cmd/link: avoid deadcode of global map vars for programs using plugins

If a program imports the plugin package, the mechanisms in place for
detecting and deleting unused global map variables are no longer safe,
since it's possibly for a given global map var to be unreferenced in
the main program but referenced by a plugin. This patch changes the
linker to test for plugin use and to avoid removing any unused global
map variables if the main program could possibly load up a plugin.

Fixes #62430.

Change-Id: Ie00b18b681cb0d259e3c859ac947ade5778cd6c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/526115
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/cmd/cgo/internal/testplugin/plugin_test.go

@@ -388,3 +388,10 @@ func TestSymbolNameMangle(t *testing.T) {
 	globalSkip(t)
 	goCmd(t, "build", "-buildmode=plugin", "-o", "mangle.so", "./mangle/plugin.go")
 }
+
+func TestIssue62430(t *testing.T) {
+	globalSkip(t)
+	goCmd(t, "build", "-buildmode=plugin", "-o", "issue62430.so", "./issue62430/plugin.go")
+	goCmd(t, "build", "-o", "issue62430.exe", "./issue62430/main.go")
+	run(t, "./issue62430.exe")
+}

src/cmd/cgo/internal/testplugin/testdata/issue62430/main.go

@@ -0,0 +1,35 @@
+// Copyright 2023 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 62430: a program that uses plugins may appear
+// to have no references to an initialized global map variable defined
+// in some stdlib package (ex: unicode), however there
+// may be references to that map var from a plugin that
+// gets loaded.
+
+package main
+
+import (
+	"fmt"
+	"plugin"
+	"unicode"
+)
+
+func main() {
+	p, err := plugin.Open("issue62430.so")
+	if err != nil {
+		panic(err)
+	}
+	s, err := p.Lookup("F")
+	if err != nil {
+		panic(err)
+	}
+
+	f := s.(func(string) *unicode.RangeTable)
+	if f("C") == nil {
+		panic("unicode.Categories not properly initialized")
+	} else {
+		fmt.Println("unicode.Categories properly initialized")
+	}
+}

src/cmd/cgo/internal/testplugin/testdata/issue62430/plugin.go

@@ -0,0 +1,11 @@
+package main
+
+import (
+	"unicode"
+)
+
+func F(s string) *unicode.RangeTable {
+	return unicode.Categories[s]
+}
+
+func main() {}

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

@@ -143,10 +143,26 @@ func (d *deadcodePass) flood() {
 		methods = methods[:0]
 		for i := 0; i < relocs.Count(); i++ {
 			r := relocs.At(i)
-			// When build with "-linkshared", we can't tell if the interface
-			// method in itab will be used or not. Ignore the weak attribute.
-			if r.Weak() && !(d.ctxt.linkShared && d.ldr.IsItab(symIdx)) {
-				continue
+			if r.Weak() {
+				convertWeakToStrong := false
+				// When build with "-linkshared", we can't tell if the
+				// interface method in itab will be used or not.
+				// Ignore the weak attribute.
+				if d.ctxt.linkShared && d.ldr.IsItab(symIdx) {
+					convertWeakToStrong = true
+				}
+				// If the program uses plugins, we can no longer treat
+				// relocs from pkg init functions to outlined map init
+				// fragments as weak, since doing so can cause package
+				// init clashes between the main program and the
+				// plugin. See #62430 for more details.
+				if d.ctxt.canUsePlugins && r.Type().IsDirectCall() {
+					convertWeakToStrong = true
+				}
+				if !convertWeakToStrong {
+					// skip this reloc
+					continue
+				}
 			}
 			t := r.Type()
 			switch t {