[bug #56897] Boot fails for ext4 filesystems with certain features enabled

[Jim Bray]

Follow-up Comment #1, bug #56897 (project grub):

Details from report of this bug for metadata_csum_seed:

https://bugs.launchpad.net/ubuntu/+source/grub2/+bug/1844012

/usr/lib/grub/x86_64-efi/ext2.mod, from:

grub-efi-amd64-bin:
  Installed: 2.02-2ubuntu8.13
  Candidate: 2.02-2ubuntu8.13

doesn't recognize an ext4 filesystem with metadata_csum_seed set.

This is true in command-line grub, not just the update-grub software.
Attempting

linux (hdx,gptx)/ etc

on an ext4 fs with metadata_csum enabled fails; removing the
metadata_csum_seed option fixes this.

The failure is "unknown filesystem" for any ext4 fs with metadata_csum_seed
set. Apparently saving the metadata checksum seed in the superblock changes it
beyond recognition for grub.

 The message appears to be coming from
grub-2.04/grub-core/kern /fs.c:

grub_fs_t
grub_fs_probe (grub_device_t device)
{....
 grub_error (GRUB_ERR_UNKNOWN_FS, N_("unknown filesystem"));
}
which appears to probe for filesystems by calling the fs modules to attempt to
mount it.

I tried to install latest grub from eaon to see if upstream has fixed this,
but it depended on too much other unreleased software such as libc6.

I pulled the 2.04 source. There is nothing in the Changelog that appears to
address this.

from `man ext4`:
 metadata_csum_seed
              This feature allows the filesystem to store the metadata
checksum seed in the superblock, which allows the administrator to change the
UUID of a filesystem using the metadata_csum feature while it is mounted.

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