debug/elf: support reading debug info from 32-bit PPC objects

Fixes #10118.

Change-Id: I4a2e6748db609c6eed1d68c824b81c59bd7b875c
Reviewed-on: https://go-review.googlesource.com/7590Reviewed-by: Minux Ma <minux@golang.org>

src/debug/elf/file.go

@@ -533,6 +533,8 @@ func (f *File) applyRelocations(dst []byte, rels []byte) error {
 		return f.applyRelocationsARM(dst, rels)
 	case f.Class == ELFCLASS64 && f.Machine == EM_AARCH64:
 		return f.applyRelocationsARM64(dst, rels)
+	case f.Class == ELFCLASS32 && f.Machine == EM_PPC:
+		return f.applyRelocationsPPC(dst, rels)
 	case f.Class == ELFCLASS64 && f.Machine == EM_PPC64:
 		return f.applyRelocationsPPC64(dst, rels)
 	default:
@@ -713,6 +715,46 @@ func (f *File) applyRelocationsARM64(dst []byte, rels []byte) error {
 	return nil
 }
 
+func (f *File) applyRelocationsPPC(dst []byte, rels []byte) error {
+	// 12 is the size of Rela32.
+	if len(rels)%12 != 0 {
+		return errors.New("length of relocation section is not a multiple of 12")
+	}
+
+	symbols, _, err := f.getSymbols(SHT_SYMTAB)
+	if err != nil {
+		return err
+	}
+
+	b := bytes.NewReader(rels)
+	var rela Rela32
+
+	for b.Len() > 0 {
+		binary.Read(b, f.ByteOrder, &rela)
+		symNo := rela.Info >> 8
+		t := R_PPC(rela.Info & 0xff)
+
+		if symNo == 0 || symNo > uint32(len(symbols)) {
+			continue
+		}
+		sym := &symbols[symNo-1]
+		if SymType(sym.Info&0xf) != STT_SECTION {
+			// We don't handle non-section relocations for now.
+			continue
+		}
+
+		switch t {
+		case R_PPC_ADDR32:
+			if rela.Off+4 >= uint32(len(dst)) || rela.Addend < 0 {
+				continue
+			}
+			f.ByteOrder.PutUint32(dst[rela.Off:rela.Off+4], uint32(rela.Addend))
+		}
+	}
+
+	return nil
+}
+
 func (f *File) applyRelocationsPPC64(dst []byte, rels []byte) error {
 	// 24 is the size of Rela64.
 	if len(rels)%24 != 0 {

src/debug/elf/file_test.go

@@ -272,6 +272,12 @@ var relocationTests = []relocationTest{
 			{0, &dwarf.Entry{Offset: 0xb, Tag: dwarf.TagCompileUnit, Children: true, Field: []dwarf.Field{{Attr: dwarf.AttrProducer, Val: "GNU C 4.9.2 20141224 (prerelease) -march=armv7-a -mfloat-abi=hard -mfpu=vfpv3-d16 -mtls-dialect=gnu -g"}, {Attr: dwarf.AttrLanguage, Val: int64(1)}, {Attr: dwarf.AttrName, Val: "go-relocation-test-gcc492.c"}, {Attr: dwarf.AttrCompDir, Val: "/root/go/src/debug/elf/testdata"}, {Attr: dwarf.AttrLowpc, Val: uint64(0x0)}, {Attr: dwarf.AttrHighpc, Val: int64(0x28)}, {Attr: dwarf.AttrStmtList, Val: int64(0)}}}},
 		},
 	},
+	{
+		"testdata/go-relocation-test-gcc5-ppc.obj",
+		[]relocationTestEntry{
+			{0, &dwarf.Entry{Offset: 0xb, Tag: dwarf.TagCompileUnit, Children: true, Field: []dwarf.Field{dwarf.Field{Attr: dwarf.AttrProducer, Val: "GNU C11 5.0.0 20150116 (experimental) -Asystem=linux -Asystem=unix -Asystem=posix -g"}, dwarf.Field{Attr: dwarf.AttrLanguage, Val: int64(12)}, dwarf.Field{Attr: dwarf.AttrName, Val: "go-relocation-test-gcc5-ppc.c"}, dwarf.Field{Attr: dwarf.AttrCompDir, Val: "/tmp"}, dwarf.Field{Attr: dwarf.AttrLowpc, Val: uint64(0x0)}, dwarf.Field{Attr: dwarf.AttrHighpc, Val: int64(0x44)}, dwarf.Field{Attr: dwarf.AttrStmtList, Val: int64(0)}}}},
+		},
+	},
 	{
 		"testdata/go-relocation-test-gcc482-ppc64le.obj",
 		[]relocationTestEntry{