UBI read only file system

By Toradex Hu Shanfeng

In the previous article, we have introduced the use of  eMMC  File system with read-only attribute configured on the module of , And the use of  squashfs and overlayfs  Mount writable partition .Toradex  In addition to using  eMMC  Out of storage , The other part is to adopt  Nand Flash, for example  Colibri iMX7  and  Colibri iMX6ULL. The following will be  Colibri iMX7  As an example, this paper introduces how to configure a file system with read-only attribute .

Due to different storage media ,Nand Flash  Generally, such as  jffs2、UBI  Equal format file system .Toradex  Of  Linux  System use  UBI  file system .

stay  Colibri iMX7  Of  Nand Flash  The following plan will be adopted .Nand Flash  In general, it is divided into two parts . The front of  raw  Part does not use any file system , Direct storage module hardware information  bcb,u-boot  and  u-boot  environment variable . The second part uses  UBI, establish  5  individual  volume, Used to store kernel files （kernel）、 Device tree file （dtb）、M4  The firmware of （m4-fw）、Linux  file system （rootfs）、 User files （userdata）. among  rootfs  Set to self reading property , and  userdata  You can write data .

Toradex Easy Installer  Can pass  image.json  File easily modify partition , Thus avoiding the use of command tools . First download from here for  Colibri iMX7S  Of  Linux BSP v5.x  The installation files . After decompression  image.json  Add  userdata  Related configuration of .

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             {

                    "name": "rootfs",

                    "content": {

                        "filesystem_type": "ubifs",

                        "filename": "Reference-Minimal-Image-colibri-imx7.tar.xz",

                        "uncompressed_size": 108.1171875

                    },

                    "size_kib": 102400

                },

                {

                    "name": "userdata",

                    "content": {

                        "filesystem_type": "ubifs",

                        "filename": "app.tar.xz",

                        "uncompressed_size": 0.1171875

                    }

                }

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here  name  Appoint  ubi volume  Name ,filesystem_type  Is used to specify the  ubifs  File format ,filename  It includes the need to burn to  userdata volume  The file of , These are user applications and configurations ,uncompressed_size  Refer to  app.tar.xz  Uncompressed size , Used for display  Toradex Easy Installer  Installation progress bar . More about  image.json  Please refer to here .

Use  Toradex Easy Installer  Burn the image modified above to  Colibri iMX7S  that will do . Start and enter  u-boot, Use the following name to see the created  volume.

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Colibri iMX7 # ubi part ubi

Colibri iMX7 # ubi info  layout

Volume information dump:

        vol_id          0

......

        name            kernel

Volume information dump:

        vol_id          1

......

        skip_check      0

        name            dtb

Volume information dump:

        vol_id          2

......

        skip_check      0

        name            m4firmware

Volume information dump:

        vol_id          3

......

        skip_check      0

        name            rootfs

Volume information dump:

        vol_id          4

......

        skip_check      0

        name            userdata

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Activate access  Linux  after ,userdata  It will not be automatically mounted , You need to add the following to  /etc/fstab  In file . Now?  rootfs  The root directory has not been set as read-only , You can create  /home/root/userdata  The directory is used to mount  userdata  volume .

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ubi:userdata         /home/root/userdata  ubifs      defaults,noatime,rw   1  1

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At the same time , You can also configure the system . For example, add a startup application . The application  write_to_file  When running, it will go  /home/root/userdata  Write a file . stay  /etc/systemd/system/  Create under directory  user-demo.service.

user-demo.service

-----------------------------------

[Unit]

Description=launch user's demo on dedicated partition

ConditionFileIsExecutable=/home/root/userdata/write_to_file

After=multi-user.target

[Service]

WorkingDirectory=/home/root/userdata

Type=simple

ExecStart=/home/root/userdata/write_to_file

[Install]

WantedBy=multi-user.target

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Run the following command to make  user-demo.service  Boot operation . Then restart the system .

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~# systemctl enable user-demo.service

~# reboot

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here , Use  mount  Command to view the mounted volume , Among them is  ubi:userdata.

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~# mount -l

tmpfs on /var/volatile type tmpfs (rw,relatime)

ubi:userdata on /home/root/userdata type ubifs (rw,noatime,assert=read-only,ubi=0,vol=4)

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stay  /home/root/userdata  You can also see  write_to_file  Written file  file.txt.

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~/userdata# ls

file.txt       write_to_file

~/userdata# cat file.txt

This is a writing file test

~/userdata# systemctl status user-demo.service

* user-demo.service - launch user's demo on dedicated partition

     Loaded: loaded (/etc/systemd/system/user-demo.service; enabled; vendor preset: disabled)

     Active: inactive (dead) since Wed 2022-07-06 06:09:44 UTC; 14min ago

    Process: 316 ExecStart=/home/root/userdata/write_to_file (code=exited, status=0/SUCCESS)

   Main PID: 316 (code=exited, status=0/SUCCESS)

Jul 06 06:09:44 colibri-imx7-02873356 systemd[1]: Started launch user's demo on dedicated partition.

Jul 06 06:09:44 colibri-imx7-02873356 systemd[1]: user-demo.service: Succeeded.

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Finally, it needs to be modified again  /etc/fstab  take  rootfs  The root directory is set to read-only property ,noatime  Add later  ro.

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/dev/root            /                    auto       noatime,ro            1  1

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Restart the system , Get into  u-boot  Command mode , Configure lower parameters .

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setenv ubiargs "ubi.mtd=ubi root=ubi0:rootfs ro rootfstype=ubifs ubi.fm_autoconvert=1"

saveenv

reset

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Restart access  Linux  System . root directory  /  It is already read-only , Unable to create file . and  /home/root/userdata  Applications under the directory can still execute normally and write files .

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:~# mount -l

ubi0:rootfs on / type ubifs (ro,noatime,assert=read-only,ubi=0,vol=3)

~# mkdir test

mkdir: can't create directory 'test': Read-only file system

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summary

By way of Linux  The system file of is set to read-only , It can reduce the probability of failure to start due to file system corruption . For more demanding applications , You can even use external storage as a backup , For restoring files .