[Buildroot] Buildroot way to populate additional partition

[Arnout Vandecappelle]

 Hi Andrey,

On 02/07/2021 15:04, Andrey Nechypurenko wrote:
> Hi Buildrooters,
> 
> I am building a system where rootfs should be read-only. However, it
> should be possible to store user data (application specific data, wifi
> configurations, etc.). For these purposes, I decided to use overlayfs.
> In particular, Buildroot is configured not to remount rootfs with
> read/write permission and /etc /var and /home should be mounted using
> overlayfs with the upper filesystem located on the separate writable
> partition. Here is how my fstab looks like:
> 
> # <file system>    <mount pt>    <type>    <options>    <dump>    <pass>
> /dev/root    /        ext4    ro,noauto    0    1
> proc        /proc        proc    defaults    0    0
> devpts        /dev/pts    devpts
> defaults,gid=5,mode=620,ptmxmode=0666    0    0
> tmpfs        /dev/shm    tmpfs    mode=0777    0    0
> tmpfs        /tmp        tmpfs    mode=1777    0    0
> tmpfs        /run        tmpfs    mode=0755,nosuid,nodev    0    0
> sysfs        /sys        sysfs    defaults    0    0
> /dev/mmcblk0p6    /overlay    ext4    rw        0    1
> overlay        /etc        overlay
> lowerdir=/etc,upperdir=/overlay/etc,workdir=/overlay/work_etc    0
> 2
> overlay        /var        overlay
> lowerdir=/var,upperdir=/overlay/var,workdir=/overlay/work_var    0
> 2
> overlay        /home        overlay
> lowerdir=/home,upperdir=/overlay/home,workdir=/overlay/work_home    0
> 
> And this is my genimage.cfg.template:
> image sdcard.img {
>     hdimage {
>         gpt = "true"    }
>     partition fsbl1 {
>         image = "%ATFBIN%"    }
>     partition fsbl2 {
>         image = "%ATFBIN%"    }
>     partition ssbl {
>         image = "u-boot.stm32"    }
>     partition env {
>         size = 16K    }
>     partition rootfs {
>         image = "rootfs.ext4"
>         bootable = "yes"    }
>     partition data {
>         image = "data.img"    }
> }
> image data.img {
>     name = "data"
>     ext4 {
>         label = "data"
>         features = "extents,uninit_bg,dir_index,has_journal,filetype"
>     }
>     size = 32M
> }
> 
> Here is a data partition created with ext4 data.img and is a target
> for user data via overlayfs.
> 
> As you can see from fstab, there should be a set of directories
> pre-created on the data partition:
> /etc, /work_etc, /var, /work_var, /home and /work_home. And this is my
> problem currently - I do not know how to pre-create them during the
> build in a Buildroot-way.

 There is not really a "Buildroot-way" to do this.

 I've used two different approaches in the past:

1. In the post-build script, create the directories etc. in some temporary
location and create the "data" filesystem by hand before calling genimage.
Actually, if you don't use genimage.sh, you can simply use the --rootpath option
of genimage to point to the data directory (so for the "data" image you would
sent mountpoint to /).

2. Create an init script to populate the data partition. This requires bypassing
fstab and instead manually mounting the overlays as part of that init script.
The advantage of this approach is that you can do "factory reset" by simply
formatting the data partition.

I suspect that there's a systemd way to do the second option with tmpfiles and
such without requiring an init script.


 Regards,
 Arnout

> 
> I understand that the primary way to customize the rootfs is using
> Buildroot's overlays. However, since I am not specifying the mount
> point for data partition (because it should be mounted with
> overlayfs), I have no idea how to pre-create directories mentioned
> above. Even though I can probably find some hack-ish way to do it (for
> example by mounting data.img and modify it before the final image will
> be created) I am wondering what is the right way [tm] to do it with
> Buildroot.
> 
> Thank you,
> Andrey.
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