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package builder
import (
"bytes"
"debug/elf"
"debug/pe"
"encoding/binary"
"errors"
"fmt"
"io"
"os"
"path/filepath"
"time"
"github.com/blakesmith/ar"
)
// makeArchive creates an arcive for static linking from a list of object files
// given as a parameter. It is equivalent to the following command:
//
// ar -rcs <archivePath> <objs...>
func makeArchive(arfile *os.File, objs []string) error {
// Open the archive file.
arwriter := ar.NewWriter(arfile)
err := arwriter.WriteGlobalHeader()
if err != nil {
return &os.PathError{Op: "write ar header", Path: arfile.Name(), Err: err}
}
// Open all object files and read the symbols for the symbol table.
symbolTable := []struct {
name string // symbol name
fileIndex int // index into objfiles
}{}
archiveOffsets := make([]int32, len(objs))
for i, objpath := range objs {
objfile, err := os.Open(objpath)
if err != nil {
return err
}
// Read the symbols and add them to the symbol table.
if dbg, err := elf.NewFile(objfile); err == nil {
symbols, err := dbg.Symbols()
if err != nil {
return err
}
for _, symbol := range symbols {
bind := elf.ST_BIND(symbol.Info)
if bind != elf.STB_GLOBAL && bind != elf.STB_WEAK {
// Don't include local symbols (STB_LOCAL).
continue
}
if elf.ST_TYPE(symbol.Info) != elf.STT_FUNC && elf.ST_TYPE(symbol.Info) != elf.STT_OBJECT {
// Not a function.
continue
}
// Include in archive.
symbolTable = append(symbolTable, struct {
name string
fileIndex int
}{symbol.Name, i})
}
} else if dbg, err := pe.NewFile(objfile); err == nil {
for _, symbol := range dbg.Symbols {
if symbol.StorageClass != 2 {
continue
}
if symbol.SectionNumber == 0 {
continue
}
symbolTable = append(symbolTable, struct {
name string
fileIndex int
}{symbol.Name, i})
}
} else {
return fmt.Errorf("failed to open file %s as ELF or PE/COFF: %w", objpath, err)
}
// Close file, to avoid issues with too many open files (especially on
// MacOS X).
objfile.Close()
}
// Create the symbol table buffer.
// For some (sparse) details on the file format:
// https://en.wikipedia.org/wiki/Ar_(Unix)#System_V_(or_GNU)_variant
buf := &bytes.Buffer{}
binary.Write(buf, binary.BigEndian, int32(len(symbolTable)))
for range symbolTable {
// This is a placeholder index, it will be updated after all files have
// been written to the archive (see the end of this function).
err = binary.Write(buf, binary.BigEndian, int32(0))
if err != nil {
return err
}
}
for _, sym := range symbolTable {
_, err := buf.Write([]byte(sym.name + "\x00"))
if err != nil {
return err
}
}
for buf.Len()%2 != 0 {
// The symbol table must be aligned.
// This appears to be required by lld.
buf.WriteByte(0)
}
// Write the symbol table.
err = arwriter.WriteHeader(&ar.Header{
Name: "/",
ModTime: time.Unix(0, 0),
Uid: 0,
Gid: 0,
Mode: 0,
Size: int64(buf.Len()),
})
if err != nil {
return err
}
// Keep track of the start of the symbol table.
symbolTableStart, err := arfile.Seek(0, os.SEEK_CUR)
if err != nil {
return err
}
// Write symbol table contents.
_, err = arfile.Write(buf.Bytes())
if err != nil {
return err
}
// Add all object files to the archive.
var copyBuf bytes.Buffer
for i, objpath := range objs {
objfile, err := os.Open(objpath)
if err != nil {
return err
}
defer objfile.Close()
// Store the start index, for when we'll update the symbol table with
// the correct file start indices.
offset, err := arfile.Seek(0, os.SEEK_CUR)
if err != nil {
return err
}
if int64(int32(offset)) != offset {
return errors.New("large archives (4GB+) not supported: " + arfile.Name())
}
archiveOffsets[i] = int32(offset)
// Write the file header.
st, err := objfile.Stat()
if err != nil {
return err
}
err = arwriter.WriteHeader(&ar.Header{
Name: filepath.Base(objfile.Name()),
ModTime: time.Unix(0, 0),
Uid: 0,
Gid: 0,
Mode: 0644,
Size: st.Size(),
})
if err != nil {
return err
}
// Copy the file contents into the archive.
// First load all contents into a buffer, then write it all in one go to
// the archive file. This is a bit complicated, but is necessary because
// io.Copy can't deal with files that are of an odd size.
copyBuf.Reset()
n, err := io.Copy(©Buf, objfile)
if err != nil {
return fmt.Errorf("could not copy object file into ar file: %w", err)
}
if n != st.Size() {
return errors.New("file modified during ar creation: " + arfile.Name())
}
_, err = arwriter.Write(copyBuf.Bytes())
if err != nil {
return fmt.Errorf("could not copy object file into ar file: %w", err)
}
// File is not needed anymore.
objfile.Close()
}
// Create symbol indices.
indicesBuf := &bytes.Buffer{}
for _, sym := range symbolTable {
err = binary.Write(indicesBuf, binary.BigEndian, archiveOffsets[sym.fileIndex])
if err != nil {
return err
}
}
// Overwrite placeholder indices.
_, err = arfile.WriteAt(indicesBuf.Bytes(), symbolTableStart+4)
return err
}
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