base/android/linker/linker_jni.cc

// Copyright 2015 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.

// This is the Android-specific Chromium linker, a tiny shared library
// implementing a custom dynamic linker that can be used to load the
// real Chromium libraries.

// The main point of this linker is to be able to share the RELRO
// section of libchrome.so (or equivalent) between renderer processes.

// This source code *cannot* depend on anything from base/ or the C++
// STL, to keep the final library small, and avoid ugly dependency issues.

#include "base/android/linker/linker_jni.h"

#include <jni.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>

namespace chromium_android_linker {

// Variable containing LibInfo for the loaded library.
LibInfo_class s_lib_info_fields;

// Simple scoped UTF String class constructor.
String::String(JNIEnv* env, jstring str) {
  size_ = env->GetStringUTFLength(str);
  ptr_ = static_cast<char*>(::malloc(size_ + 1));

  // Note: This runs before browser native code is loaded, and so cannot
  // rely on anything from base/. This means that we must use
  // GetStringUTFChars() and not base::android::ConvertJavaStringToUTF8().
  //
  // GetStringUTFChars() suffices because the only strings used here are
  // paths to APK files or names of shared libraries, all of which are
  // plain ASCII, defined and hard-coded by the Chromium Android build.
  //
  // For more: see
  //   https://crbug.com/508876
  //
  // Note: GetStringUTFChars() returns Java UTF-8 bytes. This is good
  // enough for the linker though.
  const char* bytes = env->GetStringUTFChars(str, nullptr);
  ::memcpy(ptr_, bytes, size_);
  ptr_[size_] = '\0';

  env->ReleaseStringUTFChars(str, bytes);
}

// Find the jclass JNI reference corresponding to a given |class_name|.
// |env| is the current JNI environment handle.
// On success, return true and set |*clazz|.
bool InitClassReference(JNIEnv* env, const char* class_name, jclass* clazz) {
  *clazz = env->FindClass(class_name);
  if (!*clazz) {
    LOG_ERROR("Could not find class for %s", class_name);
    return false;
  }
  return true;
}

// Initialize a jfieldID corresponding to the field of a given |clazz|,
// with name |field_name| and signature |field_sig|.
// |env| is the current JNI environment handle.
// On success, return true and set |*field_id|.
bool InitFieldId(JNIEnv* env,
                 jclass clazz,
                 const char* field_name,
                 const char* field_sig,
                 jfieldID* field_id) {
  *field_id = env->GetFieldID(clazz, field_name, field_sig);
  if (!*field_id) {
    LOG_ERROR("Could not find ID for field '%s'", field_name);
    return false;
  }
  LOG_INFO("Found ID %p for field '%s'", *field_id, field_name);
  return true;
}

// Use Android ASLR to create a random address into which we expect to be
// able to load libraries. Note that this is probabilistic; we unmap the
// address we get from mmap and assume we can re-map into it later. This
// works the majority of the time. If it doesn't, client code backs out and
// then loads the library normally at any available address.
// |env| is the current JNI environment handle, and |clazz| a class.
// Returns the address selected by ASLR, or 0 on error.
JNI_GENERATOR_EXPORT jlong
Java_org_chromium_base_library_1loader_Linker_nativeGetRandomBaseLoadAddress(
    JNIEnv* env,
    jclass clazz) {
  size_t bytes = kAddressSpaceReservationSize;

  void* address =
      mmap(nullptr, bytes, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
  if (address == MAP_FAILED) {
    LOG_INFO("Random base load address not determinable");
    return 0;
  }
  munmap(address, bytes);

  LOG_INFO("Random base load address is %p", address);
  return static_cast<jlong>(reinterpret_cast<uintptr_t>(address));
}

}  // namespace chromium_android_linker