src/cmd/link/internal/ld/objfile.go

// Copyright 2013 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package ld

// Reading of Go object files.
//
// Originally, Go object files were Plan 9 object files, but no longer.
// Now they are more like standard object files, in that each symbol is defined
// by an associated memory image (bytes) and a list of relocations to apply
// during linking. We do not (yet?) use a standard file format, however.
// For now, the format is chosen to be as simple as possible to read and write.
// It may change for reasons of efficiency, or we may even switch to a
// standard file format if there are compelling benefits to doing so.
// See golang.org/s/go13linker for more background.
//
// The file format is:
//
//	- magic header: "\x00\x00go13ld"
//	- byte 1 - version number
//	- sequence of strings giving dependencies (imported packages)
//	- empty string (marks end of sequence)
//	- sequence of symbol references used by the defined symbols
//	- byte 0xff (marks end of sequence)
//	- sequence of integer lengths:
//		- total data length
//		- total number of relocations
//		- total number of pcdata
//		- total number of automatics
//		- total number of funcdata
//		- total number of files
//	- data, the content of the defined symbols
//	- sequence of defined symbols
//	- byte 0xff (marks end of sequence)
//	- magic footer: "\xff\xffgo13ld"
//
// All integers are stored in a zigzag varint format.
// See golang.org/s/go12symtab for a definition.
//
// Data blocks and strings are both stored as an integer
// followed by that many bytes.
//
// A symbol reference is a string name followed by a version.
//
// A symbol points to other symbols using an index into the symbol
// reference sequence. Index 0 corresponds to a nil LSym* pointer.
// In the symbol layout described below "symref index" stands for this
// index.
//
// Each symbol is laid out as the following fields (taken from LSym*):
//
//	- byte 0xfe (sanity check for synchronization)
//	- type [int]
//	- name & version [symref index]
//	- flags [int]
//		1 dupok
//	- size [int]
//	- gotype [symref index]
//	- p [data block]
//	- nr [int]
//	- r [nr relocations, sorted by off]
//
// If type == STEXT, there are a few more fields:
//
//	- args [int]
//	- locals [int]
//	- nosplit [int]
//	- flags [int]
//		1<<0 leaf
//		1<<1 C function
//		1<<2 function may call reflect.Type.Method
//	- nlocal [int]
//	- local [nlocal automatics]
//	- pcln [pcln table]
//
// Each relocation has the encoding:
//
//	- off [int]
//	- siz [int]
//	- type [int]
//	- add [int]
//	- sym [symref index]
//
// Each local has the encoding:
//
//	- asym [symref index]
//	- offset [int]
//	- type [int]
//	- gotype [symref index]
//
// The pcln table has the encoding:
//
//	- pcsp [data block]
//	- pcfile [data block]
//	- pcline [data block]
//	- npcdata [int]
//	- pcdata [npcdata data blocks]
//	- nfuncdata [int]
//	- funcdata [nfuncdata symref index]
//	- funcdatasym [nfuncdata ints]
//	- nfile [int]
//	- file [nfile symref index]
//
// The file layout and meaning of type integers are architecture-independent.
//
// TODO(rsc): The file format is good for a first pass but needs work.
//	- There are SymID in the object file that should really just be strings.

import (
	"bufio"
	"bytes"
	"cmd/internal/obj"
	"io"
	"log"
	"strconv"
	"strings"
)

const (
	startmagic = "\x00\x00go13ld"
	endmagic   = "\xff\xffgo13ld"
)

var emptyPkg = []byte(`"".`)

// objReader reads Go object files.
type objReader struct {
	rd   *bufio.Reader
	ctxt *Link
	pkg  string
	pn   string
	// List of symbol references for the file being read.
	dupSym *LSym

	// rdBuf is used by readString and readSymName as scratch for reading strings.
	rdBuf []byte

	refs        []*LSym
	data        []byte
	reloc       []Reloc
	pcdata      []Pcdata
	autom       []Auto
	funcdata    []*LSym
	funcdataoff []int64
	file        []*LSym
}

func LoadObjFile(ctxt *Link, f *obj.Biobuf, pkg string, length int64, pn string) {
	start := obj.Boffset(f)
	r := &objReader{
		rd:     f.Reader(),
		pkg:    pkg,
		ctxt:   ctxt,
		pn:     pn,
		dupSym: &LSym{Name: ".dup"},
	}
	r.loadObjFile()
	if obj.Boffset(f) != start+length {
		log.Fatalf("%s: unexpected end at %d, want %d", pn, int64(obj.Boffset(f)), int64(start+length))
	}
}

func (r *objReader) loadObjFile() {
	// Increment context version, versions are used to differentiate static files in different packages
	r.ctxt.IncVersion()

	// Magic header
	var buf [8]uint8
	r.readFull(buf[:])
	if string(buf[:]) != startmagic {
		log.Fatalf("%s: invalid file start %x %x %x %x %x %x %x %x", r.pn, buf[0], buf[1], buf[2], buf[3], buf[4], buf[5], buf[6], buf[7])
	}

	// Version
	c, err := r.rd.ReadByte()
	if err != nil || c != 1 {
		log.Fatalf("%s: invalid file version number %d", r.pn, c)
	}

	// Autolib
	for {
		lib := r.readString()
		if lib == "" {
			break
		}
		addlib(r.ctxt, r.pkg, r.pn, lib)
	}

	// Symbol references
	r.refs = []*LSym{nil} // zeroth ref is nil
	for {
		c, err := r.rd.Peek(1)
		if err != nil {
			log.Fatalf("%s: peeking: %v", r.pn, err)
		}
		if c[0] == 0xff {
			r.rd.ReadByte()
			break
		}
		r.readRef()
	}

	// Lengths
	r.readSlices()

	// Data section
	r.readFull(r.data)

	// Defined symbols
	for {
		c, err := r.rd.Peek(1)
		if err != nil {
			log.Fatalf("%s: peeking: %v", r.pn, err)
		}
		if c[0] == 0xff {
			break
		}
		r.readSym()
	}

	// Magic footer
	buf = [8]uint8{}
	r.readFull(buf[:])
	if string(buf[:]) != endmagic {
		log.Fatalf("%s: invalid file end", r.pn)
	}
}

func (r *objReader) readSlices() {
	n := r.readInt()
	r.data = make([]byte, n)
	n = r.readInt()
	r.reloc = make([]Reloc, n)
	n = r.readInt()
	r.pcdata = make([]Pcdata, n)
	n = r.readInt()
	r.autom = make([]Auto, n)
	n = r.readInt()
	r.funcdata = make([]*LSym, n)
	r.funcdataoff = make([]int64, n)
	n = r.readInt()
	r.file = make([]*LSym, n)
}

// Symbols are prefixed so their content doesn't get confused with the magic footer.
const symPrefix = 0xfe

func (r *objReader) readSym() {
	if c, err := r.rd.ReadByte(); c != symPrefix || err != nil {
		log.Fatalln("readSym out of sync")
	}
	t := r.readInt()
	s := r.readSymIndex()
	flags := r.readInt()
	dupok := flags&1 != 0
	local := flags&2 != 0
	size := r.readInt()
	typ := r.readSymIndex()
	data := r.readData()
	nreloc := r.readInt()
	isdup := false

	var dup *LSym
	if s.Type != 0 && s.Type != obj.SXREF {
		if (t == obj.SDATA || t == obj.SBSS || t == obj.SNOPTRBSS) && len(data) == 0 && nreloc == 0 {
			if s.Size < int64(size) {
				s.Size = int64(size)
			}
			if typ != nil && s.Gotype == nil {
				s.Gotype = typ
			}
			return
		}

		if (s.Type == obj.SDATA || s.Type == obj.SBSS || s.Type == obj.SNOPTRBSS) && len(s.P) == 0 && len(s.R) == 0 {
			goto overwrite
		}
		if s.Type != obj.SBSS && s.Type != obj.SNOPTRBSS && !dupok && !s.Attr.DuplicateOK() {
			log.Fatalf("duplicate symbol %s (types %d and %d) in %s and %s", s.Name, s.Type, t, s.File, r.pn)
		}
		if len(s.P) > 0 {
			dup = s
			s = r.dupSym
			isdup = true
		}
	}

overwrite:
	s.File = r.pkg
	if dupok {
		s.Attr |= AttrDuplicateOK
	}
	if t == obj.SXREF {
		log.Fatalf("bad sxref")
	}
	if t == 0 {
		log.Fatalf("missing type for %s in %s", s.Name, r.pn)
	}
	if t == obj.SBSS && (s.Type == obj.SRODATA || s.Type == obj.SNOPTRBSS) {
		t = int(s.Type)
	}
	s.Type = int16(t)
	if s.Size < int64(size) {
		s.Size = int64(size)
	}
	s.Attr.Set(AttrLocal, local)
	if typ != nil {
		s.Gotype = typ
	}
	if isdup && typ != nil { // if bss sym defined multiple times, take type from any one def
		dup.Gotype = typ
	}
	s.P = data
	if nreloc > 0 {
		s.R = r.reloc[:nreloc:nreloc]
		if !isdup {
			r.reloc = r.reloc[nreloc:]
		}

		for i := 0; i < nreloc; i++ {
			s.R[i] = Reloc{
				Off:  r.readInt32(),
				Siz:  r.readUint8(),
				Type: r.readInt32(),
				Add:  r.readInt64(),
				Sym:  r.readSymIndex(),
			}
		}
	}

	if s.Type == obj.STEXT {
		s.Args = r.readInt32()
		s.Locals = r.readInt32()
		if r.readUint8() != 0 {
			s.Attr |= AttrNoSplit
		}
		flags := r.readInt()
		if flags&(1<<2) != 0 {
			s.Attr |= AttrReflectMethod
		}
		n := r.readInt()
		s.Autom = r.autom[:n:n]
		if !isdup {
			r.autom = r.autom[n:]
		}

		for i := 0; i < n; i++ {
			s.Autom[i] = Auto{
				Asym:    r.readSymIndex(),
				Aoffset: r.readInt32(),
				Name:    r.readInt16(),
				Gotype:  r.readSymIndex(),
			}
		}

		s.Pcln = new(Pcln)
		pc := s.Pcln
		pc.Pcsp.P = r.readData()
		pc.Pcfile.P = r.readData()
		pc.Pcline.P = r.readData()
		n = r.readInt()
		pc.Pcdata = r.pcdata[:n:n]
		if !isdup {
			r.pcdata = r.pcdata[n:]
		}
		for i := 0; i < n; i++ {
			pc.Pcdata[i].P = r.readData()
		}
		n = r.readInt()
		pc.Funcdata = r.funcdata[:n:n]
		pc.Funcdataoff = r.funcdataoff[:n:n]
		if !isdup {
			r.funcdata = r.funcdata[n:]
			r.funcdataoff = r.funcdataoff[n:]
		}
		for i := 0; i < n; i++ {
			pc.Funcdata[i] = r.readSymIndex()
		}
		for i := 0; i < n; i++ {
			pc.Funcdataoff[i] = r.readInt64()
		}
		n = r.readInt()
		pc.File = r.file[:n:n]
		if !isdup {
			r.file = r.file[n:]
		}
		for i := 0; i < n; i++ {
			pc.File[i] = r.readSymIndex()
		}

		if !isdup {
			if s.Attr.OnList() {
				log.Fatalf("symbol %s listed multiple times", s.Name)
			}
			s.Attr |= AttrOnList
			if r.ctxt.Etextp != nil {
				r.ctxt.Etextp.Next = s
			} else {
				r.ctxt.Textp = s
			}
			r.ctxt.Etextp = s
		}
	}
}

func (r *objReader) readFull(b []byte) {
	_, err := io.ReadFull(r.rd, b)
	if err != nil {
		log.Fatalf("%s: error reading %s", r.pn, err)
	}
}

func (r *objReader) readRef() {
	if c, err := r.rd.ReadByte(); c != symPrefix || err != nil {
		log.Fatalf("readSym out of sync")
	}
	name := r.readSymName()
	v := r.readInt()
	if v != 0 && v != 1 {
		log.Fatalf("invalid symbol version %d", v)
	}
	if v == 1 {
		v = r.ctxt.Version
	}
	s := Linklookup(r.ctxt, name, v)
	r.refs = append(r.refs, s)

	if s == nil || v != 0 {
		return
	}
	if s.Name[0] == '$' && len(s.Name) > 5 && s.Type == 0 && len(s.P) == 0 {
		x, err := strconv.ParseUint(s.Name[5:], 16, 64)
		if err != nil {
			log.Panicf("failed to parse $-symbol %s: %v", s.Name, err)
		}
		s.Type = obj.SRODATA
		s.Attr |= AttrLocal
		switch s.Name[:5] {
		case "$f32.":
			if uint64(uint32(x)) != x {
				log.Panicf("$-symbol %s too large: %d", s.Name, x)
			}
			Adduint32(r.ctxt, s, uint32(x))
		case "$f64.", "$i64.":
			Adduint64(r.ctxt, s, x)
		default:
			log.Panicf("unrecognized $-symbol: %s", s.Name)
		}
		s.Attr.Set(AttrReachable, false)
	}
	if strings.HasPrefix(s.Name, "runtime.gcbits.") {
		s.Attr |= AttrLocal
	}
}

func (r *objReader) readInt64() int64 {
	uv := uint64(0)
	for shift := uint(0); ; shift += 7 {
		if shift >= 64 {
			log.Fatalf("corrupt input")
		}
		c, err := r.rd.ReadByte()
		if err != nil {
			log.Fatalln("error reading input: ", err)
		}
		uv |= uint64(c&0x7F) << shift
		if c&0x80 == 0 {
			break
		}
	}

	return int64(uv>>1) ^ (int64(uint64(uv)<<63) >> 63)
}

func (r *objReader) readInt() int {
	n := r.readInt64()
	if int64(int(n)) != n {
		log.Panicf("%v out of range for int", n)
	}
	return int(n)
}

func (r *objReader) readInt32() int32 {
	n := r.readInt64()
	if int64(int32(n)) != n {
		log.Panicf("%v out of range for int32", n)
	}
	return int32(n)
}

func (r *objReader) readInt16() int16 {
	n := r.readInt64()
	if int64(int16(n)) != n {
		log.Panicf("%v out of range for int16", n)
	}
	return int16(n)
}

func (r *objReader) readUint8() uint8 {
	n := r.readInt64()
	if int64(uint8(n)) != n {
		log.Panicf("%v out of range for uint8", n)
	}
	return uint8(n)
}

func (r *objReader) readString() string {
	n := r.readInt()
	if cap(r.rdBuf) < n {
		r.rdBuf = make([]byte, 2*n)
	}
	r.readFull(r.rdBuf[:n])
	return string(r.rdBuf[:n])
}

func (r *objReader) readData() []byte {
	n := r.readInt()
	p := r.data[:n:n]
	r.data = r.data[n:]
	return p
}

// readSymName reads a symbol name, replacing all "". with pkg.
func (r *objReader) readSymName() string {
	pkg := r.pkg
	n := r.readInt()
	if n == 0 {
		r.readInt64()
		return ""
	}
	origName, err := r.rd.Peek(n)
	if err != nil {
		log.Fatalf("%s: unexpectedly long symbol name", r.pn)
	}
	if cap(r.rdBuf) < n {
		r.rdBuf = make([]byte, 2*n)
	}
	adjName := r.rdBuf[:0]
	for {
		i := bytes.Index(origName, emptyPkg)
		if i == -1 {
			s := string(append(adjName, origName...))
			// Read past the peeked origName, now that we're done with it,
			// using the rfBuf (also no longer used) as the scratch space.
			// TODO: use bufio.Reader.Discard if available instead?
			r.readFull(r.rdBuf[:n])
			r.rdBuf = adjName[:0] // in case 2*n wasn't enough
			return s
		}
		adjName = append(adjName, origName[:i]...)
		adjName = append(adjName, pkg...)
		adjName = append(adjName, '.')
		origName = origName[i+len(emptyPkg):]
	}
}

// Reads the index of a symbol reference and resolves it to a symbol
func (r *objReader) readSymIndex() *LSym {
	i := r.readInt()
	return r.refs[i]
}