android动态分区AB升级,Android A/B System OTA分析（四）系统的启动和升级
Android從7.0開始引入新的OTA升級方式，A/B System Updates，這里將其叫做A/B系統，涉及的內容較多，分多篇對A/B系統的各個方面進行分析。本文為第四篇，系統的啟動和升級。
本文基于AOSP 7.1.1_r23 (NMF27D)代碼進行分析。
1. 系統的啟動
1.1 bootloader檢查slot metadata
系統復位后，bootloader會去讀取boot_control私有的存儲數slot metadata并進行解析，以此確定從哪一個slot啟動。
以下是Android官方的一個bootloader加載流程圖：
1.2 linux系統的啟動
上一步中，bootloader會根據slot metadata確定讀取哪一個slot的boot分區進行啟動。
每一個slot上有兩個rootfs：
boot分區自帶recovery mode的ramdisk;
system分區包含了Android系統的rootfs;
啟動中，如何選擇加載boot分區的ramdisk還是system分區的rootfs呢？
答案是由kernel的命令行參數skip_initramfs來決定。
下面來看skip_initramfs參數是如何起作用的。
系統同時包含init\noinitramfs.c和init\initramfs.c的代碼，并在initramfs.c模塊中定義并解析skip_initramfs參數：
# init\initramfs.c
static int __initdata do_skip_initramfs;
static int __init skip_initramfs_param(char *str)
{
if (*str)
return 0;
# 設置do_skip_initramfs標志
do_skip_initramfs = 1;
return 1;
}
# 用于解析命令行的`skip_initramfs`參數
__setup("skip_initramfs", skip_initramfs_param);
如果命令行設置了skip_initramfs，則do_skip_initramfs會被設置為1。
linux調用populate_rootfs默認會并加載boot分區自帶的ramdisk，但如果do_skip_initramfs被
設置為1，則調用default_rootfs生成一個極小的rootfs：
# init\initramfs.c
static int __init populate_rootfs(void)
{
char *err;
# 如果do_skip_initramfs置1，則調用default_rootfs生成一個極小的rootfs
if (do_skip_initramfs)
return default_rootfs();
# 沒有設置do_skip_initramfs的情況下，才會解析并加載`boot`分區所包含的`ramdisk`
err = unpack_to_rootfs(__initramfs_start, __initramfs_size);
if (err)
panic("%s", err); /* Failed to decompress INTERNAL initramfs */
...
return 0;
}
default_rootfs的內容很簡單，用于在內存中生成一個極小的rootfs，僅包含/dev和root兩個文件夾以及一個設備節點/dev/console：
# init\noinitramfs.c
/*
* Create a simple rootfs that is similar to the default initramfs
*/
static int __init default_rootfs(void)
{
int err;
# 創建/dev文件夾用于存放/dev/console設備節點
err = sys_mkdir((const char __user __force *) "/dev", 0755);
if (err < 0)
goto out;
# 創建/dev/console設備節點
err = sys_mknod((const char __user __force *) "/dev/console",
S_IFCHR | S_IRUSR | S_IWUSR,
new_encode_dev(MKDEV(5, 1)));
if (err < 0)
goto out;
# 創建/root目錄，作為根用戶root的home
err = sys_mkdir((const char __user __force *) "/root", 0700);
if (err < 0)
goto out;
return 0;
out:
printk(KERN_WARNING "Failed to create a rootfs\n");
return err;
}
因此skip_initramfs參數決定了加載哪一個rootfs，進入哪一個系統。
加載android系統的命令行參數
skip_initramfs rootwait ro init=/init root="/dev/dm-0 dm=system none ro,0 1 android-verity "
例如Broadcom的7252SSFFDR3參考平臺的啟動Android系統的參數為：
mem=1016m@0m mem=1024m@2048m bmem=339m@669m bmem=237m@2048m \
brcm_cma=784m@2288m \
ramoops.mem_address=0x3F800000 ramoops.mem_size=0x800000 ramoops.console_size=0x400000 \
buildvariant=userdebug \
veritykeyid=id:7e4333f9bba00adfe0ede979e28ed1920492b40f buildvariant=eng \
rootwait init=/init ro \
root=/dev/dm-0 dm="system none ro,0 1 android-verity PARTUUID=c49e0acb-1b38-95e5-548a-2b7260e704a4" skip_initramfs
除去rootfs不相關的參數，看起來是這樣的：
rootwait init=/init ro root=/dev/dm-0 dm="system none ro,0 1 android-verity PARTUUID=c49e0acb-1b38-95e5-548a-2b7260e704a4" skip_initramfs
命令行中，文件系統的root設備由參數root="/dev/dm-0 dm=system none ro,0 1 android-verity "指定，顯然，這里的root參數設置將設備名設置為/dev/dm-0，至于設備/dev/dm-0是一個什么設備，作用為何，屬于另一個話題dm-verity，此處不再展開討論。
加載recovery系統的命令行參數
rootwait init=/init ro
例如Broadcom的7252SSFFDR3參考平臺的啟動Recovery的參數為：
mem=1016m@0m mem=1024m@2048m bmem=339m@669m bmem=237m@2048m \
brcm_cma=784m@2288m \
ramoops.mem_address=0x3F800000 ramoops.mem_size=0x800000 ramoops.console_size=0x400000 \
rootwait init=/init ro \
buildvariant=userdebug veritykeyid=id:7e4333f9bba00adfe0ede979e28ed1920492b40f buildvariant=eng
除去rootfs不相關的參數，看起來是這樣的：
rootwait init=/init ro
1.3 Android系統的啟動
linux啟動后，通過dm-verify機制校驗system分區，完成后加載system分區內包含的rootfs，通過/init程序解析/init.rc腳本，完成Android系統的啟動。
這部分的啟動過程跟傳統的系統啟動是一樣的。
1.4 Recovery系統的啟動
linux啟動后，根據參數，加載boot分區的ramdisk，通過/init程序解析/init.rc腳本，啟動Recovery系統。
這部分的啟動過程跟傳統的Recovery系統啟動是一樣的。
2. 系統的升級
A/B系統升級包的制作方式跟傳統系統升級包制作方式基本一致，主要分為兩步：
編譯系統文件
制作升級包
升級方式根據升級包的內容分為兩種：
完整升級，升級包包含完整的系統，對內容進行全新升級；
增量升級/差分升級，升級包僅包含跟當前系統不一樣的內容，對系統進行打補丁式升級；
2.1 完整升級
升級包的制作
第一步，編譯系統
$ source build/envsetup.sh
$ lunch bcm7252ssffdr4-userdebug
$ mkdir dist_output
$ make -j32 dist DIST_DIR=dist_output
[...]
$ ls -lh dist-output/*target_files*
-rw-r--r-- 1 ygu users 566M May 21 14:49 bcm7252ssffdr4-target_files-eng.ygu.zip
第二步，制作安裝包
$ ./build/tools/releasetools/ota_from_target_files dist-output/bcm7252ssffdr4-target_files-eng.ygu.zip full-ota.zip
$ ls -lh dist-output
-rw-r--r-- 1 ygu users 270M May 21 14:51 full-ota.zip
升級包的內容
解壓縮full-ota.zip可以看到其內容：
$ mkdir full-ota
$ unzip full-ota.zip -d full-ota
Archive: full-ota.zip
signed by SignApk
extracting: full-ota/payload.bin
inflating: full-ota/META-INF/com/android/metadata
inflating: full-ota/care_map.txt
inflating: full-ota/payload_properties.txt
inflating: full-ota/META-INF/com/android/otacert
inflating: full-ota/META-INF/MANIFEST.MF
inflating: full-ota/META-INF/CERT.SF
inflating: full-ota/META-INF/CERT.RSA
$ ls -lh full-ota
total 270M
drwxr-sr-x 3 ygu users 4.0K May 21 18:14 META-INF
-rw-r--r-- 1 ygu users 36 Jan 1 2009 care_map.txt
-rw-r--r-- 1 ygu users 270M Jan 1 2009 payload.bin
-rw-r--r-- 1 ygu users 154 Jan 1 2009 payload_properties.txt
$ tree -l full-ota
full-ota
|-- META-INF
| |-- CERT.RSA
| |-- CERT.SF
| |-- MANIFEST.MF
| `-- com
| `-- android
| |-- metadata
| `-- otacert
|-- care_map.txt
|-- payload.bin
`-- payload_properties.txt
3 directories, 8 files
其中，payload.bin是系統要更新的數據文件，payload_properties.txt包含了升級內容的一些屬性信息，如下：
$ cat full-ota/payload_properties.txt
FILE_HASH=ozGgyQEcnkI5ZaX+Wbjo5I/PCR7PEZka9fGd0nWa+oY=
FILE_SIZE=282164983
METADATA_HASH=GLIKfE6KRwylWMHsNadG/Q8iy5f7ENWTatvMdBlpoPg=
METADATA_SIZE=21023
升級時會使用到payload_properties.txt里面的信息。
系統包的使用
A/B系統在debug模式下會包含升級應用update_engine_client，其參數如下：
bcm7252ssffdr4:/ # which update_engine_client
/system/bin/update_engine_client
bcm7252ssffdr4:/ # update_engine_client --help
Android Update Engine Client
--cancel (Cancel the ongoing update and exit.) type: bool default: false
--follow (Follow status update changes until a final state is reached. Exit status is 0 if the update succeeded, and 1 otherwise.) type: bool default: false
--headers (A list of key-value pairs, one element of the list per line. Used when --update is passed.) type: string default: ""
--help (Show this help message) type: bool default: false
--offset (The offset in the payload where the CrAU update starts. Used when --update is passed.) type: int64 default: 0
--payload (The URI to the update payload to use.) type: string default: "http://127.0.0.1:8080/payload"
--reset_status (Reset an already applied update and exit.) type: bool default: false
--resume (Resume a suspended update.) type: bool default: false
--size (The size of the CrAU part of the payload. If 0 is passed, it will be autodetected. Used when --update is passed.) type: int64 default: 0
--suspend (Suspend an ongoing update and exit.) type: bool default: false
--update (Start a new update, if no update in progress.) type: bool default: false
將payload.bin文件放到可以通過http訪問的地方，然后在命令行調用update_engine_client進行升級：
bcm7252ssffdr4:/ # update_engine_client \
--payload=http://stbszx-bld-5/public/android/full-ota/payload.bin \
--update \
--headers="\
FILE_HASH=ozGgyQEcnkI5ZaX+Wbjo5I/PCR7PEZka9fGd0nWa+oY= \
FILE_SIZE=282164983
METADATA_HASH=GLIKfE6KRwylWMHsNadG/Q8iy5f7ENWTatvMdBlpoPg= \
METADATA_SIZE=21023 \
"
其中headers選項需要填寫payload_properties.txt文件的內容。
2.2 增量升級/差分升級
差分包升級與完整包升級除了升級包的制作不一樣之外，生成的升級包文件內容一樣，使用update_engine_client進行升級的操作也完全一樣，因此這里僅說明差分包的制作。
差分升級包的制作
第一步，對android進行改動并編譯系統
差分升級時，需要保留原有系統的生成文件，然后修改后生成新的系統文件，這里假定原有系統生成文件位于：dist_output，修改后生成的系統文件位于dist_output-new，編譯方式跟完整包的生成方式一樣。
$ source build/envsetup.sh
$ lunch bcm7252ssffdr4-userdebug
$ mkdir dist_output
$ make -j32 dist DIST_DIR=dist_output
[...]
$ ls -lh dist-output-new/*target_files*
-rw-r--r-- 1 ygu users 566M May 21 15:27 bcm7252ssffdr4-target_files-eng.ygu.zip
第二步，制作安裝包
對比原有系統文件和修改后的系統文件生成差分包，這里通過-i指定差分包生成的基線(baseline)。
$./build/tools/releasetools/ota_from_target_files \
-i dist-output/bcm7252ssffdr4-target_files-eng.ygu.zip \
dist-output-new/bcm7252ssffdr4-target_files-eng.ygu.zip \
incremental-ota.zip
2.3 升級日志樣本
調用update_engine_client進行升級后可以通過logcat查看其升級日志，如：
bcm7252ssffdr4:/ # update_engine_client \
--payload=http://stbszx-bld-5/public/android/full-ota/payload.bin \
--update \
--headers="\
FILE_HASH=ozGgyQEcnkI5ZaX+Wbjo5I/PCR7PEZka9fGd0nWa+oY= \
FILE_SIZE=282164983 \
METADATA_HASH=GLIKfE6KRwylWMHsNadG/Q8iy5f7ENWTatvMdBlpoPg= \
METADATA_SIZE=21023 \
"
bcm7252ssffdr4:/ #
bcm7252ssffdr4:/ # logcat -s update_engine:v
--------- beginning of main
--------- beginning of system
I update_engine: [INFO:main.cc(113)] Chrome OS Update Engine starting
I update_engine: [INFO:boot_control_android.cc(78)] Loaded boot_control HAL 'boot control hal for bcm platform' version 0.1 authored by 'Broadcom'.
I update_engine: [INFO:daemon_state_android.cc(43)] Booted in dev mode.
I update_engine: [INFO:update_attempter_android.cc(199)] Using this install plan:
I update_engine: [INFO:install_plan.cc(71)] InstallPlan: new_update, payload type: unknown, source_slot: A, target_slot: B, url: http://stbszx-bld-5/public/android/full-ota/payload.bin, payload size: 282164983, payload hash: ozGgyQEcnkI5ZaX+Wbjo5I/PCR7PEZka9fGd0nWa+oY=, metadata size: 21023, metadata signature: , hash_checks_mandatory: true, powerwash_required: false
W update_engine: [WARNING:hardware_android.cc(126)] STUB: Assuming OOBE is complete.
I update_engine: [INFO:cpu_limiter.cc(71)] Setting cgroup cpu shares to 2
E update_engine: [ERROR:utils.cc(199)] 0 == writer.Open(path, O_WRONLY | O_CREAT | O_TRUNC, 0600) failed: No such file or directory
E update_engine: [ERROR:cpu_limiter.cc(74)] Failed to change cgroup cpu shares to 2 using /sys/fs/cgroup/cpu/update-engine/cpu.shares
I update_engine: [INFO:update_attempter_android.cc(379)] Marking booted slot as good.
I update_engine: [INFO:update_attempter_android.cc(394)] Scheduling an action processor start.
I update_engine: [INFO:action_processor.cc(46)] ActionProcessor: starting InstallPlanAction
I update_engine: [INFO:action_processor.cc(116)] ActionProcessor: finished InstallPlanAction with code ErrorCode::kSuccess
I update_engine: [INFO:action_processor.cc(143)] ActionProcessor: starting DownloadAction
I update_engine: [INFO:install_plan.cc(71)] InstallPlan: new_update, payload type: unknown, source_slot: A, target_slot: B, url: http://stbszx-bld-5/public/android/full-ota/payload.bin, payload size: 282164983, payload hash: ozGgyQEcnkI5ZaX+Wbjo5I/PCR7PEZka9fGd0nWa+oY=, metadata size: 21023, metadata signature: , hash_checks_mandatory: true, powerwash_required: false
I update_engine: [INFO:download_action.cc(178)] Marking new slot as unbootable
I update_engine: [INFO:multi_range_http_fetcher.cc(45)] starting first transfer
I update_engine: [INFO:multi_range_http_fetcher.cc(73)] starting transfer of range 0+282164983
I update_engine: [INFO:libcurl_http_fetcher.cc(94)] Starting/Resuming transfer
I update_engine: [INFO:libcurl_http_fetcher.cc(106)] Using proxy: no
I update_engine: [INFO:libcurl_http_fetcher.cc(237)] Setting up curl options for HTTP
I update_engine: [INFO:delta_performer.cc(196)] Completed 0/? operations, 14169/282164983 bytes downloaded (0%), overall progress 0%
I update_engine: [INFO:delta_performer.cc(536)] Manifest size in payload matches expected value from Omaha
I update_engine: [INFO:delta_performer.cc(1396)] Verifying metadata hash signature using public key: /etc/update_engine/update-payload-key.pub.pem
I update_engine: [INFO:payload_verifier.cc(93)] signature blob size = 264
I update_engine: [INFO:payload_verifier.cc(112)] Verified correct signature 1 out of 1 signatures.
I update_engine: [INFO:delta_performer.cc(1439)] Metadata hash signature matches value in Omaha response.
I update_engine: [INFO:delta_performer.cc(1459)] Detected a 'full' payload.
I update_engine: [INFO:delta_performer.cc(374)] PartitionInfo old boot sha256: size: 0
I update_engine: [INFO:delta_performer.cc(374)] PartitionInfo new boot sha256: dZpLY9KsQYa2B14B0oBzfUKxVFIH7ocbgT70JavheSc= size: 19480576
I update_engine: [INFO:delta_performer.cc(374)] PartitionInfo old system sha256: size: 0
I update_engine: [INFO:delta_performer.cc(374)] PartitionInfo new system sha256: kFXbYzaM47PifNjuL+Plz1zTMEp1MoajOuXZuCh9yw0= size: 769654784
I update_engine: [INFO:delta_performer.cc(359)] Applying 10 operations to partition "boot"
I update_engine: [INFO:delta_performer.cc(647)] Starting to apply update payload operations
I update_engine: [INFO:delta_performer.cc(359)] Applying 367 operations to partition "system"
I update_engine: [INFO:delta_performer.cc(196)] Completed 23/377 operations (6%), 40302425/282164983 bytes downloaded (14%), overall progress 10%
I update_engine: [INFO:delta_performer.cc(196)] Completed 48/377 operations (12%), 79017817/282164983 bytes downloaded (28%), overall progress 20%
I update_engine: [INFO:delta_performer.cc(196)] Completed 74/377 operations (19%), 118519641/282164983 bytes downloaded (42%), overall progress 30%
I update_engine: [INFO:delta_performer.cc(196)] Completed 98/377 operations (25%), 158021465/282164983 bytes downloaded (56%), overall progress 40%
I update_engine: [INFO:delta_performer.cc(196)] Completed 121/377 operations (32%), 192001881/282164983 bytes downloaded (68%), overall progress 50%
I update_engine: [INFO:delta_performer.cc(196)] Completed 145/377 operations (38%), 231389017/282164983 bytes downloaded (82%), overall progress 60%
I update_engine: [INFO:delta_performer.cc(196)] Completed 171/377 operations (45%), 270890841/282164983 bytes downloaded (96%), overall progress 70%
I update_engine: [INFO:delta_performer.cc(196)] Completed 242/377 operations (64%), 273413977/282164983 bytes downloaded (96%), overall progress 80%
I update_engine: [INFO:delta_performer.cc(196)] Completed 317/377 operations (84%), 273430361/282164983 bytes downloaded (96%), overall progress 90%
I update_engine: [INFO:delta_performer.cc(196)] Completed 377/377 operations (100%), 282164983/282164983 bytes downloaded (100%), overall progress 100%
I update_engine: [INFO:delta_performer.cc(1336)] Extracted signature data of size 264 at 282143432
I update_engine: [INFO:multi_range_http_fetcher.cc(111)] terminating transfer
I update_engine: [INFO:multi_range_http_fetcher.cc(171)] Received transfer terminated.
I update_engine: [INFO:multi_range_http_fetcher.cc(123)] TransferEnded w/ code 206
I update_engine: [INFO:multi_range_http_fetcher.cc(157)] Done w/ all transfers
I update_engine: [INFO:delta_performer.cc(1596)] Verifying payload using public key: /etc/update_engine/update-payload-key.pub.pem
I update_engine: [INFO:payload_verifier.cc(93)] signature blob size = 264
I update_engine: [INFO:payload_verifier.cc(112)] Verified correct signature 1 out of 1 signatures.
I update_engine: [INFO:delta_performer.cc(1633)] Payload hash matches value in payload.
I update_engine: [INFO:action_processor.cc(116)] ActionProcessor: finished DownloadAction with code ErrorCode::kSuccess
I update_engine: [INFO:action_processor.cc(143)] ActionProcessor: starting FilesystemVerifierAction
I update_engine: [INFO:filesystem_verifier_action.cc(157)] Hashing partition 0 (boot) on device /dev/block/by-name/boot_e
I update_engine: [INFO:filesystem_verifier_action.cc(248)] Hash of boot: dZpLY9KsQYa2B14B0oBzfUKxVFIH7ocbgT70JavheSc=
I update_engine: [INFO:filesystem_verifier_action.cc(157)] Hashing partition 1 (system) on device /dev/block/by-name/system_e
I update_engine: [INFO:filesystem_verifier_action.cc(248)] Hash of system: kFXbYzaM47PifNjuL+Plz1zTMEp1MoajOuXZuCh9yw0=
I update_engine: [INFO:action_processor.cc(116)] ActionProcessor: finished FilesystemVerifierAction with code ErrorCode::kSuccess
I update_engine: [INFO:action_processor.cc(143)] ActionProcessor: starting PostinstallRunnerAction
I update_engine: [INFO:postinstall_runner_action.cc(341)] All post-install commands succeeded
I update_engine: [INFO:action_processor.cc(116)] ActionProcessor: finished last action PostinstallRunnerAction with code ErrorCode::kSuccess
I update_engine: [INFO:update_attempter_android.cc(282)] Processing Done.
I update_engine: [INFO:update_attempter_android.cc(291)] Update successfully applied, waiting to reboot.
以上logcat信息已經去掉了時間戳，原始的log信息請參考：update_engine_client log
update_engine更新操作成功后會提示Update successfully applied, waiting to reboot.，要求系統進行重啟，重啟后會設置相應分區slot的屬性為successful，表明系統能夠成功啟動。
重啟系統，檢查Android系統的編譯版本和時間戳，驗證升級是否成功。
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