH=$(pwd) go build -gcflags "$GO_GCFLAGS" -buildmode=plugin -o issue22175_plugin2.so src/issue22175/plugin2.go
+GOPATH=$(pwd) go build -gcflags "$GO_GCFLAGS" -o issue22175 src/issue22175/main.go
+./issue22175

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

@@ -641,6 +641,13 @@ func (ctxt *Link) symtab() {
 		moduledata.AddUint(ctxt.Arch, uint64(len(ctxt.Shlibs)))
 	}
 
+	hasmain := ctxt.BuildMode == BuildModeExe || ctxt.BuildMode == BuildModePIE
+	if hasmain {
+		moduledata.AddUint8(1)
+	} else {
+		moduledata.AddUint8(0)
+	}
+
 	// The rest of moduledata is zero initialized.
 	// When linking an object that does not contain the runtime we are
 	// creating the moduledata from scratch and it does not have a

src/runtime/symtab.go

@@ -375,6 +375,8 @@ type moduledata struct {
 	modulename   string
 	modulehashes []modulehash
 
+	hasmain uint8 // 1 if module contains the main function, 0 otherwise
+
 	gcdatamask, gcbssmask bitvector
 
 	typemap map[typeOff]*_type // offset to *_rtype in previous module
@@ -472,9 +474,8 @@ func modulesinit() {
 	// contains the main function.
 	//
 	// See Issue #18729.
-	mainText := funcPC(main_main)
 	for i, md := range *modules {
-		if md.text <= mainText && mainText <= md.etext {
+		if md.hasmain != 0 {
 			(*modules)[0] = md
 			(*modules)[i] = &firstmoduledata
 			break