阅读完上一篇文章： http://blog.csdn.net/andyhuabing/article/details/9226569
我们已经清楚了如何进入正常模式和Recovery模式已有深刻理解了，假设进入了Recovery模式，那么其核心代码是怎么做的呢？
代码路径在 android 源码的根路径： bootable\recovery 其入口文件就是 recovery.c 中 main函数
下面就开始逐步了解其Recovery的设计思想：
static const char *COMMAND_FILE = "/cache/recovery/command";
static const char *INTENT_FILE = "/cache/recovery/intent";
static const char *LOG_FILE = "/cache/recovery/log";
注解里面描述的相当清楚：
* The recovery tool communicates with the main system through /cache files.
* /cache/recovery/command - INPUT - command line for tool, one arg per line
* /cache/recovery/log - OUTPUT - combined log file from recovery run(s)
* /cache/recovery/intent - OUTPUT - intent that was passed in
static const char *LAST_LOG_FILE = "/cache/recovery/last_log";
static const char *LAST_INSTALL_FILE = "/cache/recovery/last_install";
static const char *CACHE_ROOT = "/cache";
static const char *SDCARD_ROOT = "/sdcard";
下面的描述针对写入的 command 有大致的介绍：
* The arguments which may be supplied in the recovery.command file:
* --send_intent=anystring - write the text out to recovery.intent
* --update_package=path - verify install an OTA package file
* --wipe_data - erase user data (and cache), then reboot
* --wipe_cache - wipe cache (but not user data), then reboot
* --set_encrypted_filesystem=on|off - enables / diasables encrypted fs
两种升级模式步骤说明：
* After completing, we remove /cache/recovery/command and reboot.
* Arguments may also be supplied in the bootloader control block (BCB).
* These important scenarios must be safely restartable at any point:
*
* FACTORY RESET
* 1. user selects "factory reset"
* 2. main system writes "--wipe_data" to /cache/recovery/command
* 3. main system reboots into recovery
* 4. get_args() writes BCB with "boot-recovery" and "--wipe_data"
* -- after this, rebooting will restart the erase --
* 5. erase_volume() reformats /data
* 6. erase_volume() reformats /cache
* 7. finish_recovery() erases BCB
* -- after this, rebooting will restart the main system --
* 8. main() calls reboot() to boot main system
*
* OTA INSTALL
* 1. main system downloads OTA package to /cache/some-filename.zip
* 2. main system writes "--update_package=/cache/some-filename.zip"
* 3. main system reboots into recovery
* 4. get_args() writes BCB with "boot-recovery" and "--update_package=..."
* -- after this, rebooting will attempt to reinstall the update --
* 5. install_package() attempts to install the update
* NOTE: the package install must itself be restartable from any point
* 6. finish_recovery() erases BCB
* -- after this, rebooting will (try to) restart the main system --
* 7. ** if install failed **
* 7a. prompt_and_wait() shows an error icon and waits for the user
* 7b; the user reboots (pulling the battery, etc) into the main system
* 8. main() calls maybe_install_firmware_update()
* ** if the update contained radio/hboot firmware **:
* 8a. m_i_f_u() writes BCB with "boot-recovery" and "--wipe_cache"
* -- after this, rebooting will reformat cache & restart main system --
* 8b. m_i_f_u() writes firmware image into raw cache partition
* 8c. m_i_f_u() writes BCB with "update-radio/hboot" and "--wipe_cache"
* -- after this, rebooting will attempt to reinstall firmware --
* 8d. bootloader tries to flash firmware
* 8e. bootloader writes BCB with "boot-recovery" (keeping "--wipe_cache")
* -- after this, rebooting will reformat cache & restart main system --
* 8f. erase_volume() reformats /cache
* 8g. finish_recovery() erases BCB
* -- after this, rebooting will (try to) restart the main system --
* 9. main() calls reboot() to boot main system
从上面的几段注解中，基本上就明白的 Recovery 是如何工作的啦。下面就从具体代码开始一步步分析。
1、recovery main 函数
int
main(int argc, char **argv) {
time_t start = time(NULL); // If these fail, there's not really anywhere to complain...
freopen(TEMPORARY_LOG_FILE, "a", stdout); setbuf(stdout, NULL);
freopen(TEMPORARY_LOG_FILE, "a", stderr); setbuf(stderr, NULL);
printf("Starting recovery on %s", ctime(&start));
将标准输出和标准错误输出重定位到
"/tmp/recovery.log",
如果是
eng
模式，就可以通过
adb pull /tmp/recovery.log,
看到当前的
log
信息，这为我们提供了有效的调试手段。
ui_init();
一个简单的基于framebuffer的ui系统，叫miniui 主要建立了图像部分（gglInit、gr_init_font、framebuffer）及进度条和事件处理（input_callback）
load_volume_table();
根据 /etc/recovery.fstab 建立分区表
// command line args come from, in decreasing precedence:
// - the actual command line
// - the bootloader control block (one per line, after "recovery")
// - the contents of COMMAND_FILE (one per line)
get_args(&argc, &argv);
从misc 分区以及 CACHE:recovery/command 文件中读入参数，写入到argc, argv （get_bootloader_message） 并有可能写回 misc 分区（set_bootloader_message）
做完以上事情后就开始解析具体参数：
while ((arg = getopt_long(argc, argv, "", OPTIONS, NULL)) != -1) {
switch (arg) {
case 'p': previous_runs = atoi(optarg); break;
case 's': send_intent = optarg; break;
case 'u': update_package = optarg; break;
case 'w': wipe_data = wipe_cache = 1; break;
case 'c': wipe_cache = 1; break;
case 't': ui_show_text(1); break;
case '?':
LOGE("Invalid command argument\n");
continue;
}
}
printf("Command:");
for (arg = 0; arg < argc; arg++) {
printf(" \"%s\"", argv[arg]);
}
printf("\n");
以上仅仅是打印表明进入到哪一步，方便调试情况的掌握
下面的代码就是具体干的事情了：
if (update_package != NULL) {
status = install_package(update_package, &wipe_cache, TEMPORARY_INSTALL_FILE);
if (status == INSTALL_SUCCESS && wipe_cache) {
if (erase_volume("/cache")) {
LOGE("Cache wipe (requested by package) failed.");
}
}
if (status != INSTALL_SUCCESS) ui_print("Installation aborted.\n");
} else if (wipe_data) {
if (device_wipe_data()) status = INSTALL_ERROR;
if (erase_volume("/data")) status = INSTALL_ERROR;
if (wipe_cache && erase_volume("/cache")) status = INSTALL_ERROR;
if (status != INSTALL_SUCCESS) ui_print("Data wipe failed.\n");
clear_sdcard_update_bootloader_message();
} else if (wipe_cache) {
if (wipe_cache && erase_volume("/cache")) status = INSTALL_ERROR;
if (status != INSTALL_SUCCESS) ui_print("Cache wipe failed.\n");
clear_sdcard_update_bootloader_message();
} else {
status = update_by_key(); // No command specified
}
根据用户提供参数，调用各项功能，比如，安装一个升级包，擦除cache分区, 擦除user data分区等等，后在会将继续详细分解。
if (status != INSTALL_SUCCESS) prompt_and_wait();
如果前面做的操作成功则进入重启流程，否则由用户操作，可选操作为： reboot, 安装update.zip，除cache分区, 擦除user data分区
// Otherwise, get ready to boot the main system...
finish_recovery(send_intent);
先看函数注解：
// clear the recovery command and prepare to boot a (hopefully working) system,
// copy our log file to cache as well (for the system to read), and
// record any intent we were asked to communicate back to the system.
// this function is idempotent: call it as many times as you like.
其实主要的就是如下函数操作：
// Remove the command file, so recovery won't repeat indefinitely.
if (ensure_path_mounted(COMMAND_FILE) != 0 ||
(unlink(COMMAND_FILE) && errno != ENOENT)) {
LOGW("Can't unlink %s\n", COMMAND_FILE);
}
将指定分区mounted 成功并 unlink 删除一个文件的目录项并减少它的链接数
ensure_path_unmounted(CACHE_ROOT);
将指定分区 unmounted
sync(); // For good measure.
对于上面的代码总结：
它的功能如下：
1、将前面定义的intent字符串写入（如果有的话）：CACHE:recovery/command
2、将 /tmp/recovery.log 复制到 "CACHE:recovery/log";
3、清空 misc 分区，这样重启就不会进入recovery模式
4、删除command 文件：CACHE:recovery/command;
最后重启机器
ui_print("Rebooting...\n");
android_reboot(ANDROID_RB_RESTART, 0, 0);
2、factory reset 核心代码实现
按照前面所列的8条步骤，其中1-6及7-8都与 main 通用流程一样，不再复述。
* 5. erase_volume() reformats /data
* 6. erase_volume() reformats /cache
这两个操作是如何做到的呢？
if (erase_volume("/data")) status = INSTALL_ERROR;
if (erase_volume("/cache")) status = INSTALL_ERROR;
最后就是
clear_sdcard_update_bootloader_message();
看看 erase_volume() 函数先：
static int
erase_volume(const char *volume) {
ui_set_background(BACKGROUND_ICON_INSTALLING);
ui_show_indeterminate_progress();
ui_print("Formatting %s...\n", volume); ensure_path_unmounted(volume); if (strcmp(volume, "/cache") == 0) {
// Any part of the log we'd copied to cache is now gone.
// Reset the pointer so we copy from the beginning of the temp
// log.
tmplog_offset = 0;
} return format_volume(volume);
}
上面红字标明的是重要函数调用
int ensure_path_unmounted(const char* path) {
Volume* v = volume_for_path(path);
result = scan_mounted_volumes();
return unmount_mounted_volume(mv);
}
就是将指定的path中径mount point进行卸载掉，而 format_volume的主要功能就是：
MtdWriteContext *write = mtd_write_partition(partition);
mtd_erase_blocks(write, -1);
mtd_write_close(write);
不要细说了吧，就是将整个分区数据全清掉。
最后一个函数：
void
clear_sdcard_update_bootloader_message() {
struct bootloader_message boot;
memset(&boot, 0, sizeof(boot));
set_bootloader_message(&boot);
}
就是将misc分区数据重置清0
这样子就完成的恢复出厂设置的情况了。将 data/cache分区erase擦掉就好了。
3、OTA 安装 核心代码实现
主要函数就是如何安装 Package ：
* 5. install_package() attempts to install the update
* NOTE: the package install must itself be restartable from any point
int
install_package(const char* path, int* wipe_cache, const char* install_file)
-->
static int
really_install_package(const char *path, int* wipe_cache){
clear_sdcard_update_bootloader_message();
ui_set_background(BACKGROUND_ICON_INSTALLING);
ui_print("Finding update package...\n");
ui_show_indeterminate_progress();
LOGI("Update location: %s\n", path);
更新 ui 显示
for(;((i < 5)&&(ensure_path_mounted(path) != 0));i++){
LOGE("Can't mount %s\n",path);
sleep(1);
}
if((i >= 5)&&(ensure_path_mounted(path) != 0)){
return INSTALL_CORRUPT;
}
确保升级包所在分区已经mount,通常为 cache 分区或者 SD 分区
RSAPublicKey* loadedKeys = load_keys(PUBLIC_KEYS_FILE, &numKeys);
// Look for an RSA signature embedded in the .ZIP file comment given
// the path to the zip. Verify it matches one of the given public
// keys.
//
// Return VERIFY_SUCCESS, VERIFY_FAILURE (if any error is encountered
// or no key matches the signature).
err = verify_file(path, loadedKeys, numKeys);
从/res/keys中装载公钥，并进行确认文件的合法性
/* Try to open the package.
*/
ZipArchive zip;
err = mzOpenZipArchive(path, &zip);
打开升级包，将相关信息存到ZipArchive数据机构中，便于后面处理。
/* Verify and install the contents of the package.
*/
ui_print("Installing update...\n");
return try_update_binary(path, &zip, wipe_cache);
进行最后的安装包文件
}
// If the package contains an update binary, extract it and run it.
static int
try_update_binary(const char *path, ZipArchive *zip, int* wipe_cache) {
const ZipEntry* binary_entry =
mzFindZipEntry(zip, ASSUMED_UPDATE_BINARY_NAME);
char* binary = "/tmp/update_binary";
unlink(binary);
int fd = creat(binary, 0755);
bool ok = mzExtractZipEntryToFile(zip, binary_entry, fd);
close(fd);
mzCloseZipArchive(zip);
将升级包内文件META-INF/com/google/android/update-binary 复制为/tmp/update_binary
// When executing the update binary contained in the package, the
// arguments passed are:
//
// - the version number for this interface
//
// - an fd to which the program can write in order to update the
// progress bar. The program can write single-line commands:
int pipefd[2];
pipe(pipefd);
char** args = malloc(sizeof(char*) * 5);
args[0] = binary;
args[1] = EXPAND(RECOVERY_API_VERSION); // defined in Android.mk
args[2] = malloc(10);
sprintf(args[2], "%d", pipefd[1]);
args[3] = (char*)path;
args[4] = buf_uuid;
args[5] = NULL;
组装新的进程参数
pid_t pid = fork();
if (pid == 0) { // child process
close(pipefd[0]);
execv(binary, args);
}
// parent process
close(pipefd[1]);
ui_show_progress
ui_set_progress
ui_print
总结一下代码主要行为功能：
1、将会创建新的进程，执行：/tmp/update_binary
2、同时，会给该进程传入一些参数，其中最重要的就是一个管道fd，供新进程与原进程通信。
3、新进程诞生后，原进程就变成了一个服务进程，它提供若干UI更新服务：
a) progress
b) set_progress
c) ui_print
这样，新进程就可以通过老进程的UI系统完成显示任务。而其他功能就靠它自己了。