[release-branch.go1.15] cmd/link: avoid exporting all symbols on windows buildmode=pie

Marking one functions with __declspec(dllexport) forces mingw to
create .reloc section without having to export all symbols.

See https://insights.sei.cmu.edu/cert/2018/08/when-aslr-is-not-really-aslr---the-case-of-incorrect-assumptions-and-bad-defaults.html for more info.

This change cuts 73kb of a "hello world" pie binary.

Updates #6853.
Updates #40795.
Fixes #43592.

Change-Id: I3cc57c3b64f61187550bc8751dfa085f106c8475
Reviewed-on: https://go-review.googlesource.com/c/go/+/264459
Trust: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/300692
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: David Chase <drchase@google.com>

5 files changed, 47 insertions, 3 deletions

diff --git a/src/cmd/go/go_test.go b/src/cmd/go/go_test.go
index 021930a8a81..3f790cdeabc 100644
--- a/src/cmd/go/go_test.go
+++ b/src/cmd/go/go_test.go
@@ -9,6 +9,7 @@ import (
 	"debug/elf"
 	"debug/macho"
 	"debug/pe"
+	"encoding/binary"
 	"flag"
 	"fmt"
 	"go/format"
@@ -2166,6 +2167,38 @@ func testBuildmodePIE(t *testing.T, useCgo, setBuildmodeToPIE bool) {
 		if (dc & pe.IMAGE_DLLCHARACTERISTICS_DYNAMIC_BASE) == 0 {
 			t.Error("IMAGE_DLLCHARACTERISTICS_DYNAMIC_BASE flag is not set")
 		}
+		if useCgo {
+			// Test that only one symbol is exported (#40795).
+			// PIE binaries don´t require .edata section but unfortunately
+			// binutils doesn´t generate a .reloc section unless there is
+			// at least one symbol exported.
+			// See https://sourceware.org/bugzilla/show_bug.cgi?id=19011
+			section := f.Section(".edata")
+			if section == nil {
+				t.Fatalf(".edata section is not present")
+			}
+			// TODO: deduplicate this struct from cmd/link/internal/ld/pe.go
+			type IMAGE_EXPORT_DIRECTORY struct {
+				_                 [2]uint32
+				_                 [2]uint16
+				_                 [2]uint32
+				NumberOfFunctions uint32
+				NumberOfNames     uint32
+				_                 [3]uint32
+			}
+			var e IMAGE_EXPORT_DIRECTORY
+			if err := binary.Read(section.Open(), binary.LittleEndian, &e); err != nil {
+				t.Fatalf("binary.Read failed: %v", err)
+			}
+
+			// Only _cgo_dummy_export should be exported
+			if e.NumberOfFunctions != 1 {
+				t.Fatalf("got %d exported functions; want 1", e.NumberOfFunctions)
+			}
+			if e.NumberOfNames != 1 {
+				t.Fatalf("got %d exported names; want 1", e.NumberOfNames)
+			}
+		}
 	default:
 		panic("unreachable")
 	}
diff --git a/src/cmd/link/internal/ld/lib.go b/src/cmd/link/internal/ld/lib.go
index fc897599970..d03fb6cf91b 100644
--- a/src/cmd/link/internal/ld/lib.go
+++ b/src/cmd/link/internal/ld/lib.go
@@ -1423,9 +1423,6 @@ func (ctxt *Link) hostlink() {
 			if ctxt.Arch.PtrSize >= 8 {
 				argv = append(argv, "-Wl,--high-entropy-va")
 			}
-			// Work around binutils limitation that strips relocation table for dynamicbase.
-			// See https://sourceware.org/bugzilla/show_bug.cgi?id=19011
-			argv = append(argv, "-Wl,--export-all-symbols")
 		default:
 			// ELF.
 			if ctxt.UseRelro() {
diff --git a/src/runtime/cgo/gcc_windows_386.c b/src/runtime/cgo/gcc_windows_386.c
index 9184b91393f..60cb011bf24 100644
--- a/src/runtime/cgo/gcc_windows_386.c
+++ b/src/runtime/cgo/gcc_windows_386.c
@@ -9,6 +9,7 @@
 #include <stdio.h>
 #include <errno.h>
 #include "libcgo.h"
+#include "libcgo_windows.h"
 
 static void threadentry(void*);
 
diff --git a/src/runtime/cgo/gcc_windows_amd64.c b/src/runtime/cgo/gcc_windows_amd64.c
index 7192a246315..0f8c817f0e4 100644
--- a/src/runtime/cgo/gcc_windows_amd64.c
+++ b/src/runtime/cgo/gcc_windows_amd64.c
@@ -9,6 +9,7 @@
 #include <stdio.h>
 #include <errno.h>
 #include "libcgo.h"
+#include "libcgo_windows.h"
 
 static void threadentry(void*);
 
diff --git a/src/runtime/cgo/libcgo_windows.h b/src/runtime/cgo/libcgo_windows.h
new file mode 100644
index 00000000000..0013f06baeb
--- /dev/null
+++ b/src/runtime/cgo/libcgo_windows.h
@@ -0,0 +1,12 @@
+// Copyright 2020 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.
+
+// Ensure there's one symbol marked __declspec(dllexport).
+// If there are no exported symbols, the unfortunate behavior of
+// the binutils linker is to also strip the relocations table,
+// resulting in non-PIE binary. The other option is the
+// --export-all-symbols flag, but we don't need to export all symbols
+// and this may overflow the export table (#40795).
+// See https://sourceware.org/bugzilla/show_bug.cgi?id=19011
+__declspec(dllexport) int _cgo_dummy_export;