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Re: Transfering Data from GRUB to userspace?

From: Hans de Goede
Subject: Re: Transfering Data from GRUB to userspace?
Date: Thu, 14 Nov 2019 13:42:35 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.0



Sorry for jumping in mid thread, but I just noticed this and some functionality
like this has my interest too. I especially triggered on the cpio / initrd

I have been discussing something similar with the systemd folks, see
some background see:

Where the last one goed into the cpio / appended initrd concept.

What we were discussing there is a way to pass the kbd-layout to the bootloader
and then having the bootloader pass it back to the initrd (so userspace in
a way).

The problem is that localectl updates /etc/vconsole.conf but that does not
influence that file in existing initrds so the initrd has a wrong value
after changing the kbd layout and regenerating the initrd is undesirable,
since we keep old kernel + initrds around as a fallback in case something
breaks and if it is the initrd which broke then regenerating it will
break the fallback.

So Lennart said that we should really also pass the kbdmap to the bootloader
in case the bootloader supports it (for the cmdline mode). Lennart suggested
dropping a kbdmap file next to say grubenv which contains the kbdmap info.
I instead am suggesting a "config" file which contains
key-value pairs to make the mechanism more generic.

Now the problem is, we still need to get this info to the initrd. Typically
the initrd does not mount /boot or the ESP, so it cannot directly get
to the kbdmap or config file. Lennart suggested simply passing it on
the kernel cmdline which will work for the kbdmap case, but again is not
very generic.

I instead suggested making the "config" (or whatever we are going to name ir)
file available to the initrd variant by append an on the fly generated cpio
archive to the initrd with that file in it. Which is why I triggered on the
mentioning of using cpio to pass info from the bootloader to userspace.

Please keep in mind that what I'm looking for is a bootloader agnostic
solution. Requiring changes to the bootloader is fine, but it is something
which should not be grub specific.

Anyways TL;DR: I think that being able to pass an extra on the fly
generated cpio archive appended to the initrd when booting is something
which would be useful to have.



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