[Android][Recovery]自动挂载system分区

前一篇Recovery打开adb shell里提到system目录是用来挂载系统/system分区的，所以是一个空目录。这一点是通过打开adb shell后，查看文件目录知道的。

因为原本cache目录也是空的，但是通过shell能看到/cache目录里的文件和Android系统是一致的，也就说明，这个目录是挂载进来的。因此推测system也应该是用来挂载的。

关于挂载/system，在Recovery里有实现代码动态挂载：

// bootable/recovery/recovery.cpp
case Device::MOUNT_SYSTEM:
    char system_root_image[PROPERTY_VALUE_MAX];
    property_get("ro.build.system_root_image", system_root_image, "");

    // For a system image built with the root directory (i.e.
    // system_root_image == "true"), we mount it to /system_root, and symlink /system
    // to /system_root/system to make adb shell work (the symlink is created through
    // the build system).
    // Bug: 22855115
    if (strcmp(system_root_image, "true") == 0) {
        if (ensure_path_mounted_at("/", "/system_root") != -1) {
            ui->Print("Mounted /system.\n");
        }
    } else {
        if (ensure_path_mounted("/system") != -1) {
            ui->Print("Mounted /system.\n");
        }
    }

    break;

但是实现开机挂载，我们就要去看init.rc了。

mvim /bootable/recovery/etc/init.rc

import /init.recovery.${ro.hardware}.rc

on early-init
    # Set the security context of /postinstall if present.
    restorecon /postinstall

    start ueventd
    start healthd

on init
    export ANDROID_ROOT /system
    export ANDROID_DATA /data
    export EXTERNAL_STORAGE /sdcard

    symlink /system/etc /etc

    mkdir /sdcard
    mkdir /udisk
    mkdir /system
    mkdir /data
    mkdir /cache
    mkdir /sideload
    mkdir /firmware
    mount tmpfs tmpfs /tmp

    chown root shell /tmp
    chmod 0775 /tmp

    write /proc/sys/kernel/panic_on_oops 1
    write /proc/sys/vm/max_map_count 1000000

on fs
    mkdir /dev/usb-ffs 0770 shell shell
    mkdir /dev/usb-ffs/adb 0770 shell shell
    mount functionfs adb /dev/usb-ffs/adb uid=2000,gid=2000

    write /sys/class/android_usb/android0/enable 0
    write /sys/class/android_usb/android0/idVendor 18D1
    write /sys/class/android_usb/android0/idProduct D001
    write /sys/class/android_usb/android0/f_ffs/aliases adb
    write /sys/class/android_usb/android0/functions adb
    write /sys/class/android_usb/android0/iManufacturer ${ro.product.manufacturer}
    write /sys/class/android_usb/android0/iProduct ${ro.product.model}
    write /sys/class/android_usb/android0/iSerial ${ro.serialno}

首先会去访问init.recovery.${ro.hardware}.rc的内容，这个文件不存在，然后后面的流程也没有做mount操作。

作为对比，看一下Android系统的init文件

mvim /device/qcom/xxxx/imit.target.rc

on fs
    wait /dev/block/bootdevice
    mount_all fstab.qcom
    write /proc/sys/vm/swappiness 100

好了，Android系统启动的时候做了一个mount_all操作。来看一下fstab.qcom文件

# Android fstab file.
# The filesystem that contains the filesystem checker binary (typically /system) cannot
# specify MF_CHECK, and must come before any filesystems that do specify MF_CHECK

#TODO: Add 'check' as fs_mgr_flags with data partition.
# Currently we dont have e2fsck compiled. So fs check would failed.

#                                                                         
/dev/block/bootdevice/by-name/system         /system      ext4    ro,barrier=1,discard                                wait, verify
/dev/block/bootdevice/by-name/userdata       /data        ext4    nosuid,nodev,barrier=1,noauto_da_alloc,discard      wait,forceencrypt=footer
/devices/soc/7864900.sdhci/mmc_host*        /storage/sdcard1   vfat    nosuid,nodev         wait,voldmanaged=sdcard1:auto,noemulatedsd,encryptable=footer
/devices/soc/78db000.usb/msm_hsusb_host*    /storage/usbotg    vfat    nosuid,nodev         wait,voldmanaged=usbotg:auto
/dev/block/bootdevice/by-name/config         /frp  emmc    defaults                                            defaults
/dev/block/bootdevice/by-name/misc          /misc              emmc    defaults             defaults

我们的system就在这个表里。所以要挂载system，只需要在Recovery的init.rc添加：

wait /dev/block/bootdevice/by-name/system
mount ext4 /dev/block/bootdevice/by-name/system /system wait rw barrier=1

然后还要注意，挂载点应该（没专门验证）是空目录，所以如果在Recovery开启了shell，要把它删掉。