debug/pe: .bss section must contain only zeros

.bss section has no data stored in PE file. But when .bss section data
is used by the linker it is assumed that its every byte is set to zero.
(*Section).Data returns garbage at this moment. Change (*Section).Data
so it returns slice filled with 0s.

Updates #15345

Change-Id: I1fa5138244a9447e1d59dec24178b1dd0fd4c5d7
Reviewed-on: https://go-review.googlesource.com/22544Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/debug/pe/file.go

@@ -158,7 +158,11 @@ func NewFile(r io.ReaderAt) (*File, error) {
 			NumberOfLineNumbers:  sh.NumberOfLineNumbers,
 			Characteristics:      sh.Characteristics,
 		}
-		s.sr = io.NewSectionReader(r, int64(s.SectionHeader.Offset), int64(s.SectionHeader.Size))
+		r2 := r
+		if sh.PointerToRawData == 0 { // .bss must have all 0s
+			r2 = zeroReaderAt{}
+		}
+		s.sr = io.NewSectionReader(r2, int64(s.SectionHeader.Offset), int64(s.SectionHeader.Size))
 		s.ReaderAt = s.sr
 		f.Sections[i] = s
 	}
@@ -173,6 +177,17 @@ func NewFile(r io.ReaderAt) (*File, error) {
 	return f, nil
 }
 
+// zeroReaderAt is ReaderAt that reads 0s.
+type zeroReaderAt struct{}
+
+// ReadAt writes len(p) 0s into p.
+func (w zeroReaderAt) ReadAt(p []byte, off int64) (n int, err error) {
+	for i := range p {
+		p[i] = 0
+	}
+	return len(p), nil
+}
+
 // getString extracts a string from symbol string table.
 func getString(section []byte, start int) (string, bool) {
 	if start < 0 || start >= len(section) {

src/debug/pe/file_test.go

@@ -6,6 +6,7 @@ package pe
 
 import (
 	"debug/dwarf"
+	"internal/testenv"
 	"io/ioutil"
 	"os"
 	"os/exec"
@@ -307,3 +308,75 @@ func main() {
 	}
 	t.Fatal("main.main not found")
 }
+
+func TestBSSHasZeros(t *testing.T) {
+	testenv.MustHaveExec(t)
+
+	if runtime.GOOS != "windows" {
+		t.Skip("skipping windows only test")
+	}
+	gccpath, err := exec.LookPath("gcc")
+	if err != nil {
+		t.Skip("skipping test: gcc is missing")
+	}
+
+	tmpdir, err := ioutil.TempDir("", "TestBSSHasZeros")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpdir)
+
+	srcpath := filepath.Join(tmpdir, "a.c")
+	src := `
+#include <stdio.h>
+
+int zero = 0;
+
+int
+main(void)
+{
+	printf("%d\n", zero);
+	return 0;
+}
+`
+	err = ioutil.WriteFile(srcpath, []byte(src), 0644)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	objpath := filepath.Join(tmpdir, "a.obj")
+	cmd := exec.Command(gccpath, "-c", srcpath, "-o", objpath)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("failed to build object file: %v - %v", err, string(out))
+	}
+
+	f, err := Open(objpath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+
+	var bss *Section
+	for _, sect := range f.Sections {
+		if sect.Name == ".bss" {
+			bss = sect
+			break
+		}
+	}
+	if bss == nil {
+		t.Fatal("could not find .bss section")
+	}
+	data, err := bss.Data()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(data) == 0 {
+		t.Fatalf("%s file .bss section cannot be empty", objpath)
+	}
+	for _, b := range data {
+		if b != 0 {
+			t.Fatalf(".bss section has non zero bytes: %v", data)
+		}
+	}
+}