cmd/link: set .dynsym info field

.dynsym section info field is the index of first non-local symbol, mean
the number of local symbols.

The go linker have never ever set it before, so just set it.

Fixes #33358

Change-Id: Ifde2deb7c15471b04d565861f5d81daffb0c0d3d
Reviewed-on: https://go-review.googlesource.com/c/go/+/187979
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -1942,8 +1942,17 @@ func Asmbelf(ctxt *Link, symo int64) {
 		sh.addralign = uint64(ctxt.Arch.RegSize)
 		sh.link = uint32(elfshname(".dynstr").shnum)
 
-		// sh->info = index of first non-local symbol (number of local symbols)
-		shsym(sh, ctxt.Syms.Lookup(".dynsym", 0))
+		// sh.info is the index of first non-local symbol (number of local symbols)
+		s := ctxt.Syms.Lookup(".dynsym", 0)
+		i := uint32(0)
+		for sub := s; sub != nil; sub = sub.Sub {
+			i++
+			if !sub.Attr.Local() {
+				sh.info = i
+				break
+			}
+		}
+		shsym(sh, s)
 
 		sh = elfshname(".dynstr")
 		sh.type_ = SHT_STRTAB

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

@@ -0,0 +1,79 @@
+// +build cgo
+
+// Copyright 2019 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 ld
+
+import (
+	"debug/elf"
+	"internal/testenv"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"testing"
+)
+
+func TestDynSymShInfo(t *testing.T) {
+	t.Parallel()
+	testenv.MustHaveGoBuild(t)
+	dir, err := ioutil.TempDir("", "go-build-issue33358")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+
+	const prog = `
+package main
+
+import "net"
+
+func main() {
+	net.Dial("", "")
+}
+`
+	src := filepath.Join(dir, "issue33358.go")
+	if err := ioutil.WriteFile(src, []byte(prog), 0666); err != nil {
+		t.Fatal(err)
+	}
+
+	binFile := filepath.Join(dir, "issue33358")
+	cmd := exec.Command(testenv.GoToolPath(t), "build", "-o", binFile, src)
+	if out, err := cmd.CombinedOutput(); err != nil {
+		t.Fatalf("%v: %v:\n%s", cmd.Args, err, out)
+	}
+
+	fi, err := os.Open(binFile)
+	if err != nil {
+		t.Fatalf("failed to open built file: %v", err)
+	}
+
+	elfFile, err := elf.NewFile(fi)
+	if err != nil {
+		t.Skip("The system may not support ELF, skipped.")
+	}
+
+	section := elfFile.Section(".dynsym")
+	if section == nil {
+		t.Fatal("no dynsym")
+	}
+
+	symbols, err := elfFile.DynamicSymbols()
+	if err != nil {
+		t.Fatalf("failed to get dynamic symbols: %v", err)
+	}
+
+	var numLocalSymbols uint32
+	for i, s := range symbols {
+		if elf.ST_BIND(s.Info) != elf.STB_LOCAL {
+			numLocalSymbols = uint32(i + 1)
+			break
+		}
+	}
+
+	if section.Info != numLocalSymbols {
+		t.Fatalf("Unexpected sh info, want greater than 0, got: %d", section.Info)
+	}
+}