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