// Copyright 2017 Google Inc. All rights reserved.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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// This tool extracts ELF LOAD segments from our linker binary, and produces an
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// assembly file and linker flags which will embed those segments as sections
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// in another binary.
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package main
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import (
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"bytes"
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"debug/elf"
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"flag"
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"fmt"
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"io"
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"io/ioutil"
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"log"
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"os"
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"strings"
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)
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func main() {
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var asmPath string
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var flagsPath string
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flag.StringVar(&asmPath, "s", "", "Path to save the assembly file")
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flag.StringVar(&flagsPath, "f", "", "Path to save the linker flags")
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flag.Parse()
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f, err := os.Open(flag.Arg(0))
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if err != nil {
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log.Fatalf("Error opening %q: %v", flag.Arg(0), err)
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}
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defer f.Close()
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ef, err := elf.NewFile(f)
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if err != nil {
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log.Fatalf("Unable to read elf file: %v", err)
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}
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asm := &bytes.Buffer{}
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baseLoadAddr := uint64(0x1000)
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load := 0
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linkFlags := []string{}
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fmt.Fprintln(asm, ".globl __dlwrap_linker_offset")
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fmt.Fprintf(asm, ".set __dlwrap_linker_offset, 0x%x\n", baseLoadAddr)
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for _, prog := range ef.Progs {
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if prog.Type != elf.PT_LOAD {
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continue
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}
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sectionName := fmt.Sprintf(".linker.sect%d", load)
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symName := fmt.Sprintf("__dlwrap_linker_sect%d", load)
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flags := ""
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if prog.Flags&elf.PF_W != 0 {
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flags += "w"
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}
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if prog.Flags&elf.PF_X != 0 {
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flags += "x"
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}
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fmt.Fprintf(asm, ".section %s, \"a%s\"\n", sectionName, flags)
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fmt.Fprintf(asm, ".globl %s\n%s:\n\n", symName, symName)
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linkFlags = append(linkFlags,
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fmt.Sprintf("-Wl,--undefined=%s", symName),
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fmt.Sprintf("-Wl,--section-start=%s=0x%x",
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sectionName, baseLoadAddr+prog.Vaddr))
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buffer, _ := ioutil.ReadAll(prog.Open())
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bytesToAsm(asm, buffer)
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// Fill in zeros for any BSS sections. It would be nice to keep
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// this as a true BSS, but ld/gold isn't preserving those,
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// instead combining the segments with the following segment,
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// and BSS only exists at the end of a LOAD segment. The
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// linker doesn't use a lot of BSS, so this isn't a huge
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// problem.
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if prog.Memsz > prog.Filesz {
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fmt.Fprintf(asm, ".fill 0x%x, 1, 0\n", prog.Memsz-prog.Filesz)
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}
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fmt.Fprintln(asm)
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load += 1
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}
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if asmPath != "" {
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if err := ioutil.WriteFile(asmPath, asm.Bytes(), 0777); err != nil {
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log.Fatalf("Unable to write %q: %v", asmPath, err)
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}
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}
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if flagsPath != "" {
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flags := strings.Join(linkFlags, " ")
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if err := ioutil.WriteFile(flagsPath, []byte(flags), 0777); err != nil {
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log.Fatalf("Unable to write %q: %v", flagsPath, err)
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}
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}
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}
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func bytesToAsm(asm io.Writer, buf []byte) {
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for i, b := range buf {
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if i%64 == 0 {
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if i != 0 {
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fmt.Fprint(asm, "\n")
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}
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fmt.Fprint(asm, ".byte ")
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} else {
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fmt.Fprint(asm, ",")
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}
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fmt.Fprintf(asm, "%d", b)
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}
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fmt.Fprintln(asm)
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}
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