debug/elf: support DWARF that needs relocs for 386

It's not clear how widespread this issue is, but we do have a
test case generated by a development version of clang.

I don't know whether this should go into 1.3 or not; happy to
hear arguments either way.

LGTM=rsc
R=golang-codereviews, bradfitz, rsc
CC=golang-codereviews
https://golang.org/cl/96680045

src/pkg/debug/elf/file.go

@@ -522,13 +522,17 @@ func (f *File) applyRelocations(dst []byte, rels []byte) error {
 	if f.Class == ELFCLASS64 && f.Machine == EM_X86_64 {
 		return f.applyRelocationsAMD64(dst, rels)
 	}
+	if f.Class == ELFCLASS32 && f.Machine == EM_386 {
+		return f.applyRelocations386(dst, rels)
+	}
 
 	return errors.New("not implemented")
 }
 
 func (f *File) applyRelocationsAMD64(dst []byte, rels []byte) error {
-	if len(rels)%Sym64Size != 0 {
-		return errors.New("length of relocation section is not a multiple of Sym64Size")
+	// 24 is the size of Rela64.
+	if len(rels)%24 != 0 {
+		return errors.New("length of relocation section is not a multiple of 24")
 	}
 
 	symbols, _, err := f.getSymbols(SHT_SYMTAB)
@@ -570,6 +574,43 @@ func (f *File) applyRelocationsAMD64(dst []byte, rels []byte) error {
 	return nil
 }
 
+func (f *File) applyRelocations386(dst []byte, rels []byte) error {
+	// 8 is the size of Rel32.
+	if len(rels)%8 != 0 {
+		return errors.New("length of relocation section is not a multiple of 8")
+	}
+
+	symbols, _, err := f.getSymbols(SHT_SYMTAB)
+	if err != nil {
+		return err
+	}
+
+	b := bytes.NewReader(rels)
+	var rel Rel32
+
+	for b.Len() > 0 {
+		binary.Read(b, f.ByteOrder, &rel)
+		symNo := rel.Info >> 8
+		t := R_386(rel.Info & 0xff)
+
+		if symNo == 0 || symNo > uint32(len(symbols)) {
+			continue
+		}
+		sym := &symbols[symNo-1]
+
+		if t == R_386_32 {
+			if rel.Off+4 >= uint32(len(dst)) {
+				continue
+			}
+			val := f.ByteOrder.Uint32(dst[rel.Off : rel.Off+4])
+			val += uint32(sym.Value)
+			f.ByteOrder.PutUint32(dst[rel.Off:rel.Off+4], val)
+		}
+	}
+
+	return nil
+}
+
 func (f *File) DWARF() (*dwarf.Data, error) {
 	// There are many other DWARF sections, but these
 	// are the required ones, and the debug/dwarf package
@@ -603,6 +644,19 @@ func (f *File) DWARF() (*dwarf.Data, error) {
 		}
 	}
 
+	// When using clang we need to process relocations even for 386.
+	rel := f.Section(".rel.debug_info")
+	if rel != nil && rel.Type == SHT_REL && f.Machine == EM_386 {
+		data, err := rel.Data()
+		if err != nil {
+			return nil, err
+		}
+		err = f.applyRelocations(dat[1], data)
+		if err != nil {
+			return nil, err
+		}
+	}
+
 	abbrev, info, str := dat[0], dat[1], dat[2]
 	d, err := dwarf.New(abbrev, nil, nil, info, nil, nil, nil, str)
 	if err != nil {

src/pkg/debug/elf/file_test.go

@@ -260,6 +260,12 @@ var relocationTests = []relocationTest{
 			{0, &dwarf.Entry{Offset: 0xb, Tag: dwarf.TagCompileUnit, Children: true, Field: []dwarf.Field{{Attr: dwarf.AttrProducer, Val: "GNU C 4.2.4 (Ubuntu 4.2.4-1ubuntu4)"}, {Attr: dwarf.AttrLanguage, Val: int64(1)}, {Attr: dwarf.AttrName, Val: "go-relocation-test-gcc424.c"}, {Attr: dwarf.AttrCompDir, Val: "/tmp"}, {Attr: dwarf.AttrLowpc, Val: uint64(0x0)}, {Attr: dwarf.AttrHighpc, Val: uint64(0x6)}, {Attr: dwarf.AttrStmtList, Val: int64(0)}}}},
 		},
 	},
+	{
+		"testdata/go-relocation-test-clang-x86.obj",
+		[]relocationTestEntry{
+			{0, &dwarf.Entry{Offset: 0xb, Tag: dwarf.TagCompileUnit, Children: true, Field: []dwarf.Field{{Attr: dwarf.AttrProducer, Val: "clang version google3-trunk (trunk r209387)"}, {Attr: dwarf.AttrLanguage, Val: int64(12)}, {Attr: dwarf.AttrName, Val: "go-relocation-test-clang.c"}, {Attr: dwarf.AttrStmtList, Val: int64(0)}, {Attr: dwarf.AttrCompDir, Val: "/tmp"}}}},
+		},
+	},
 	{
 		"testdata/gcc-amd64-openbsd-debug-with-rela.obj",
 		[]relocationTestEntry{