updating grub can end up with loopback devices in grub.cfg

[Michael Hudson-Doyle]

I had (I think, I can't quite remember) the filesystem.squashfs from a live server ISO mounted when the kernel was updated. A menu entry for this OS-resembling device is now in my grub.cfg:

menuentry 'Ubuntu 18.04.1 LTS (18.04) (on /dev/loop35p1)' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'osprober-gnulinux-simple-cf05f674-791e-48eb-9b42-1bb903f4d024' {
        insmod part_msdos
        insmod ext2
        if [ x$feature_platform_search_hint = xy ]; then
          search --no-floppy --fs-uuid --set=root cf05f674-791e-48eb-9b42-1bb903f4d024
        else
          search --no-floppy --fs-uuid --set=root cf05f674-791e-48eb-9b42-1bb903f4d024
        fi
        linux /boot/vmlinuz-4.15.0-29-generic root=UUID=cf05f674-791e-48eb-9b42-1bb903f4d024 ro
        initrd /boot/initrd.img-4.15.0-29-generic
}

I didn't select it :)

Something in this setup should be skipping loopback devices, surely?

[Mathieu Trudel-Lapierre (cyphermox)]

Well, we do use loop devices and do expect them to be listed for building the images. I'm not sure if there'll be a way to differentiate between the image build case and normal case to be able to pass some parameter (--skip-loopback, whatever) as appropriate. Let's look into that.

[Michael Hudson-Doyle (mwhudson)]

Hmm that's a fair point.

[Phillip Susi (psusi)]

How is this something relied on for building images? As it stands, such a boot entry is non functional since it doesn't tell grub to set up a loop mapping to the underlying file so the "disk" won't be found. If you are building an image and later transferring that image to bare metal, then it would work ( who does this? ), but does that use case only need to get the running system set up, or does it also need to be able to have os-prober add entries for other installations too?

[Mate Kukri (mkukri)]

"If you are building an image and later transferring that image to bare metal"

this is exactly how pre-generated configs are created during ubuntu image builds, I believe.

I suppose an explicit flag could be introduced to allow loopback entry generation for use during image builds, but I don't think it is worth the trouble.