cmd/compile, cmd/link: dynamically export writable static tmps

Static tmps are private to a package, but with plugins a package
can be shared among multiple DSOs. They need to have a consistent
view of the static tmps, especially for writable ones. So export
them. (Read-only static tmps have the same values anyway, so it
doesn't matter. Also Mach-O doesn't support dynamically exporting
read-only symbols anyway.)

Fixes #44956.

Change-Id: I921e25b7ab73cd5d5347800eccdb7931e3448779
Reviewed-on: https://go-review.googlesource.com/c/go/+/301793
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

misc/cgo/testplugin/plugin_test.go

@@ -209,3 +209,10 @@ func TestMethod2(t *testing.T) {
 	goCmd(t, "build", "-o", "method2.exe", "./method2/main.go")
 	run(t, "./method2.exe")
 }
+
+func TestIssue44956(t *testing.T) {
+	goCmd(t, "build", "-buildmode=plugin", "-o", "issue44956p1.so", "./issue44956/plugin1.go")
+	goCmd(t, "build", "-buildmode=plugin", "-o", "issue44956p2.so", "./issue44956/plugin2.go")
+	goCmd(t, "build", "-o", "issue44956.exe", "./issue44956/main.go")
+	run(t, "./issue44956.exe")
+}

misc/cgo/testplugin/testdata/issue44956/base/base.go

@@ -0,0 +1,7 @@
+// Copyright 2021 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 base
+
+var X = &map[int]int{123: 456}

misc/cgo/testplugin/testdata/issue44956/main.go

@@ -0,0 +1,47 @@
+// Copyright 2021 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.
+
+// Issue 44956: writable static temp is not exported correctly.
+// In the test below, package base is
+//
+//     X = &map{...}
+//
+// which compiles to
+//
+//     X = &stmp           // static
+//     stmp = makemap(...) // in init function
+//
+// plugin1 and plugin2 both import base. plugin1 doesn't use
+// base.X, so that symbol is deadcoded in plugin1.
+//
+// plugin1 is loaded first. base.init runs at that point, which
+// initialize base.stmp.
+//
+// plugin2 is then loaded. base.init already ran, so it doesn't run
+// again. When base.stmp is not exported, plugin2's base.X points to
+// its own private base.stmp, which is not initialized, fail.
+
+package main
+
+import "plugin"
+
+func main() {
+	_, err := plugin.Open("issue44956p1.so")
+	if err != nil {
+		panic("FAIL")
+	}
+
+	p2, err := plugin.Open("issue44956p2.so")
+	if err != nil {
+		panic("FAIL")
+	}
+	f, err := p2.Lookup("F")
+	if err != nil {
+		panic("FAIL")
+	}
+	x := f.(func() *map[int]int)()
+	if x == nil || (*x)[123] != 456 {
+		panic("FAIL")
+	}
+}

misc/cgo/testplugin/testdata/issue44956/plugin1.go

@@ -0,0 +1,9 @@
+// Copyright 2021 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 _ "testplugin/issue44956/base"
+
+func main() {}

misc/cgo/testplugin/testdata/issue44956/plugin2.go

@@ -0,0 +1,11 @@
+// Copyright 2021 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 "testplugin/issue44956/base"
+
+func F() *map[int]int { return base.X }
+
+func main() {}

src/cmd/compile/internal/staticinit/sched.go

@@ -459,7 +459,6 @@ func StaticName(t *types.Type) *ir.Name {
 	statuniqgen++
 	typecheck.Declare(n, ir.PEXTERN)
 	n.SetType(t)
-	n.Linksym().Set(obj.AttrLocal, true)
 	return n
 }
 

src/cmd/compile/internal/walk/complit.go

@@ -55,11 +55,12 @@ func (c initContext) String() string {
 	return "inNonInitFunction"
 }
 
-// readonlystaticname returns a name backed by a (writable) static data symbol.
+// readonlystaticname returns a name backed by a read-only static data symbol.
 func readonlystaticname(t *types.Type) *ir.Name {
 	n := staticinit.StaticName(t)
 	n.MarkReadonly()
 	n.Linksym().Set(obj.AttrContentAddressable, true)
+	n.Linksym().Set(obj.AttrLocal, true)
 	return n
 }
 

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

@@ -31,6 +31,7 @@
 package ld
 
 import (
+	"cmd/internal/obj"
 	"cmd/internal/objabi"
 	"cmd/link/internal/loader"
 	"cmd/link/internal/sym"
@@ -140,7 +141,9 @@ func putelfsym(ctxt *Link, x loader.Sym, typ elf.SymType, curbind elf.SymBind) {
 	// One pass for each binding: elf.STB_LOCAL, elf.STB_GLOBAL,
 	// maybe one day elf.STB_WEAK.
 	bind := elf.STB_GLOBAL
-	if ldr.IsFileLocal(x) || ldr.AttrVisibilityHidden(x) || ldr.AttrLocal(x) {
+	if ldr.IsFileLocal(x) && !isStaticTmp(sname) || ldr.AttrVisibilityHidden(x) || ldr.AttrLocal(x) {
+		// Static tmp is package local, but a package can be shared among multiple DSOs.
+		// They need to have a single view of the static tmp that are writable.
 		bind = elf.STB_LOCAL
 	}
 
@@ -856,3 +859,7 @@ func setCarrierSize(typ sym.SymKind, sz int64) {
 	}
 	CarrierSymByType[typ].Size = sz
 }
+
+func isStaticTmp(name string) bool {
+	return strings.Contains(name, "."+obj.StaticNamePref)
+}