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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"
"strconv"
)
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.Fatalf("Unable to read elf file: %v", err)
}
asm := &bytes.Buffer{}
script := &bytes.Buffer{}
baseLoadAddr := uint64(0x1000)
load := 0
fmt.Fprintln(asm, ".globl __dlwrap_linker_offset")
fmt.Fprintf(asm, ".set __dlwrap_linker_offset, 0x%x\n", baseLoadAddr)
fmt.Fprintln(script, "ENTRY(__dlwrap__start)")
fmt.Fprintln(script, "SECTIONS {")
progsWithFlagsCount := make(map[string]int)
for _, prog := range ef.Progs {
if prog.Type != elf.PT_LOAD {
continue
}
progName := progNameFromFlags(prog.Flags, progsWithFlagsCount)
sectionName := ".linker_" + progName
symName := "__dlwrap_linker_" + progName
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")
fmt.Fprintln(asm, "__dlwrap_linker:")
fmt.Fprintln(asm)
}
fmt.Fprintf(asm, ".globl %s\n%s:\n\n", symName, symName)
fmt.Fprintf(script, " %s 0x%x : {\n", sectionName, baseLoadAddr+prog.Vaddr)
fmt.Fprintf(script, " KEEP(*(%s));\n", sectionName)
fmt.Fprintln(script, " }")
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)
load += 1
}
fmt.Fprintln(asm, ".globl __dlwrap_linker_end")
fmt.Fprintln(asm, "__dlwrap_linker_end:")
fmt.Fprintln(asm)
fmt.Fprintln(asm, `.section .note.android.embedded_linker,"a",%note`)
// Discard the PT_INTERP section so that the linker doesn't need to be passed the
// --no-dynamic-linker flag.
fmt.Fprintln(script, " /DISCARD/ : { *(.interp) }")
fmt.Fprintln(script, "}")
fmt.Fprintln(script, "INSERT BEFORE .note.android.embedded_linker;")
if asmPath != "" {
if err := ioutil.WriteFile(asmPath, asm.Bytes(), 0777); err != nil {
log.Fatalf("Unable to write %q: %v", asmPath, err)
}
}
if scriptPath != "" {
if err := ioutil.WriteFile(scriptPath, script.Bytes(), 0777); err != nil {
log.Fatalf("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)
}
func progNameFromFlags(flags elf.ProgFlag, progsWithFlagsCount map[string]int) string {
s := ""
if flags&elf.PF_R != 0 {
s += "r"
}
if flags&elf.PF_W != 0 {
s += "w"
}
if flags&elf.PF_X != 0 {
s += "x"
}
count := progsWithFlagsCount[s]
count++
progsWithFlagsCount[s] = count
if count > 1 {
s += strconv.Itoa(count)
}
return s
}
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