debug/elf: do not read unrelated bytes for SHT_NOBITS sections

SHT_NOBITS sections do not occupy space in the file and their offset is
"conceptual", reading their data should return all zeroes instead of
reading bytes from the section that follows them.

Change-Id: Iaa9634792c1909c3e87dab841dd646cd6dcf9027
Reviewed-on: https://go-review.googlesource.com/c/go/+/375216
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/debug/elf/elf_test.go

@@ -47,3 +47,25 @@ func TestNames(t *testing.T) {
 		}
 	}
 }
+
+func TestNobitsSection(t *testing.T) {
+	const testdata = "testdata/gcc-amd64-linux-exec"
+	f, err := Open(testdata)
+	if err != nil {
+		t.Fatalf("could not read %s: %v", testdata, err)
+	}
+	defer f.Close()
+	bss := f.Section(".bss")
+	bssData, err := bss.Data()
+	if err != nil {
+		t.Fatalf("error reading .bss section: %v", err)
+	}
+	if g, w := uint64(len(bssData)), bss.Size; g != w {
+		t.Errorf(".bss section length mismatch: got %d, want %d", g, w)
+	}
+	for i := range bssData {
+		if bssData[i] != 0 {
+			t.Fatalf("unexpected non-zero byte at offset %d: %#x", i, bssData[i])
+		}
+	}
+}

src/debug/elf/file.go

@@ -120,6 +120,9 @@ func (f *File) stringTable(link uint32) ([]byte, error) {
 // Even if the section is stored compressed in the ELF file,
 // the ReadSeeker reads uncompressed data.
 func (s *Section) Open() io.ReadSeeker {
+	if s.Type == SHT_NOBITS {
+		return io.NewSectionReader(&zeroReader{}, 0, int64(s.Size))
+	}
 	if s.Flags&SHF_COMPRESSED == 0 {
 		return io.NewSectionReader(s.sr, 0, 1<<63-1)
 	}
@@ -1453,3 +1456,12 @@ func (f *File) DynString(tag DynTag) ([]string, error) {
 	}
 	return all, nil
 }
+
+type zeroReader struct{}
+
+func (*zeroReader) ReadAt(p []byte, off int64) (n int, err error) {
+	for i := range p {
+		p[i] = 0
+	}
+	return len(p), nil
+}