君子知夫不全不粹之不足以為美也， 

故誦數以貫之，

思索以通之，

為其人以處之，

除其害者以持養之；

　　　　　　　　　　出自荀子《勸學篇》

終於OTA的升級過程的詳解來了，之前的兩篇文章OTA升級詳解（一）與OTA升級詳解（二）主要是鋪墊，

OTA升級的一些基礎知識，那這邊文章就開始揭開OTA-recovery模式升級過程的神祕面紗，需要說明的是

以下重點梳理了本人認為的關鍵、核心的流程，其他如ui部分、簽名校驗部分我並未花筆墨去描述，主要

還是講升級的核心，其他都是枝枝葉葉。Android 10 recovery原始碼分析，程式碼來源路徑：

https://www.androidos.net.cn/android/10.0.0_r6/xref

本文所講的流程程式碼路徑為：bootable/recovery/

首先從檔案層面說下升級功能的呼叫流程，說明如下：

recovery-main.cpp      升級的主入口

recovery.cpp                開始recovery升級的處理流程

install/install.cpp         執行升級的處理流程（呼叫updater）

updater/updater.cpp  完成升級的核心流程

1 主入口程式碼為：recovery-main.cpp，main入口

1.1 日誌相關的工作準備

 1 // We don't have logcat yet under recovery; so we'll print error on screen and log to stdout
 2 // (which is redirected to recovery.log) as we used to do.
 3 android::base::InitLogging(argv, &UiLogger);
 4 
 5 // Take last pmsg contents and rewrite it to the current pmsg session.
 6 static constexpr const char filter[] = "recovery/";
 7 // Do we need to rotate?
 8 bool do_rotate = false;
 9 
10 __android_log_pmsg_file_read(LOG_ID_SYSTEM, ANDROID_LOG_INFO, filter, logbasename, &do_rotate);
11 // Take action to refresh pmsg contents
12 __android_log_pmsg_file_read(LOG_ID_SYSTEM, ANDROID_LOG_INFO, filter, logrotate, &do_rotate);
13 
14 time_t start = time(nullptr);
15 
16 // redirect_stdio should be called only in non-sideload mode. Otherwise we may have two logger
17 // instances with different timestamps.
18 redirect_stdio(Paths::Get().temporary_log_file().c_str());

1.2 load_volume_table(); 載入系統分割槽資訊，注意這裡並明白掛載分割槽

.mount_point = "/tmp", .fs_type = "ramdisk", .blk_device = "ramdisk", .length = 0 

mount_point -- 掛載點    fs_type -- 分割槽型別 

blk_device     -- 裝置塊名 length  -- 分割槽大小

1.3 掛載/cache分割槽，我們的升級命令都放在這個分割槽下

1 has_cache = volume_for_mount_point(CACHE_ROOT) != nullptr;

1.4 獲取升級的引數並寫BCB塊資訊

std::vector<std::string> args = get_args(argc, argv);

if (!update_bootloader_message(options, &err)) {
    LOG(ERROR) << "Failed to set BCB message: " << err;
}

a、讀取misc分割槽分割槽，並將recovery模式升級的標記寫到misc分割槽中，這樣做的目的是斷電續升，

升級中掉電之後，如果下次開機重啟，在bootloader中會讀取此標記，並重新進入到recovery模式中

update_bootloader_message函式完成此功能。

b、從/cache/recovery/command 中讀取升級引數，這裡recovery啟動程序是未帶入引數時，command

檔案的介面其實有很詳細的解釋

 * The arguments which may be supplied in the recovery.command file:
 *   --update_package=path - verify install an OTA package file
 *   --wipe_data - erase user data (and cache), then reboot
 *   --prompt_and_wipe_data - prompt the user that data is corrupt, with their consent erase user
 *       data (and cache), then reboot
 *   --wipe_cache - wipe cache (but not user data), then reboot
 *   --show_text - show the recovery text menu, used by some bootloader (e.g. http://b/36872519).
 *   --set_encrypted_filesystem=on|off - enables / diasables encrypted fs
 *   --just_exit - do nothing; exit and reboot

1.5 載入recovery_ui_ext.so，完成升級中與螢幕資訊的顯示，升級進度，升級結果等。這裡就不多說了。

static constexpr const char* kDefaultLibRecoveryUIExt = "librecovery_ui_ext.so";
  // Intentionally not calling dlclose(3) to avoid potential gotchas (e.g. `make_device` may have
  // handed out pointers to code or static [or thread-local] data and doesn't collect them all back
  // in on dlclose).
  void* librecovery_ui_ext = dlopen(kDefaultLibRecoveryUIExt, RTLD_NOW);

  using MakeDeviceType = decltype(&make_device);
  MakeDeviceType make_device_func = nullptr;
  if (librecovery_ui_ext == nullptr) {
    printf("Failed to dlopen %s: %s\n", kDefaultLibRecoveryUIExt, dlerror());
  } else {
    reinterpret_cast<void*&>(make_device_func) = dlsym(librecovery_ui_ext, "make_device");
    if (make_device_func == nullptr) {
      printf("Failed to dlsym make_device: %s\n", dlerror());
    }
  }

1.6 非fastboot模式升級就開始了recovery模式升級，start_recovery

ret = fastboot ? StartFastboot(device, args) : start_recovery(device, args);

2 進入 recovery.cpp 

2.1 引數解析，這些引數其實就是來源於/cache/recovery/command， 上面已經通過get_arg，

讀取到了args中

2.2 介面的各種ui資訊顯示，點事電量的檢查等待輔助動作。

2.3 函式名為安裝升級包，其實還未真正開始進行升級包的安裝

1 status = install_package(update_package, should_wipe_cache, true, retry_count, ui);

2.4 安裝結束之後由finish_recovery()完成收尾工作，儲存日誌、清除BCB中的標記，裝置重啟。

 1 static void finish_recovery() {
 2   std::string locale = ui->GetLocale();
 3   // Save the locale to cache, so if recovery is next started up without a '--locale' argument
 4   // (e.g., directly from the bootloader) it will use the last-known locale.
 5   if (!locale.empty() && has_cache) {
 6     LOG(INFO) << "Saving locale \"" << locale << "\"";
 7     if (ensure_path_mounted(LOCALE_FILE) != 0) {
 8       LOG(ERROR) << "Failed to mount " << LOCALE_FILE;
 9     } else if (!android::base::WriteStringToFile(locale, LOCALE_FILE)) {
10       PLOG(ERROR) << "Failed to save locale to " << LOCALE_FILE;
11     }
12   }
13 
14   copy_logs(save_current_log, has_cache, sehandle);
15 
16   // Reset to normal system boot so recovery won't cycle indefinitely.
17   std::string err;
18   if (!clear_bootloader_message(&err)) {
19     LOG(ERROR) << "Failed to clear BCB message: " << err;
20   }
21 
22   // Remove the command file, so recovery won't repeat indefinitely.
23   if (has_cache) {
24     if (ensure_path_mounted(COMMAND_FILE) != 0 || (unlink(COMMAND_FILE) && errno != ENOENT)) {
25       LOG(WARNING) << "Can't unlink " << COMMAND_FILE;
26     }
27     ensure_path_unmounted(CACHE_ROOT);
28   }
29 
30   sync();  // For good measure.
31 }

3 install/install.cpp

3.1 install.cpp其實就進入了安裝升級包的準備動作，剛上的install_package,是假的，這裡才是

really_install_package

1 really_install_package(path, &updater_wipe_cache, needs_mount, &log_buffer,
2                                     retry_count, &max_temperature, ui);

3.2 really_install_package 關鍵地方已加註釋

 1 static int really_install_package(const std::string& path, bool* wipe_cache, bool needs_mount,
 2                                   std::vector<std::string>* log_buffer, int retry_count,
 3                                   int* max_temperature, RecoveryUI* ui) {
 4   ui->SetBackground(RecoveryUI::INSTALLING_UPDATE);
 5   ui->Print("Finding update package...\n");
 6   // Give verification half the progress bar...
 7   ui->SetProgressType(RecoveryUI::DETERMINATE);
 8   ui->ShowProgress(VERIFICATION_PROGRESS_FRACTION, VERIFICATION_PROGRESS_TIME);
 9   LOG(INFO) << "Update location: " << path;
10 
11   // Map the update package into memory.
12   ui->Print("Opening update package...\n");
13 
14   if (needs_mount) {
15     if (path[0] == '@') {
16       ensure_path_mounted(path.substr(1));
17     } else {
18       ensure_path_mounted(path);
19     }
20   }
21 
22   /* 將zip對映到記憶體中 */
23   auto package = Package::CreateMemoryPackage(
24       path, std::bind(&RecoveryUI::SetProgress, ui, std::placeholders::_1));
25   if (!package) {
26     log_buffer->push_back(android::base::StringPrintf("error: %d", kMapFileFailure));
27     return INSTALL_CORRUPT;
28   }
29 
30   // Verify package.進行zip包進行簽名校驗
31   if (!verify_package(package.get(), ui)) {
32     log_buffer->push_back(android::base::StringPrintf("error: %d", kZipVerificationFailure));
33     return INSTALL_CORRUPT;
34   }
35 
36   // Try to open the package.開啟zip包
37   ZipArchiveHandle zip = package->GetZipArchiveHandle();
38   if (!zip) {
39     log_buffer->push_back(android::base::StringPrintf("error: %d", kZipOpenFailure));
40     return INSTALL_CORRUPT;
41   }
42 
43   // Additionally verify the compatibility of the package if it's a fresh install.
44   if (retry_count == 0 && !verify_package_compatibility(zip)) {
45     log_buffer->push_back(android::base::StringPrintf("error: %d", kPackageCompatibilityFailure));
46     return INSTALL_CORRUPT;
47   }
48 
49   // Verify and install the contents of the package.
50   ui->Print("Installing update...\n");
51   if (retry_count > 0) {
52     ui->Print("Retry attempt: %d\n", retry_count);
53   }
54   ui->SetEnableReboot(false);
55   int result =
56       /* 執行升級updater程序進行升級 */
57       try_update_binary(path, zip, wipe_cache, log_buffer, retry_count, max_temperature, ui);
58   ui->SetEnableReboot(true);
59   ui->Print("\n");
60 
61   return result;
62 }

3.3 try_update_binary

從升級包中讀取元資料資訊

1 ReadMetadataFromPackage(zip, &metadata)

3.4 從升級包中讀取updater程序

 1 int SetUpNonAbUpdateCommands(const std::string& package, ZipArchiveHandle zip, int retry_count,
 2                              int status_fd, std::vector<std::string>* cmd) {
 3   CHECK(cmd != nullptr);
 4 
 5   // In non-A/B updates we extract the update binary from the package.
 6   static constexpr const char* UPDATE_BINARY_NAME = "META-INF/com/google/android/update-binary";
 7   ZipString binary_name(UPDATE_BINARY_NAME);
 8   ZipEntry binary_entry;
 9   if (FindEntry(zip, binary_name, &binary_entry) != 0) {
10     LOG(ERROR) << "Failed to find update binary " << UPDATE_BINARY_NAME;
11     return INSTALL_CORRUPT;
12   }
13 
14   const std::string binary_path = Paths::Get().temporary_update_binary();
15   unlink(binary_path.c_str());
16   android::base::unique_fd fd(
17       open(binary_path.c_str(), O_CREAT | O_WRONLY | O_TRUNC | O_CLOEXEC, 0755));
18   if (fd == -1) {
19     PLOG(ERROR) << "Failed to create " << binary_path;
20     return INSTALL_ERROR;
21   }
22 
23   int32_t error = ExtractEntryToFile(zip, &binary_entry, fd);
24   if (error != 0) {
25     LOG(ERROR) << "Failed to extract " << UPDATE_BINARY_NAME << ": " << ErrorCodeString(error);
26     return INSTALL_ERROR;
27   }
28 
29   // When executing the update binary contained in the package, the arguments passed are:
30   //   - the version number for this interface
31   //   - an FD to which the program can write in order to update the progress bar.
32   //   - the name of the package zip file.
33   //   - an optional argument "retry" if this update is a retry of a failed update attempt.
34   *cmd = {
35     binary_path,
36     std::to_string(kRecoveryApiVersion),
37     std::to_string(status_fd),
38     package,
39   };
40   if (retry_count > 0) {
41     cmd->push_back("retry");
42   }
43   return 0;
44 }

3.5 建立管道，這裡子程序關閉了讀端，父程序關閉了寫端，這樣就是保證從單向的資訊通訊，從

子程序傳入資訊到父程序中。

1 android::base::Pipe(&pipe_read, &pipe_write, 0)

3.6 建立子程序，在子程序中執行update-binary程序

 1 if (pid == 0) {
 2     umask(022);
 3     pipe_read.reset();
 4 
 5     // Convert the std::string vector to a NULL-terminated char* vector suitable for execv.
 6     auto chr_args = StringVectorToNullTerminatedArray(args);
 7     /* chr_args[0] 其實就是升級包中的 META-INF/com/google/android/update-binary */
 8     execv(chr_args[0], chr_args.data());
 9     // We shouldn't use LOG/PLOG in the forked process, since they may cause the child process to
10     // hang. This deadlock results from an improperly copied mutex in the ui functions.
11     // (Bug: 34769056)
12     fprintf(stdout, "E:Can't run %s (%s)\n", chr_args[0], strerror(errno));
13     _exit(EXIT_FAILURE);
14   }

3.7 recovery獲取子程序的資訊並顯示，進度、ui_print 等等。

1 FILE* from_child = android::base::Fdopen(std::move(pipe_read), "r");
2 while (fgets(buffer, sizeof(buffer), from_child) != nullptr)

4 execv執行升級程序之後，工作在updater/updater.cpp中完成。

4.1 這裡的主要核心就是構造指令碼解析器對updater-script中的命令進行執行，至於這個指令碼解析器

是如何構造的，如何執行的， 其實我也搞的不是很清楚。

4.2 安裝升級包的核心程式就是Configure edify's functions. 中的那些註冊回撥函式

  1 int main(int argc, char** argv) {
  2 // Various things log information to stdout or stderr more or less
  3 // at random (though we've tried to standardize on stdout).  The
  4 // log file makes more sense if buffering is turned off so things
  5 // appear in the right order.
  6   setbuf(stdout, nullptr);
  7   setbuf(stderr, nullptr);
  8 // We don't have logcat yet under recovery. Update logs will always be written to stdout
  9 // (which is redirected to recovery.log).
 10   android::base::InitLogging(argv, &UpdaterLogger);
 11 if (argc != 4 && argc != 5) {
 12     LOG(ERROR) << "unexpected number of arguments: " << argc;
 13 return 1;
 14   }
 15 /* 支援的版本檢查 */
 16 char* version = argv[1];
 17 if ((version[0] != '1' && version[0] != '2' && version[0] != '3') || version[1] != '\0') {
 18 // We support version 1, 2, or 3.
 19     LOG(ERROR) << "wrong updater binary API; expected 1, 2, or 3; got " << argv[1];
 20 return 2;
 21   }
 22 // Set up the pipe for sending commands back to the parent process.
 23 int fd = atoi(argv[2]);
 24   FILE* cmd_pipe = fdopen(fd, "wb");
 25   setlinebuf(cmd_pipe);
 26 // Extract the script from the package.
 27 /* 從包中提取指令碼 */
 28 const char* package_filename = argv[3];
 29   MemMapping map;
 30 if (!map.MapFile(package_filename)) {
 31     LOG(ERROR) << "failed to map package " << argv[3];
 32 return 3;
 33   }
 34   ZipArchiveHandle za;
 35 int open_err = OpenArchiveFromMemory(map.addr, map.length, argv[3], &za);
 36 if (open_err != 0) {
 37     LOG(ERROR) << "failed to open package " << argv[3] << ": " << ErrorCodeString(open_err);
 38     CloseArchive(za);
 39 return 3;
 40   }
 41 ZipString script_name(SCRIPT_NAME);
 42   ZipEntry script_entry;
 43 int find_err = FindEntry(za, script_name, &script_entry);
 44 if (find_err != 0) {
 45     LOG(ERROR) << "failed to find " << SCRIPT_NAME << " in " << package_filename << ": "
 46                << ErrorCodeString(find_err);
 47     CloseArchive(za);
 48 return 4;
 49   }
 50 std::string script;
 51   script.resize(script_entry.uncompressed_length);
 52 int extract_err = ExtractToMemory(za, &script_entry, reinterpret_cast<uint8_t*>(&script[0]),
 53                                     script_entry.uncompressed_length);
 54 if (extract_err != 0) {
 55     LOG(ERROR) << "failed to read script from package: " << ErrorCodeString(extract_err);
 56     CloseArchive(za);
 57 return 5;
 58   }
 59 // Configure edify's functions.
 60 /* 註冊updater-script中的回撥函式 這裡主要是一些斷言函式 abort assert*/
 61   RegisterBuiltins();
 62 /* 這裡主要是一些安裝升級包的函式 主要是對有檔案系統的分割槽來說*/
 63   RegisterInstallFunctions();
 64 /* 這裡主要註冊對裸分割槽進行升級的函式 */
 65   RegisterBlockImageFunctions();
 66   RegisterDynamicPartitionsFunctions();
 67   RegisterDeviceExtensions();
 68 // Parse the script.
 69 std::unique_ptr<Expr> root;
 70 int error_count = 0;
 71 int error = ParseString(script, &root, &error_count);
 72 if (error != 0 || error_count > 0) {
 73     LOG(ERROR) << error_count << " parse errors";
 74     CloseArchive(za);
 75 return 6;
 76   }
 77   sehandle = selinux_android_file_context_handle();
 78   selinux_android_set_sehandle(sehandle);
 79 if (!sehandle) {
 80 fprintf(cmd_pipe, "ui_print Warning: No file_contexts\n");
 81   }
 82 // Evaluate the parsed script.
 83   UpdaterInfo updater_info;
 84   updater_info.cmd_pipe = cmd_pipe;
 85   updater_info.package_zip = za;
 86   updater_info.version = atoi(version);
 87   updater_info.package_zip_addr = map.addr;
 88   updater_info.package_zip_len = map.length;
 89 State state(script, &updater_info);
 90 if (argc == 5) {
 91 if (strcmp(argv[4], "retry") == 0) {
 92       state.is_retry = true;
 93     } else {
 94 printf("unexpected argument: %s", argv[4]);
 95     }
 96   }
 97 std::string result;
 98 bool status = Evaluate(&state, root, &result);
 99 if (!status) {
100 if (state.errmsg.empty()) {
101       LOG(ERROR) << "script aborted (no error message)";
102 fprintf(cmd_pipe, "ui_print script aborted (no error message)\n");
103     } else {
104       LOG(ERROR) << "script aborted: " << state.errmsg;
105 const std::vector<std::string> lines = android::base::Split(state.errmsg, "\n");
106 for (const std::string& line : lines) {
107 // Parse the error code in abort message.
108 // Example: "E30: This package is for bullhead devices."
109 if (!line.empty() && line[0] == 'E') {
110 if (sscanf(line.c_str(), "E%d: ", &state.error_code) != 1) {
111             LOG(ERROR) << "Failed to parse error code: [" << line << "]";
112           }
113         }
114 fprintf(cmd_pipe, "ui_print %s\n", line.c_str());
115       }
116     }
117 // Installation has been aborted. Set the error code to kScriptExecutionFailure unless
118 // a more specific code has been set in errmsg.
119 if (state.error_code == kNoError) {
120       state.error_code = kScriptExecutionFailure;
121     }
122 fprintf(cmd_pipe, "log error: %d\n", state.error_code);
123 // Cause code should provide additional information about the abort.
124 if (state.cause_code != kNoCause) {
125 fprintf(cmd_pipe, "log cause: %d\n", state.cause_code);
126 if (state.cause_code == kPatchApplicationFailure) {
127         LOG(INFO) << "Patch application failed, retry update.";
128 fprintf(cmd_pipe, "retry_update\n");
129       } else if (state.cause_code == kEioFailure) {
130         LOG(INFO) << "Update failed due to EIO, retry update.";
131 fprintf(cmd_pipe, "retry_update\n");
132       }
133     }
134 if (updater_info.package_zip) {
135       CloseArchive(updater_info.package_zip);
136     }
137 return 7;
138   } else {
139 fprintf(cmd_pipe, "ui_print script succeeded: result was [%s]\n", result.c_str());
140   }
141 if (updater_info.package_zip) {
142     CloseArchive(updater_info.package_zip);
143   }
144 return 0;
145 }

以上就是基於Android的OTA的Recovery模式升級流程。我這裡主要是梳理整個升級流程的主要，

很多地方還是寫的不夠細，望讀者理解，我認為比較核心與關鍵的地方有以下幾點吧

• 主系統與recovery升級系統，升級訊息的傳遞通過cache；

• BCB塊中寫資訊來保證斷電續升；

• 主系統中fork子程序進行升級程序的執行，並通過pipe管道進行資訊互動；

• updater中使用命令與執行的分離，命令在updater-script中，執行在update-binary中；

• 升級程式通過升級包帶入的，那麼核心升級流程是每次都有機會變更或者優化的，
• 這樣就比那些將升級流程預置在系統中的要靈活的很多；

　　　　　　　　長按二維碼關注【嵌入式C部落】，獲取更多程式設計資料及精華文章

&n