1 files changed, 154 insertions, 0 deletions
diff --git a/cmd/extract_linker/main.go b/cmd/extract_linker/main.go
new file mode 100644
index 000000000..8530b4aa5
--- /dev/null
+++ b/cmd/extract_linker/main.go
@@ -0,0 +1,154 @@
+// 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)
+}