cmd/internal/objfile: add Disasm

This was missing from CL 167320043.
Happy to apply comments in a followup.
TBR to fix build.

TBR=r
CC=golang-codereviews
https://golang.org/cl/171260043

src/cmd/internal/objfile/disasm.go

+package objfile
+
+import (
+	"bufio"
+	"debug/gosym"
+	"encoding/binary"
+	"fmt"
+	"io"
+	"regexp"
+	"sort"
+	"strings"
+	"text/tabwriter"
+
+	"cmd/internal/rsc.io/arm/armasm"
+	"cmd/internal/rsc.io/x86/x86asm"
+)
+
+// Disasm is a disassembler for a given File.
+type Disasm struct {
+	syms      []Sym
+	pcln      *gosym.Table
+	text      []byte
+	textStart uint64
+	textEnd   uint64
+	goarch    string
+	disasm    disasmFunc
+	byteOrder binary.ByteOrder
+}
+
+// Disasm returns a disassembler for the file f.
+func (f *File) Disasm() (*Disasm, error) {
+	syms, err := f.Symbols()
+	if err != nil {
+		return nil, err
+	}
+
+	pcln, err := f.PCLineTable()
+	if err != nil {
+		return nil, err
+	}
+
+	textStart, textBytes, err := f.Text()
+	if err != nil {
+		return nil, err
+	}
+
+	goarch := f.GOARCH()
+	disasm := disasms[goarch]
+	byteOrder := byteOrders[goarch]
+	if disasm == nil || byteOrder == nil {
+		return nil, fmt.Errorf("unsupported architecture")
+	}
+
+	// Filter out section symbols, overwriting syms in place.
+	keep := syms[:0]
+	for _, sym := range syms {
+		switch sym.Name {
+		case "runtime.text", "text", "_text", "runtime.etext", "etext", "_etext":
+			// drop
+		default:
+			keep = append(keep, sym)
+		}
+	}
+	syms = keep
+	d := &Disasm{
+		syms:      syms,
+		pcln:      pcln,
+		text:      textBytes,
+		textStart: textStart,
+		textEnd:   textStart + uint64(len(textBytes)),
+		goarch:    goarch,
+		disasm:    disasm,
+		byteOrder: byteOrder,
+	}
+
+	return d, nil
+}
+
+// lookup finds the symbol name containing addr.
+func (d *Disasm) lookup(addr uint64) (name string, base uint64) {
+	i := sort.Search(len(d.syms), func(i int) bool { return addr < d.syms[i].Addr })
+	if i > 0 {
+		s := d.syms[i-1]
+		if s.Addr != 0 && s.Addr <= addr && addr < s.Addr+uint64(s.Size) {
+			return s.Name, s.Addr
+		}
+	}
+	return "", 0
+}
+
+// base returns the final element in the path.
+// It works on both Windows and Unix paths.
+func base(path string) string {
+	path = path[strings.LastIndex(path, "/")+1:]
+	path = path[strings.LastIndex(path, `\`)+1:]
+	return path
+}
+
+// Print prints a disassembly of the file to w.
+// If filter is non-nil, the disassembly only includes functions with names matching filter.
+// The disassembly only includes functions that overlap the range [start, end).
+func (d *Disasm) Print(w io.Writer, filter *regexp.Regexp, start, end uint64) {
+	if start < d.textStart {
+		start = d.textStart
+	}
+	if end > d.textEnd {
+		end = d.textEnd
+	}
+	printed := false
+	bw := bufio.NewWriter(w)
+	for _, sym := range d.syms {
+		symStart := sym.Addr
+		symEnd := sym.Addr + uint64(sym.Size)
+		if sym.Code != 'T' && sym.Code != 't' ||
+			symStart < d.textStart ||
+			symEnd <= start || end <= symStart ||
+			filter != nil && !filter.MatchString(sym.Name) {
+			continue
+		}
+		if printed {
+			fmt.Fprintf(bw, "\n")
+		}
+		printed = true
+
+		file, _, _ := d.pcln.PCToLine(sym.Addr)
+		fmt.Fprintf(bw, "TEXT %s(SB) %s\n", sym.Name, file)
+
+		tw := tabwriter.NewWriter(bw, 1, 8, 1, '\t', 0)
+		if symEnd > end {
+			symEnd = end
+		}
+		code := d.text[:end-d.textStart]
+		d.Decode(symStart, symEnd, func(pc, size uint64, file string, line int, text string) {
+			i := pc - d.textStart
+			fmt.Fprintf(tw, "\t%s:%d\t%#x\t", base(file), line, pc)
+			if size%4 != 0 || d.goarch == "386" || d.goarch == "amd64" {
+				// Print instruction as bytes.
+				fmt.Fprintf(tw, "%x", code[i:i+size])
+			} else {
+				// Print instruction as 32-bit words.
+				for j := uint64(0); j < size; j += 4 {
+					if j > 0 {
+						fmt.Fprintf(tw, " ")
+					}
+					fmt.Fprintf(tw, "%08x", d.byteOrder.Uint32(code[i+j:]))
+				}
+			}
+			fmt.Fprintf(tw, "\t%s\n", text)
+		})
+		tw.Flush()
+	}
+	bw.Flush()
+}
+
+// Decode disassembles the text segment range [start, end), calling f for each instruction.
+func (d *Disasm) Decode(start, end uint64, f func(pc, size uint64, file string, line int, text string)) {
+	if start < d.textStart {
+		start = d.textStart
+	}
+	if end > d.textEnd {
+		end = d.textEnd
+	}
+	code := d.text[:end-d.textStart]
+	lookup := d.lookup
+	for pc := start; pc < end; {
+		i := pc - d.textStart
+		text, size := d.disasm(code[i:], pc, lookup)
+		file, line, _ := d.pcln.PCToLine(pc)
+		f(pc, uint64(size), file, line, text)
+		pc += uint64(size)
+	}
+}
+
+type lookupFunc func(addr uint64) (sym string, base uint64)
+type disasmFunc func(code []byte, pc uint64, lookup lookupFunc) (text string, size int)
+
+func disasm_386(code []byte, pc uint64, lookup lookupFunc) (string, int) {
+	return disasm_x86(code, pc, lookup, 32)
+}
+
+func disasm_amd64(code []byte, pc uint64, lookup lookupFunc) (string, int) {
+	return disasm_x86(code, pc, lookup, 64)
+}
+
+func disasm_x86(code []byte, pc uint64, lookup lookupFunc, arch int) (string, int) {
+	inst, err := x86asm.Decode(code, 64)
+	var text string
+	size := inst.Len
+	if err != nil || size == 0 || inst.Op == 0 {
+		size = 1
+		text = "?"
+	} else {
+		text = x86asm.Plan9Syntax(inst, pc, lookup)
+	}
+	return text, size
+}
+
+type textReader struct {
+	code []byte
+	pc   uint64
+}
+
+func (r textReader) ReadAt(data []byte, off int64) (n int, err error) {
+	if off < 0 || uint64(off) < r.pc {
+		return 0, io.EOF
+	}
+	d := uint64(off) - r.pc
+	if d >= uint64(len(r.code)) {
+		return 0, io.EOF
+	}
+	n = copy(data, r.code[d:])
+	if n < len(data) {
+		err = io.ErrUnexpectedEOF
+	}
+	return
+}
+
+func disasm_arm(code []byte, pc uint64, lookup lookupFunc) (string, int) {
+	inst, err := armasm.Decode(code, armasm.ModeARM)
+	var text string
+	size := inst.Len
+	if err != nil || size == 0 || inst.Op == 0 {
+		size = 4
+		text = "?"
+	} else {
+		text = armasm.Plan9Syntax(inst, pc, lookup, textReader{code, pc})
+	}
+	return text, size
+}
+
+var disasms = map[string]disasmFunc{
+	"386":   disasm_386,
+	"amd64": disasm_amd64,
+	"arm":   disasm_arm,
+}
+
+var byteOrders = map[string]binary.ByteOrder{
+	"386":       binary.LittleEndian,
+	"amd64":     binary.LittleEndian,
+	"arm":       binary.LittleEndian,
+	"power64":   binary.BigEndian,
+	"power64le": binary.LittleEndian,
+}