debug/elf: add support for GOARCH=mips{,le}

Change-Id: Ia6f8ae7e56a49ad66b60a24c4afb606f3cfe5efd
Reviewed-on: https://go-review.googlesource.com/31482
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/debug/elf/file.go

@@ -579,7 +579,7 @@ func (f *File) Section(name string) *Section {
 }
 
 // applyRelocations applies relocations to dst. rels is a relocations section
-// in RELA format.
+// in REL or RELA format.
 func (f *File) applyRelocations(dst []byte, rels []byte) error {
 	switch {
 	case f.Class == ELFCLASS64 && f.Machine == EM_X86_64:
@@ -594,6 +594,8 @@ func (f *File) applyRelocations(dst []byte, rels []byte) error {
 		return f.applyRelocationsPPC(dst, rels)
 	case f.Class == ELFCLASS64 && f.Machine == EM_PPC64:
 		return f.applyRelocationsPPC64(dst, rels)
+	case f.Class == ELFCLASS32 && f.Machine == EM_MIPS:
+		return f.applyRelocationsMIPS(dst, rels)
 	case f.Class == ELFCLASS64 && f.Machine == EM_MIPS:
 		return f.applyRelocationsMIPS64(dst, rels)
 	case f.Class == ELFCLASS64 && f.Machine == EM_S390:
@@ -863,6 +865,44 @@ func (f *File) applyRelocationsPPC64(dst []byte, rels []byte) error {
 	return nil
 }
 
+func (f *File) applyRelocationsMIPS(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_MIPS(rel.Info & 0xff)
+
+		if symNo == 0 || symNo > uint32(len(symbols)) {
+			continue
+		}
+		sym := &symbols[symNo-1]
+
+		switch t {
+		case R_MIPS_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) applyRelocationsMIPS64(dst []byte, rels []byte) error {
 	// 24 is the size of Rela64.
 	if len(rels)%24 != 0 {

src/debug/elf/file_test.go

@@ -510,6 +510,44 @@ var relocationTests = []relocationTest{
 			}},
 		},
 	},
+	{
+		"testdata/go-relocation-test-gcc492-mipsle.obj",
+		[]relocationTestEntry{
+			{0, &dwarf.Entry{
+				Offset:   0xb,
+				Tag:      dwarf.TagCompileUnit,
+				Children: true,
+				Field: []dwarf.Field{
+					{Attr: dwarf.AttrProducer, Val: "GNU C 4.9.2 -mel -march=mips2 -mtune=mips32 -mllsc -mno-shared -mabi=32 -g", Class: dwarf.ClassString},
+					{Attr: dwarf.AttrLanguage, Val: int64(1), Class: dwarf.ClassConstant},
+					{Attr: dwarf.AttrName, Val: "hello.c", Class: dwarf.ClassString},
+					{Attr: dwarf.AttrCompDir, Val: "/tmp", Class: dwarf.ClassString},
+					{Attr: dwarf.AttrLowpc, Val: uint64(0x0), Class: dwarf.ClassAddress},
+					{Attr: dwarf.AttrHighpc, Val: int64(0x58), Class: dwarf.ClassConstant},
+					{Attr: dwarf.AttrStmtList, Val: int64(0), Class: dwarf.ClassLinePtr},
+				},
+			}},
+		},
+	},
+	{
+		"testdata/go-relocation-test-gcc540-mips.obj",
+		[]relocationTestEntry{
+			{0, &dwarf.Entry{
+				Offset:   0xb,
+				Tag:      dwarf.TagCompileUnit,
+				Children: true,
+				Field: []dwarf.Field{
+					{Attr: dwarf.AttrProducer, Val: "GNU C11 5.4.0 20160609 -meb -mips32 -mtune=mips32r2 -mfpxx -mllsc -mno-shared -mabi=32 -g -gdwarf-2", Class: dwarf.ClassString},
+					{Attr: dwarf.AttrLanguage, Val: int64(12), Class: dwarf.ClassConstant},
+					{Attr: dwarf.AttrName, Val: "hello.c", Class: dwarf.ClassString},
+					{Attr: dwarf.AttrCompDir, Val: "/tmp", Class: dwarf.ClassString},
+					{Attr: dwarf.AttrLowpc, Val: uint64(0x0), Class: dwarf.ClassAddress},
+					{Attr: dwarf.AttrHighpc, Val: uint64(0x5c), Class: dwarf.ClassAddress},
+					{Attr: dwarf.AttrStmtList, Val: int64(0), Class: dwarf.ClassLinePtr},
+				},
+			}},
+		},
+	},
 	{
 		"testdata/go-relocation-test-gcc493-mips64le.obj",
 		[]relocationTestEntry{