[OE-core] [PATCH 2/2] kernel-initramfs.bbclass: Handle built-in /dev/console dependence

[Andrea Adami]

On Wed, Nov 7, 2018 at 1:16 PM Nathan Rossi <nathan at nathanrossi.com> wrote:
>
> The kernel has two differing code paths when handling a built-in cpio
> versus a cpio passed to the kernel from a boot loader. When the kernel
> is booted with the built-in cpio it expects the /dev/console node to
> exist and will panic if it does not exist.
>
>   Kernel panic - not syncing: /dev/console is missing or not a character device!
>
> When the cpio is passed to the kernel via a bootloader, this behaviour
> is not observed.
>

Well, of course once booted you have devtmpfs or you have provided
your devices-list.
We fight since long with this early issues and I think this approach
is wrong: there are more things needed, not only /dev/consoleI'd say
the responsability to check for devtmpfs or recreate the devices is up
to init manager of the image.

We don't use busybox, just a single binary running as init: kexecboot
will exactly do that, check and recreate devices if missing:
http://cgit.openembedded.org/meta-openembedded/tree/meta-initramfs/recipes-bsp/kexecboot/kexecboot_git.bb

Please don't inject anything in the cpio, let the image-engineer decide.

This poor image is regularly attacked by new changes filling the cpio
with undesidered stuff:
http://cgit.openembedded.org/meta-openembedded/tree/meta-initramfs/recipes-bsp/images/initramfs-kexecboot-image.bb

Maybe we are on the corner-case, size-contraints limit us to be within
1MiB, neverthless I think it is a good exercise to keep things small
and clean ;)

As for the previous patch touching initramfs, I hold my comments for
the moment: let me first test it.
Cheers

Andrea

> To resolve this issue, ensure any cpio that is built into the kernel is
> prepared with a /dev/console node. This is done by appending to the
> copied cpio of the INITRAMFS_IMAGE. In order to create the node to
> append, the task must be executed with root permission (to run mknod).
> As such this change creates and intermediate _prepare task between the
> existing _bundle and _copy tasks.
>
> Note: The default/minimal initramfs that the kernel sources
> gen_initramfs_list.sh generates contains this device node.
>
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/usr/gen_initramfs_list.sh?h=master#n55
>
> Signed-off-by: Nathan Rossi <nathan at nathanrossi.com>
> ---
>  meta/classes/kernel-initramfs.bbclass | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/meta/classes/kernel-initramfs.bbclass b/meta/classes/kernel-initramfs.bbclass
> index b23fb51495..135f4c8949 100644
> --- a/meta/classes/kernel-initramfs.bbclass
> +++ b/meta/classes/kernel-initramfs.bbclass
> @@ -16,7 +16,8 @@ python __anonymous () {
>      if image and bundle:
>          # add all the tasks
>          bb.build.addtask('do_initramfs_copy', 'do_initramfs_bundle', 'do_install', d)
> -        bb.build.addtask('do_initramfs_bundle', 'do_deploy', 'do_initramfs_copy', d)
> +        bb.build.addtask('do_initramfs_prepare', 'do_initramfs_bundle', 'do_initramfs_copy', d)
> +        bb.build.addtask('do_initramfs_bundle', 'do_deploy', 'do_initramfs_prepare', d)
>
>          # make the do_initramfs_copy task depend on the image do_image_complete task
>          d.appendVarFlag('do_initramfs_copy', 'depends', ' ${INITRAMFS_IMAGE}:do_image_complete')
> @@ -67,6 +68,18 @@ do_initramfs_copy () {
>      echo "Finished copy of initramfs into ./usr"
>  }
>
> +do_initramfs_prepare[dirs] = "${B}"
> +fakeroot do_initramfs_prepare () {
> +    echo "Preparing initramfs by creating required /dev/ nodes"
> +    # append a /dev/console node, this node must be created in the rootfs
> +    # for built-in initramfs cpios otherwise it will error with:
> +    #   Kernel panic - not syncing: /dev/console is missing or not a character device!
> +    rm -rf ${B}/usr/append
> +    mkdir -p ${B}/usr/append/dev
> +    mknod -m 600 ${B}/usr/append/dev/console c 5 1
> +    (cd  ${B}/usr/append && echo "./dev/console" | cpio -oA -H newc -F ${B}/usr/${INITRAMFS_IMAGE_NAME}.cpio)
> +}
> +
>  do_initramfs_bundle[dirs] = "${B}"
>  do_initramfs_bundle () {
>      echo "Creating a kernel image with a bundled initramfs..."
> ---
> 2.19.1
> --
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