Android 启动时判断overlay fs是否挂载

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一、背景

Android新版本使用super分区替代原来的system、vendor后，就采用了overlayfs文件系统。这种文件系统在执行adb remount 后，修改system 、vendor分区内容并不是真正存储在原来的位置，而是单独利用super剩余空间或data分区存了一份新的，原来的文件并没有改变。系统使用时判断有overlay的就用新的，没有就是有原文件。具体概念可查看https://www.cnblogs.com/loongson-artc-lyc/p/15981855.html 了解。

这就带来一个问题，调试时使用adb push 更新了需要开机下载的固件，但是开机过程中不是一开始就挂载overlay fs的，这就可能导致加载的还是老的固件。我们需要在kernel中判断何时挂载了overlay fs再去加载固件，就可以解决这个问题。

二、Init阶段overlay fs挂载前后阶段分析

adb remount log

adb remount[libfs_mgr]fs_mgr_do_format: Format /dev/block/dm-5 as 'f2fs'
Using overlayfs for /vendor
Now reboot your device for settings to take effect
remount succeeded

init 启动代码

system/core/init/main.cppint main(int argc, char** argv) {
#if __has_feature(address_sanitizer)__asan_set_error_report_callback(AsanReportCallback);
#endifif (!strcmp(basename(argv[0]), "ueventd")) {return ueventd_main(argc, argv);}if (argc > 1) {if (!strcmp(argv[1], "subcontext")) {android::base::InitLogging(argv, &android::base::KernelLogger);const BuiltinFunctionMap& function_map = GetBuiltinFunctionMap();return SubcontextMain(argc, argv, &function_map);}if (!strcmp(argv[1], "selinux_setup")) {return SetupSelinux(argv);}if (!strcmp(argv[1], "second_stage")) {return SecondStageMain(argc, argv);}}return FirstStageMain(argc, argv);
}

 上面函数会执行多次，执行FirstStageMain时会调用DoFirstStageMount跟据fstab挂载物理分区

和overlay fs

system/core/init/first_stage_init.cppint FirstStageMain(int argc, char** argv) {if (REBOOT_BOOTLOADER_ON_PANIC) {InstallRebootSignalHandlers();}boot_clock::time_point start_time = boot_clock::now();....if (access("/force_debuggable", F_OK) == 0) {std::error_code ec;  // to invoke the overloaded copy_file() that won't throw.if (!fs::copy_file("/adb_debug.prop", kDebugRamdiskProp, ec) ||!fs::copy_file("/userdebug_plat_sepolicy.cil", kDebugRamdiskSEPolicy, ec)) {LOG(ERROR) << "Failed to setup debug ramdisk";} else {// setenv for second-stage init to read above kDebugRamdisk* files.setenv("INIT_FORCE_DEBUGGABLE", "true", 1);}}if (!DoFirstStageMount()) {LOG(FATAL) << "Failed to mount required partitions early ...";}...}system/core/init/first_stage_mount.cppbool FirstStageMount::DoFirstStageMount() {if (!IsDmLinearEnabled() && fstab_.empty()) {// Nothing to mount.LOG(INFO) << "First stage mount skipped (missing/incompatible/empty fstab in device tree)";return true;}if (!InitDevices()) return false;if (!MountPartitions()) return false;return true;
}bool FirstStageMount::MountPartitions() {...// If we don't see /system or / in the fstab, then we need to create an root entry for// overlayfs.if (!GetEntryForMountPoint(&fstab_, "/system") && !GetEntryForMountPoint(&fstab_, "/")) {FstabEntry root_entry;if (GetRootEntry(&root_entry)) {fstab_.emplace_back(std::move(root_entry));}}// heads up for instantiating required device(s) for overlayfs logicauto init_devices = [this](std::set<std::string> devices) -> bool {for (auto iter = devices.begin(); iter != devices.end();) {if (android::base::StartsWith(*iter, "/dev/block/dm-")) {if (!block_dev_init_.InitDmDevice(*iter)) {return false;}iter = devices.erase(iter);} else {iter++;}}return InitRequiredDevices(std::move(devices));};...}

执行SecondStageMain会创建/dev/.booting和/dev/__properties__文件

system/core/init/init.cppint SecondStageMain(int argc, char** argv) {...close(open("/dev/.booting", O_WRONLY | O_CREAT | O_CLOEXEC, 0000));...PropertyInit();//此函数会创建/dev/__properties__...}

 所以只需要我们在init的第二个阶段去加载固件就可以保证overlay fs已挂载，加载的是新的固件。

判断是否处于第二阶段可以判断/dev/.booting或/dev/__properties__是否存在即可。

三、内核增加判断示例

//#define SECOND_INIT_STAGE_FILE "/dev/.booting" //this file will be deleted when firmware load
#define SECOND_INIT_STAGE_FILE "/dev/__properties__" //this is a dir created at second initfile = filp_open(SECOND_INIT_STAGE_FILE, O_RDONLY, 0);
if (IS_ERR(file)){dev_info(sdev->dev, "%s: open '%s' failed, ret %d\n",__func__, SECOND_INIT_STAGE_FILE, ret);} else {filp_close(file,NULL);
}或bool folder_exists(struct path *path, const char *name) {struct path lookup_path = *path; // 初始路径struct qstr qname = QSTR_INIT(name, strlen(name));int err = path_lookupat(lookup_path.dentry, lookup_path.mnt, qname, &lookup_path);if (err == 0) {// 文件夹存在path_put(&lookup_path); // 释放路径return true;}return false; // 文件夹不存在
}或error = vfs_stat(path,&stat)if(error) {return -ENOENT;//路径不存在
}
if(S_ISDIR(stat.mode)){//是目录
}
else if(S_ISREG(stat.mode)) {//是文件
}else {//其他类型
}

参考：Android 动态分区详解(七) overlayfs 与 adb remount 操作_android overlayfs-CSDN博客

android 系统相关_android super分区-CSDN博客

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