cmd/link: don't export all symbols for ELF external linking

Since this may add a large number of --export-dynamic-symbol options,
use a response file if the command line gets large.

Fixes #53579

Change-Id: Ic226bf372bf1e177a3dae886d1c48f4ce3569c0e
Reviewed-on: https://go-review.googlesource.com/c/go/+/414654
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Joedian Reid <joedian@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

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

@@ -1605,7 +1605,13 @@ func (ctxt *Link) hostlink() {
 
 	// Force global symbols to be exported for dlopen, etc.
 	if ctxt.IsELF {
-		argv = append(argv, "-rdynamic")
+		if ctxt.DynlinkingGo() || ctxt.BuildMode == BuildModeCShared || !linkerFlagSupported(ctxt.Arch, argv[0], altLinker, "-Wl,--export-dynamic-symbol=main") {
+			argv = append(argv, "-rdynamic")
+		} else {
+			ctxt.loader.ForAllCgoExportDynamic(func(s loader.Sym) {
+				argv = append(argv, "-Wl,--export-dynamic-symbol="+ctxt.loader.SymExtname(s))
+			})
+		}
 	}
 	if ctxt.HeadType == objabi.Haix {
 		fileName := xcoffCreateExportFile(ctxt)
@@ -1748,7 +1754,7 @@ func (ctxt *Link) hostlink() {
 		// case used has specified "-fuse-ld=...".
 		extld := ctxt.extld()
 		name, args := extld[0], extld[1:]
-		args = append(args, flagExtldflags...)
+		args = append(args, trimLinkerArgv(flagExtldflags)...)
 		args = append(args, "-Wl,--version")
 		cmd := exec.Command(name, args...)
 		usingLLD := false
@@ -1775,6 +1781,8 @@ func (ctxt *Link) hostlink() {
 		argv = append(argv, peimporteddlls()...)
 	}
 
+	argv = ctxt.passLongArgsInResponseFile(argv, altLinker)
+
 	if ctxt.Debugvlog != 0 {
 		ctxt.Logf("host link:")
 		for _, v := range argv {
@@ -1885,6 +1893,47 @@ func (ctxt *Link) hostlink() {
 	}
 }
 
+// passLongArgsInResponseFile writes the arguments into a file if they
+// are very long.
+func (ctxt *Link) passLongArgsInResponseFile(argv []string, altLinker string) []string {
+	c := 0
+	for _, arg := range argv {
+		c += len(arg)
+	}
+
+	if c < sys.ExecArgLengthLimit {
+		return argv
+	}
+
+	// Only use response files if they are supported.
+	response := filepath.Join(*flagTmpdir, "response")
+	if err := os.WriteFile(response, nil, 0644); err != nil {
+		log.Fatalf("failed while testing response file: %v", err)
+	}
+	if !linkerFlagSupported(ctxt.Arch, argv[0], altLinker, "@"+response) {
+		if ctxt.Debugvlog != 0 {
+			ctxt.Logf("not using response file because linker does not support one")
+		}
+		return argv
+	}
+
+	var buf bytes.Buffer
+	for _, arg := range argv[1:] {
+		// The external linker response file supports quoted strings.
+		fmt.Fprintf(&buf, "%q\n", arg)
+	}
+	if err := os.WriteFile(response, buf.Bytes(), 0644); err != nil {
+		log.Fatalf("failed while writing response file: %v", err)
+	}
+	if ctxt.Debugvlog != 0 {
+		ctxt.Logf("response file %s contents:\n%s", response, buf.Bytes())
+	}
+	return []string{
+		argv[0],
+		"@" + response,
+	}
+}
+
 var createTrivialCOnce sync.Once
 
 func linkerFlagSupported(arch *sys.Arch, linker, altLinker, flag string) bool {
@@ -1895,6 +1944,28 @@ func linkerFlagSupported(arch *sys.Arch, linker, altLinker, flag string) bool {
 		}
 	})
 
+	flags := hostlinkArchArgs(arch)
+
+	moreFlags := trimLinkerArgv(append(flagExtldflags, ldflag...))
+	flags = append(flags, moreFlags...)
+
+	if altLinker != "" {
+		flags = append(flags, "-fuse-ld="+altLinker)
+	}
+	flags = append(flags, flag, "trivial.c")
+
+	cmd := exec.Command(linker, flags...)
+	cmd.Dir = *flagTmpdir
+	cmd.Env = append([]string{"LC_ALL=C"}, os.Environ()...)
+	out, err := cmd.CombinedOutput()
+	// GCC says "unrecognized command line option ‘-no-pie’"
+	// clang says "unknown argument: '-no-pie'"
+	return err == nil && !bytes.Contains(out, []byte("unrecognized")) && !bytes.Contains(out, []byte("unknown"))
+}
+
+// trimLinkerArgv returns a new copy of argv that does not include flags
+// that are not relevant for testing whether some linker option works.
+func trimLinkerArgv(argv []string) []string {
 	flagsWithNextArgSkip := []string{
 		"-F",
 		"-l",
@@ -1921,10 +1992,10 @@ func linkerFlagSupported(arch *sys.Arch, linker, altLinker, flag string) bool {
 		"-target",
 	}
 
-	flags := hostlinkArchArgs(arch)
+	var flags []string
 	keep := false
 	skip := false
-	for _, f := range append(flagExtldflags, ldflag...) {
+	for _, f := range argv {
 		if keep {
 			flags = append(flags, f)
 			keep = false
@@ -1945,19 +2016,7 @@ func linkerFlagSupported(arch *sys.Arch, linker, altLinker, flag string) bool {
 			}
 		}
 	}
-
-	if altLinker != "" {
-		flags = append(flags, "-fuse-ld="+altLinker)
-	}
-	flags = append(flags, flag, "trivial.c")
-
-	cmd := exec.Command(linker, flags...)
-	cmd.Dir = *flagTmpdir
-	cmd.Env = append([]string{"LC_ALL=C"}, os.Environ()...)
-	out, err := cmd.CombinedOutput()
-	// GCC says "unrecognized command line option ‘-no-pie’"
-	// clang says "unknown argument: '-no-pie'"
-	return err == nil && !bytes.Contains(out, []byte("unrecognized")) && !bytes.Contains(out, []byte("unknown"))
+	return flags
 }
 
 // hostlinkArchArgs returns arguments to pass to the external linker

src/cmd/link/internal/loader/loader.go

@@ -1041,6 +1041,14 @@ func (l *Loader) SetAttrCgoExportDynamic(i Sym, v bool) {
 	}
 }
 
+// ForAllAttrCgoExportDynamic calls f for every symbol that has been
+// marked with the "cgo_export_dynamic" compiler directive.
+func (l *Loader) ForAllCgoExportDynamic(f func(Sym)) {
+	for s := range l.attrCgoExportDynamic {
+		f(s)
+	}
+}
+
 // AttrCgoExportStatic returns true for a symbol that has been
 // specially marked via the "cgo_export_static" directive
 // written by cgo.

src/cmd/link/link_test.go

@@ -17,6 +17,8 @@ import (
 	"runtime"
 	"strings"
 	"testing"
+
+	"cmd/internal/sys"
 )
 
 var AuthorPaidByTheColumnInch struct {
@@ -1150,3 +1152,46 @@ func TestUnlinkableObj(t *testing.T) {
 		t.Errorf("link failed: %v. output:\n%s", err, out)
 	}
 }
+
+// TestResponseFile tests that creating a response file to pass to the
+// external linker works correctly.
+func TestResponseFile(t *testing.T) {
+	t.Parallel()
+
+	testenv.MustHaveGoBuild(t)
+
+	// This test requires -linkmode=external. Currently all
+	// systems that support cgo support -linkmode=external.
+	testenv.MustHaveCGO(t)
+
+	tmpdir := t.TempDir()
+
+	src := filepath.Join(tmpdir, "x.go")
+	if err := os.WriteFile(src, []byte(`package main; import "C"; func main() {}`), 0666); err != nil {
+		t.Fatal(err)
+	}
+
+	cmd := testenv.Command(t, testenv.GoToolPath(t), "build", "-o", "output", "x.go")
+	cmd.Dir = tmpdir
+
+	// Add enough arguments to push cmd/link into creating a response file.
+	var sb strings.Builder
+	sb.WriteString(`'-ldflags=all="-extldflags=`)
+	for i := 0; i < sys.ExecArgLengthLimit/len("-g"); i++ {
+		if i > 0 {
+			sb.WriteString(" ")
+		}
+		sb.WriteString("-g")
+	}
+	sb.WriteString(`"'`)
+	cmd = testenv.CleanCmdEnv(cmd)
+	cmd.Env = append(cmd.Env, "GOFLAGS="+sb.String())
+
+	out, err := cmd.CombinedOutput()
+	if len(out) > 0 {
+		t.Logf("%s", out)
+	}
+	if err != nil {
+		t.Error(err)
+	}
+}