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Re: Grub, UEFI Secure Boot and netboot - help!

On Mon, Jun 10, 2019 at 03:37:41AM +0100, Steve McIntyre wrote:
> Hey folks,


> We have a bit of a problem with PXE booting Grub in Buster, as shown
> in #928750:
>  * On all supported arches, we *used* to generate a Grub netboot image
>    inside d-i, with a prefix setting of
>    "debian-installer/$arch/grub". The prefix is important, as it's how
>    Grub finds its config file, modules etc that it loads. Things are
>    still like this in Stretch.
>  * In Buster, we can no longer do this on arches which support Secure
>    Boot. To keep the SB signature, we now re-use the existing signed
>    binaries that have come directly from the Grub build (and the
>    Debian signing infrastructure). There's just one minor problem with
>    this - this means that we're stuck with the hard-coded prefix baked
>    into the grubnetXXX.efi binary. This is currently set to "/grub",
>    and this means that to get a functional amd64 (say) PXE
>    installation working the user has to add a "/grub" symlink on their
>    TFTP server, something like:
>      /grub -> /debian-installer/amd64/grub
>    (assuming that /debian-installer is the root of the netboot tree).
> I can see a couple of options here, but I'm not sure either of them
> are good. Comments would be most welcome!
>   1. Update the docs to mention this - this is a new thing needed to
>      get netboot working with Buster. It's *currently* inconsistent,
>      as ia64 and armhf (as non-SB arches) are still using the old
>      prefix setting. For the sake of consistency (in docs etc.), I
>      propose to also update the d-i build for those arches to use the
>      same prefix. But I acknowledge that will break existing
>      setups. :-(

These are two options:

  1a. docs update (does not break anything)
  1b. d-i build update (might break something)

>   2. Alternatively, we could tweak the netboot prefix setting as built
>      by grub. I'm worried that this may also break things for some
>      users. Do we assume (can we?) that all our grub netboot users are
>      installer users (so we could use /debian-installer/$ARCH/grub)?
>      If so, that might be a way to go. But is it a valid assumption?
>      We'd be forcing all our grub netboot binaries to only sensibly
>      work for d-i, and that worries me too.
> Any other suggestions on what we could do? Let me know what you
> think...

Is this question for Buster or Buster+x?  Those solutions requiring a
change in grub (and a new signature from M$) are not be ok for Buster.

For Buster I find acceptable only 1a. I prefer a sub-optimal solution
now to an improved solution later, especially under release.

For Buster+x, is it possible to make grub search multiple prefixes?


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