[Bug 1945874] Re: 21.10 beta, errors in 10-linux and 10_linux_zfs

[Mason Loring Bliss]

I haven't had a chance to dig deeper, but I just noticed this same issue in 
Focal Fossa.

If I get a chance to debug this I'll submit a patch here. I might get a 
chance over the next week, during Thanksgiving break.

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Title:
  21.10 beta, errors in 10-linux and 10_linux_zfs

Status in grub2 package in Ubuntu:
  New

Bug description:
  In a custom install of Ubuntu 21.10 beta, both hardware and VM installs
  suffer from a bug in the grub.d/10_linux and 10_linux_zfs scripts. (For
  comparison, Debian Bullseye, running a similar version of grub, doesn't
  have this issue.)

  Unique to Ubuntu, there's this block in 10_linux:

      xzfs)
          # We have a more specialized ZFS handler, with multiple system in
          # 10_linux_zfs.
          if [ -e "`dirname $(readlink -f $0)`/10_linux_zfs" ]; then
            exit 0
          fi

  This looks at the root filesystem, and if it's ZFS, it shunts kernel
  discovery and entry population off to 10_linux_zfs. This subsequent
  script assumes that the default/automated Ubuntu ZFS layout is in
  effect, and if it's not, the end result is that 10_linux doesn't add an
  entry because there is ZFS present, and 10_linux_zfs doesn't add a
  kernel stanza either, evidently because /boot isn't in a pool. (I
  haven't tracked the logic in 10_linux_zfs fully but given time pressure
  I wanted to get this bug in so someone could look at it.) With this
  combination of events, the resulting grub.cfg has no kernel stanzas at
  all, leaving the user at a grub> prompt. Manual configuration and
  booting from the prompt works from this point, but it's obviously not
  ideal.

  In testing, commenting out the "exit" in the code block noted above
  resulted in correct stanzas being generated, in this case with /boot
  being on ext4 atop MD-RAID1. Rather than exiting if the root is on ZFS,
  correct behaviour would occur in more cases if we check for /boot being
  on ZFS or not. A simple check (untested) might be:

          if ! grep -q '[[:space:]]/boot[[:space:]]' /etc/fstab; then
            exit 0
          fi

  This doesn't check for 10_linux_zfs existing, but that check is perhaps
  redundant given that the file is installed alongside 10_linux and thus
  will always exist, as packaged. This instead checks to see if there's a
  /boot in /etc/fstab, which if present should indicate that /boot is not
  going to be found on a ZFS dataset. (Certainly, traditional filesystems
  can exist on zvols and legacy-mount datasets exist, and both of these
  can appear in fstab, but neither of those is possible for a working
  /boot.)

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