Add a host tool to pack R_ARM_RELATIVE relocations in libchrome.<ver>.so.

tools/relocation_packer/src/main.cc

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Tool to pack and unpack R_ARM_RELATIVE relocations in a shared library.
+//
+// Packing removes R_ARM_RELATIVE relocations from .rel.dyn and writes them
+// in a more compact form to .android.rel.dyn.  Unpacking does the reverse.
+//
+// Invoke with -v to trace actions taken when packing or unpacking.
+// Invoke with -p to pad removed relocations with R_ARM_NONE.  Suppresses
+// shrinking of .rel.dyn.
+// See PrintUsage() below for full usage details.
+//
+// NOTE: Breaks with libelf 0.152, which is buggy.  libelf 0.158 works.
+
+// TODO(simonb): Extend for 64-bit target libraries.
+
+#include <errno.h>
+#include <fcntl.h>
+#include <getopt.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/types.h>
+#include <unistd.h>
+#include <string>
+
+#include "debug.h"
+#include "elf_file.h"
+#include "libelf.h"
+#include "packer.h"
+
+void PrintUsage(const char* argv0) {
+  std::string temporary = argv0;
+  const size_t last_slash = temporary.find_last_of("/");
+  if (last_slash != temporary.npos) {
+    temporary.erase(0, last_slash + 1);
+  }
+  const char* basename = temporary.c_str();
+
+  printf(
+      "Usage: %s [-u] [-v] [-p] file\n"
+      "Pack or unpack R_ARM_RELATIVE relocations in a shared library.\n\n"
+      "  -u, --unpack   unpack previously packed R_ARM_RELATIVE relocations\n"
+      "  -v, --verbose  trace object file modifications (for debugging)\n"
+      "  -p, --pad      do not shrink .rel.dyn, but pad (for debugging)\n\n"
+      "Extracts R_ARM_RELATIVE relocations from the .rel.dyn section, packs\n"
+      "them into a more compact format, and stores the packed relocations in\n"
+      ".android.rel.dyn.  Expands .android.rel.dyn to hold the packed data,\n"
+      "and shrinks .rel.dyn by the amount of unpacked data removed from it.\n\n"
+      "Before being packed, a shared library needs to be prepared by adding\n"
+      "a null .android.rel.dyn section.  A complete packing process is:\n\n"
+      "    echo -n 'NULL' >/tmp/small\n"
+      "    arm-linux-gnueabi-objcopy \\\n"
+      "        --add-section .android.rel.dyn=/tmp/small \\\n"
+      "        libchrome.<version>.so\n"
+      "    rm /tmp/small\n"
+      "    %s libchrome.<version>.so\n\n"
+      "To unpack and restore the shared library to its original state:\n\n"
+      "    %s -u libchrome.<version>.so\n"
+      "    arm-linux-gnueabi-objcopy \\\n"
+      "        --remove-section=.android.rel.dyn libchrome.<version>.so\n\n"
+      "Debug sections are not handled, so packing should not be used on\n"
+      "shared libraries compiled for debugging or otherwise unstripped.\n",
+      basename, basename, basename);
+}
+
+int main(int argc, char* argv[]) {
+  bool is_unpacking = false;
+  bool is_verbose = false;
+  bool is_padding = false;
+
+  static const option options[] = {
+    {"unpack", 0, 0, 'u'}, {"verbose", 0, 0, 'v'}, {"pad", 0, 0, 'p'},
+    {"help", 0, 0, 'h'}, {NULL, 0, 0, 0}
+  };
+  bool has_options = true;
+  while (has_options) {
+    int c = getopt_long(argc, argv, "uvph", options, NULL);
+    switch (c) {
+      case 'u':
+        is_unpacking = true;
+        break;
+      case 'v':
+        is_verbose = true;
+        break;
+      case 'p':
+        is_padding = true;
+        break;
+      case 'h':
+        PrintUsage(argv[0]);
+        return 0;
+      case '?':
+        LOG("Try '%s --help' for more information.\n", argv[0]);
+        return 1;
+      case -1:
+        has_options = false;
+        break;
+      default:
+        NOTREACHED();
+    }
+  }
+  if (optind != argc - 1) {
+    LOG("Try '%s --help' for more information.\n", argv[0]);
+    return 1;
+  }
+
+  if (elf_version(EV_CURRENT) == EV_NONE) {
+    LOG("WARNING: Elf Library is out of date!\n");
+  }
+
+  const char* file = argv[argc - 1];
+  const int fd = open(file, O_RDWR);
+  if (fd == -1) {
+    LOG("%s: %s\n", file, strerror(errno));
+    return 1;
+  }
+
+  relocation_packer::Logger::SetVerbose(is_verbose);
+
+  relocation_packer::ElfFile elf_file(fd);
+  elf_file.SetPadding(is_padding);
+
+  bool status;
+  if (is_unpacking)
+    status = elf_file.UnpackRelocations();
+  else
+    status = elf_file.PackRelocations();
+
+  close(fd);
+
+  if (!status) {
+    LOG("ERROR: %s: failed to pack/unpack file\n", file);
+    return 1;
+  }
+
+  return 0;
+}