Fix non conforming partition table

[gabriel-samfira]

Fix non conforming partition table

This change removes the legacy_boot flag from the EFI system partition. We already have a BIOS boot partition which should offer compatibility with legacy bios systems.

How to use

Upload the image in a fresh OpenStack devstack deploy and boot.

Testing done

• Downloaded the already existing image
• Removed the legacy_boot flag
• booted the image and it worked.

• [x ] Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update)
• Inspected CI output for image differences: /boot and /usr size, packages, list files for any missing binaries, kernel modules, config files, kernel modules, etc.

/update-sdk

[chewi]

It looks straightforward, but I'll kick off a Jenkins run against it.

[github-actions]

Build action triggered: https://github.com/flatcar/scripts/actions/runs/13444442241

[gabriel-samfira]

Let me also test it out manually. Will ping back once I confirm if it works.

[gabriel-samfira]

Converted to draft. While using parted 3.6 (version 3.4 seemed to make the image unbootable - need to investigate) on an existing image did seem to make it work, generating a new image without the hybrid flag seems to:

• Not set the bootable flag in the MBR
• makes the CHS entry invalid ([0, 1, 0] instead or [0, 2, 0])

[chewi]

I'm surprised by your issues, but I can at least report that CI passed on all platforms except OpenStack and Equinix Metal. I believe those are actually down to availability issues. Brightbox worked, and that uses OpenStack too.

[gabriel-samfira]

You need to run the tests on a fresh devstack deployment. The change that triggers issues in OpenStack was added in December.

The cgpt binary in https://github.com/flatcar/seismograph/tree/flatcar-master/src/cgpt is quite outdated. Newer versions of cgpt (like the one in ubuntu 24.04) can properly set CHS entries (along with other PMBR fields) by running:

cgpt boot -p path/to/raw.img

We then set the bootable flag in the PMBR either via fdisk or by simply flipping the bit at offset 447 in the disk to 0x80. We then have a proper bootable hybrid image that passes the OpenStack disk image tests.

[gabriel-samfira]

to set the bootable flag via fdisk we can run:

echo -e "M\na\nw\n" | fdisk flatcar_production_openstack_image.raw

or we can do it via python in the disk_util module via something like:

def EnsurePMBRBootFlag(options, config=None, partitions=None):
  # Set bootable flag
  with open(options.disk_image, 'r+b') as fd:
    # Seek to partition table offset
    fd.seek(446)
    # Read the partition entry
    partition = fd.read(16)
    partition_data = struct.unpack('<B3BB3BII', partition)
    if partition_data[0] != 0x80:
        new_partition_data = struct.pack('<B3BB3BII', 0x80, *partition_data[1:])
        # Go back to offset of start of partition
        fd.seek(446)
        # write the modified partition data
        fd.write(new_partition_data)

[gabriel-samfira]

to set the bootable flag via fdisk we can run:

echo -e "M\na\nw\n" | fdisk flatcar_production_openstack_image.raw

or we can do it via python in the disk_util module via something like:

def EnsurePMBRBootFlag(options, config=None, partitions=None):
  # Set bootable flag
  with open(options.disk_image, 'r+b') as fd:
    # Seek to partition table offset
    fd.seek(446)
    # Read the partition entry
    partition = fd.read(16)
    partition_data = struct.unpack('<B3BB3BII', partition)
    if partition_data[0] != 0x80:
        new_partition_data = struct.pack('<B3BB3BII', 0x80, *partition_data[1:])
        # Go back to offset of start of partition
        fd.seek(446)
        # write the modified partition data
        fd.write(new_partition_data)

the boot flag is not needed. To pass the OpenStack Validation, the CHS values are enough.

[gabriel-samfira]

seems to pass on OpenStack. @chewi could you kick off another test in the CI as well?