Android Native 之文件系统挂载

一、文件系统挂载流程概述

二、文件系统挂载流程细节

1、Init启动阶段

众所周知，init进程为android系统的第一个进程，也是native世界的开端，要想让整个android世界能够稳定的运行，文件系统的创建和初始化是必不可少的，这个过程需要在android世界的前面。

//aosp/system/core/init/first_stage_init.cpp
int FirstStageMain(int argc, char** argv) {LOG(INFO) << "init first stage started!";//.....bool created_devices = false;if (want_console == FirstStageConsoleParam::CONSOLE_ON_FAILURE) {if (!IsRecoveryMode()) {//第一步：创建设备created_devices = DoCreateDevices();if (!created_devices) {LOG(ERROR) << "Failed to create device nodes early";}}StartConsole(cmdline);}//......//第二步：挂载设备if (!DoFirstStageMount(!created_devices)) {LOG(FATAL) << "Failed to mount required partitions early ...";}//.....
}
//aosp/system/core/init/first_stage_mount.cpp
// Public functions公共函数
// Creates devices and logical partitions from storage devices
bool DoCreateDevices() {auto fsm = FirstStageMount::Create();if (!fsm.ok()) {LOG(ERROR) << "Failed to create FirstStageMount: " << fsm.error();return false;}//来创建设备/即初始化磁盘逻辑分区return (*fsm)->DoCreateDevices();
}
// Mounts partitions specified by fstab in device tree.
bool DoFirstStageMount(bool create_devices) {// Skips first stage mount if we're in recovery mode.if (IsRecoveryMode()) {LOG(INFO) << "First stage mount skipped (recovery mode)";return true;}auto fsm = FirstStageMount::Create();if (!fsm.ok()) {LOG(ERROR) << "Failed to create FirstStageMount " << fsm.error();return false;}if (create_devices) {if (!(*fsm)->DoCreateDevices()) return false;}//来进行文件系统的挂载return (*fsm)->DoFirstStageMount();
}
//void SetInitAvbVersionInRecovery() 第三个public函数，看起来是和recovery有关系的

Init进程的通过FirstStageMount::Create()来拿到一个fsm对象，然后依次调用fsm的DoCreateDevices和DoFirstStageMount来初始化挂载文件系统。

1.1 FirstStageMount::Create读取fstab配置表

此步骤主要是读取fstab分区配置表，具体实现逻辑其实移交给了fs_mgr

//aosp/system/core/init/first_stage_mount.cpp
using android::fs_mgr::ReadDefaultFstab;
using android::fs_mgr::ReadFstabFromDt;
Result<std::unique_ptr<FirstStageMount>> FirstStageMount::Create() {//读取fstab配置表，此表配置了各个目录支持的文件系统相关配置auto fstab = ReadFirstStageFstab();if (!fstab.ok()) {return fstab.error();}return std::make_unique<FirstStageMountVBootV2>(std::move(*fstab));
}
static Result<Fstab> ReadFirstStageFstab() {Fstab fstab;//从DT里面获取，DT好像跟内核有关系，没有具体研究if (!ReadFstabFromDt(&fstab)) {//读取默认的fstab配置表if (ReadDefaultFstab(&fstab)) {fstab.erase(std::remove_if(fstab.begin(), fstab.end(),[](const auto& entry) { return !entry.fs_mgr_flags.first_stage_mount; }),fstab.end());} else {return Error() << "failed to read default fstab for first stage mount";}}return fstab;
}

fs_mgr被编译成为静态库lib_fs_mgr，这部分逻辑其实就是读取fstab.ini配置文件并进行解析：

//aosp/system/core/fs_mgr/fs_mgr_fstab.cpp
// Loads the fstab file and combines with fstab entries passed in from device tree.
bool ReadDefaultFstab(Fstab* fstab) {fstab->clear();ReadFstabFromDt(fstab, false /* verbose */);std::string default_fstab_path;// Use different fstab paths for normal boot and recovery boot, respectivelyif ((access("/sbin/recovery", F_OK) == 0) || (access("/system/bin/recovery", F_OK) == 0)) {//recovery模式下读取/etc/recovery.fstabdefault_fstab_path = "/etc/recovery.fstab";} else { //正常模式下读取类似于/odm/etc/fstab.default_fstab_path = GetFstabPath();}Fstab default_fstab;if (!default_fstab_path.empty() && ReadFstabFromFile(default_fstab_path, &default_fstab)) {for (auto&& entry : default_fstab) {fstab->emplace_back(std::move(entry));}} else {LINFO << __FUNCTION__ << "(): failed to find device default fstab";}return !fstab->empty();
}
// Return the path to the fstab file.  There may be multiple fstab files; the
// one that is returned will be the first that exists of fstab.<fstab_suffix>,
// fstab.<hardware>, and fstab.<hardware.platform>.  The fstab is searched for
// in /odm/etc/ and /vendor/etc/, as well as in the locations where it may be in
// the first stage ramdisk during early boot.  Previously, the first stage
// ramdisk's copy of the fstab had to be located in the root directory, but now
// the system/etc directory is supported too and is the preferred location.
std::string GetFstabPath() {for (const char* prop : {"fstab_suffix", "hardware", "hardware.platform"}) {std::string suffix;if (!fs_mgr_get_boot_config(prop, &suffix)) continue;for (const char* prefix : {// late-boot/post-boot locations"/odm/etc/fstab.", "/vendor/etc/fstab.",// early boot locations"/system/etc/fstab.", "/first_stage_ramdisk/system/etc/fstab.","/fstab.", "/first_stage_ramdisk/fstab."}) {std::string fstab_path = prefix + suffix;if (access(fstab_path.c_str(), F_OK) == 0) {return fstab_path;}}}return "";
}

如下Android 14的手机的开机日志，在init阶段来读取fstab配置表的打印：这里的dt没有配置fstab，默认路径没有任何打印，但是可以了解到libfs_mgr的入口

1.2 fstab文件是什么样子的？

android系统的文件系统相关参数定义被统一放在fstab.in里面，从上面的流程可以了解到fs_mgr会去读取fstab.*文件，并根据此文件配置的内容去逐一挂载所有的分区，那么它到底长什么样子的呢？

首先cat /vendor/etc/fstab.mtxxxx内容如下：

D50:/vendor/etc # cat fsta
fstab.enableswap  fstab.mt6765      fstab.mt8768
D50:/vendor/etc # cat fstab.mt6765
# 1 "vendor/mediatek/proprietary/hardware/fstab/mt6765/fstab.in.mt6765"
# 1 "<built-in>" 1
# 1 "<built-in>" 3
# 341 "<built-in>" 3
# 1 "<command line>" 1
# 1 "<built-in>" 2
# 1 "vendor/mediatek/proprietary/hardware/fstab/mt6765/fstab.in.mt6765" 2
# 145 "vendor/mediatek/proprietary/hardware/fstab/mt6765/fstab.in.mt6765"
system /system ext4 ro wait,avb=vbmeta_system,logical,first_stage_mount,avb_keys=/avb/q-gsi.avbpubkey:/avb/r-gsi.avbpubkey:/avb/s-gsi.avbpubkey,slotselect
system_ext /system_ext ext4 ro wait,avb=vbmeta_system,logical,first_stage_mount,avb_keys=/avb/q-gsi.avbpubkey:/avb/r-gsi.avbpubkey:/avb/s-gsi.avbpubkey,slotselectvendor /vendor ext4 ro wait,avb,logical,first_stage_mount,slotselectproduct /product ext4 ro wait,avb,logical,first_stage_mount,slotselect
# 170 "vendor/mediatek/proprietary/hardware/fstab/mt6765/fstab.in.mt6765"
/dev/block/by-name/md_udc /metadata ext4 noatime,nosuid,nodev,discard wait,check,formattable,first_stage_mount/dev/block/by-name/userdata /data f2fs noatime,nosuid,nodev,discard,noflush_merge,reserve_root=134217,resgid=1065,inlinecrypt latemount,wait,check,quota,reservedsize=128M,formattable,resize,,checkpoint=fs,fileencryption=aes-256-xts:aes-256-cts:v2,keydirectory=/metadata/vold/metadata_encryption/dev/block/by-name/protect1 /mnt/vendor/protect_f ext4 noatime,nosuid,nodev,noauto_da_alloc,commit=1,nodelalloc wait,check,formattable
/dev/block/by-name/protect2 /mnt/vendor/protect_s ext4 noatime,nosuid,nodev,noauto_da_alloc,commit=1,nodelalloc wait,check,formattable
/dev/block/by-name/nvdata /mnt/vendor/nvdata ext4 noatime,nosuid,nodev,noauto_da_alloc,commit=1,nodelalloc wait,check,formattable
/dev/block/by-name/nvcfg /mnt/vendor/nvcfg ext4 noatime,nosuid,nodev,noauto_da_alloc,commit=1,nodelalloc wait,check,formattable/dev/block/by-name/persist /mnt/vendor/persist ext4 noatime,nosuid,nodev,noauto_da_alloc,commit=1,nodelalloc wait,check,formattable/devices/platform/externdevice* auto auto defaults voldmanaged=sdcard1:auto,encryptable=userdata/devices/platform/mt_usb* auto vfat defaults voldmanaged=usbotg:auto/dev/block/by-name/frp /persistent emmc defaults defaults/dev/block/by-name/nvram /nvram emmc defaults defaults
/dev/block/by-name/proinfo /proinfo emmc defaults defaults
/dev/block/by-name/lk /bootloader emmc defaults defaults
/dev/block/by-name/lk2 /bootloader2 emmc defaults defaults
/dev/block/by-name/para /misc emmc defaults defaults/dev/block/by-name/boot /boot emmc defaults first_stage_mount,nofail,slotselect
# 210 "vendor/mediatek/proprietary/hardware/fstab/mt6765/fstab.in.mt6765"
/dev/block/by-name/vbmeta_vendor /vbmeta_vendor emmc defaults first_stage_mount,nofail,slotselect
/dev/block/by-name/vbmeta_system /vbmeta_system emmc defaults first_stage_mount,nofail,slotselect,avb=vbmeta/dev/block/by-name/logo /logo emmc defaults defaults
/dev/block/by-name/expdb /expdb emmc defaults defaults
/dev/block/by-name/seccfg /seccfg emmc defaults defaults/dev/block/by-name/tee1 /tee1 emmc defaults defaults
/dev/block/by-name/tee2 /tee2 emmc defaults defaults/dev/block/by-name/scp1 /scp1 emmc defaults defaults
/dev/block/by-name/scp2 /scp2 emmc defaults defaults/dev/block/by-name/sspm_1 /sspm_1 emmc defaults defaults
/dev/block/by-name/sspm_2 /sspm_2 emmc defaults defaults/dev/block/by-name/md1img /md1img emmc defaults defaults
/dev/block/by-name/md1dsp /md1dsp emmc defaults defaults
/dev/block/by-name/md1arm7 /md1arm7 emmc defaults defaults
/dev/block/by-name/md3img /md3img emmc defaults defaults/dev/block/by-name/gz1 /gz1 emmc defaults defaults
/dev/block/by-name/gz2 /gz2 emmc defaults defaults/dev/block/by-name/spmfw /spmfw emmc defaults defaults/dev/block/by-name/boot_para /boot_para emmc defaults defaults
/dev/block/by-name/odmdtbo /odmdtbo emmc defaults defaults
/dev/block/by-name/dtbo /dtbo emmc defaults defaults/dev/block/by-name/vbmeta /vbmeta emmc defaults defaults
D50:/vendor/etc #

如上格式，此文件可以解析如下三部分

那么我们在源代码是如何配置的呢？MTK可以参考如下逻辑，后文详细解读各大配置参数

1.3 FirstStageMount::DoCreateDevices

//aosp/system/core/init/first_stage_mount.cpp
bool FirstStageMount::DoCreateDevices() {if (!InitDevices()) return false;// Mount /metadata before creating logical partitions, since we need to// know whether a snapshot merge is in progress.auto metadata_partition = std::find_if(fstab_.begin(), fstab_.end(), [](const auto& entry) {//从fstab配置表中寻找/metadata分区的信息，此分区很重要存储了一些元数据和秘钥相关的东西return entry.mount_point == "/metadata";});if (metadata_partition != fstab_.end()) {//首先需要挂载/metadata分区，因为它太重要了if (MountPartition(metadata_partition, true /* erase_same_mounts */)) {// Copies DSU AVB keys from the ramdisk to /metadata.// Must be done before the following TrySwitchSystemAsRoot().// Otherwise, ramdisk will be inaccessible after switching root.//它为什么重要，就是因为拷贝AVB Key到这个目录，详细的待后续研究CopyDsuAvbKeys();}}//创建逻辑分区if (!CreateLogicalPartitions()) return false;return true;
}

流程1：如上逻辑首先挂载了/metadata分区，为什么要先挂载它？

流程2：/metadata分区挂载流程对应日志：注意这里调用了metadata_partition函数传递了参数，所以只挂载了一个分区

流程3：创建逻辑分区，那么何为逻辑分区？从下面日志来看个人理解它类似与PC的C盘来区别于其他磁盘，因此这里的逻辑分区通常为system/vendor几个目录

如上日志对应逻辑代码如下：

1.4 FirstStageMount::DoFirstStageMount

//aosp/system/core/init/first_stage_mount.cpp
bool FirstStageMount::DoFirstStageMount() {if (!IsDmLinearEnabled() && fstab_.empty()) {// Nothing to mount.LOG(INFO) << "First stage mount skipped (missing/incompatible/empty fstab in device tree)";return true;}//挂载分区if (!MountPartitions()) return false;    return true;
}

这里的主要流程还是去调用MountPartitions()去挂载分区，注意这里不像metadata哪里传递了参数，因此这里是根据fstab表去挂载所有其他分区，代码如下：

2、fstab文件参数解读

3、fs_mgr挂载分区

接着init的FirstStageMount::MountPartition通过fstab表来挂载单个分区，如下逻辑，在对底层设备块相关初始化之后通过fs_mgr来进行单个分区的挂载。

//aosp/system/core/init/first_stage_mount.cpp
bool FirstStageMount::MountPartition(const Fstab::iterator& begin, bool erase_same_mounts, Fstab::iterator* end) {// Sets end to begin + 1, so we can just return on failure below.if (end) {*end = begin + 1;}if (!fs_mgr_create_canonical_mount_point(begin->mount_point)) {return false;}//跟底层设备块有关系，暂时没有深入研究if (begin->fs_mgr_flags.logical) {if (!fs_mgr_update_logical_partition(&(*begin))) {return false;}if (!block_dev_init_.InitDmDevice(begin->blk_device)) {return false;}}if (!SetUpDmVerity(&(*begin))) {PLOG(ERROR) << "Failed to setup verity for '" << begin->mount_point << "'";return false;}//核心代码，通过fs_mgr去进行挂载bool mounted = (fs_mgr_do_mount_one(*begin) == 0);// Try other mounts with the same mount point.Fstab::iterator current = begin + 1;for (; current != fstab_.end() && current->mount_point == begin->mount_point; current++) {if (!mounted) {// blk_device is already updated to /dev/dm-<N> by SetUpDmVerity() above.// Copy it from the begin iterator.current->blk_device = begin->blk_device;mounted = (fs_mgr_do_mount_one(*current) == 0);}}if (erase_same_mounts) {current = fstab_.erase(begin, current);}if (end) {*end = current;}return mounted;
}
//aosp/system/core/fs_mgr/fs_mgr.cpp
// wrapper to __mount() and expects a fully prepared fstab_rec,
// unlike fs_mgr_do_mount which does more things with avb / verity etc.
int fs_mgr_do_mount_one(const FstabEntry& entry, const std::string& alt_mount_point) {// First check the filesystem if requested.if (entry.fs_mgr_flags.wait && !WaitForFile(entry.blk_device, 20s)) {LERROR << "Skipping mounting '" << entry.blk_device << "'";}auto& mount_point = alt_mount_point.empty() ? entry.mount_point : alt_mount_point;//步骤1：挂载前的准备工作，其实就是解析fstab分区配置的各种参数int ret = prepare_fs_for_mount(entry.blk_device, entry, mount_point);// Wiped case doesn't require to try __mount below.if (ret & FS_STAT_INVALID_MAGIC) {return FS_MGR_DOMNT_FAILED;}//步骤2：正式进行文件分区的挂载ret = __mount(entry.blk_device, mount_point, entry);if (ret) {ret = (errno == EBUSY) ? FS_MGR_DOMNT_BUSY : FS_MGR_DOMNT_FAILED;}return ret;
}

如上代码进行总结如下：