[UNSOLVABLE] GRUB fails loopback booting Batocera IMG

wormz

Hey guys, I need your help for (I know it’s unsupported) booting batocera-x86_64-vv-dddddddd.img (gUnzipped) via GRUB.

I really hope that some kind soul is willing to give me a hand! 🧡

I feel I’m so close to the solution!! ⚡

Why not the Ventoy way?

Well, because:

I dont’ want an USB port to be occupied by BatoceraUSB
Due to lack of space, I cannot and neither do I want to take the risk of messing up partitions
I need the main OS to be operational and easily accessible

My HDD layout is composed like this:

TABLE: GPT
1st GPT partition: /dev/sda1
2nd GPT partition: /dev/sda2
3rd GPT partition: /dev/sda3

Partitions:

/:		/dev/sda2	[ext4, ROOT]
/boot/efi:	/dev/sda1	[fat32, ESP]
swap:		/dev/sda3	[linux-swap, SWAP]

ArchLinux is installed on /dev/sda2 and GRUB2 on /dev/sda1.

Directory structure:

/bin
/bato/
	bato.img
/boot/
	efi/
/dev
/etc
/home
## [...]
## truncated output
## [...]

I’ve already expanded BATOCERA partition:

$ sudo qemu-img resize /bato/bato.img +8G
$ sudo losetup --find --show /bato/bato.img.img
loop0
├─loop0p1   vfat    FAT32 BATOCERA        ABCD-1234
└─loop0p2   ext4    1.0   SHARE           12345678-abcd-1234-abcd-0123456789ab

So far I have been able to start the kernel (partition1[vfat]/boot/linux), but it then fails booting.

Here’s the steps I’m trying at GRUB’s console:

grub> loopback bato (hd0,gpt2)/bato/bato.img
grub> probe --fs-uuid (bato,gpt1)
ABCD-1234
grub> probe --label (bato,gpt1)
BATOCERA
grub> insmod cpio
grub> insmod fat
grub> insmod gzio
grub> insmod part_gpt
grub> insmod part_msdos
grub> linux (bato,gpt1)/boot/linux root=UUID=ABCD-1234 label=BATOCERA loglevel=9
grub> initrd (bato,gpt1)/boot/initrd.gz
grub> boot

At this stage linux is correctly loaded, but immediately fails because of this:

Waiting for the root device
LABEL=BATOCERA: Can't open blockdev
LABEL=BATOCERA: Can't open blockdev
LABEL=BATOCERA: Can't open blockdev
LABEL=BATOCERA: Can't open blockdev
LABEL=BATOCERA: Can't open blockdev
mount: mounting LABEL=BATOCERA on /boot_root failed: No such file or directory

Atari

Could just chainload to syslinux instead and have that sort it out.

wormz

Atari

Nothing changes, still the exact same error as when booting via GRUB:

Waiting for the root device
LABEL=BATOCERA: Can't open blockdev
LABEL=BATOCERA: Can't open blockdev
LABEL=BATOCERA: Can't open blockdev
LABEL=BATOCERA: Can't open blockdev
LABEL=BATOCERA: Can't open blockdev
mount: mounting LABEL=BATOCERA on /boot_root failed: No such file or directory

wormz

So… Going on with “debugging”…
Having a look Batocera’s initrd.gz contents, I found this inside the init (ash)script:

while ! do_mount "${MOUNTARG}"
    do
        echo "Waiting for the root device"
        sleep 1
    done

^Here’s where the boot stage fails.

The do_mount function tries mounting what’s declared inside "${1}" variable:

mount -o ro "${1}"		 /boot_root

And "${1}" is in turn declared by:

    # read the parameters
    read -r cmdline < /proc/cmdline
    for param in ${cmdline} ; do
        case ${param} in
            dev=*)   dev=${param#dev=};;
            label=*) label=${param#label=};;
        esac
    done

    # look for devices
    mount -t devtmpfs none /dev

    MOUNTARG=none
    test -n "${dev}"   && MOUNTARG=${dev}
    test -n "${label}" && MOUNTARG=LABEL=${label}

So I tried on my actual machine doing this:

$ sudo mkdir /mnt/batotestmount
$ sudo mount /dev/sdz99 /mnt/batotestmount

And this actually leads into the same blockdev error!!!
Here’s dmesg ouput:

/dev/sdz99: Can't open blockdev

Now I’m on a dead-end… 🤕 🤔

christophe195

Burn a version of ubuntu on a usb drive and launch it in live version (do not install it on the batocera device). Can you open the HDD?

Maybe you can access the shared data, after download it of the device do a clean install and copy everything back.

Atari

Well at least you know it’s not a problem of what Batocera is doing. Maybe it’s a hardware issue?

wormz

Sorry guys, sorry, i expressed myself wrong!
My computer is working fine and also ArchLinux.
I’ve done that test with the wrong device (/dev/sdz99) on purpose, just to be sure the error is in fact the same of when booting Batocera!
initrd starts fine but then it cannot see its partition (the root one: label=BATOCERA) when booted via loopback in GRUB.
I need to understand why this is happening.
Maybe GRUB loopback is not suitable for booting linux from raw images / partitions?

Atari

wormz You want to be booting syslinux, not linux. Linux is the kernel, not the boot code.

wormz

Atari

GRUB is a bootloader just like SYSLINUX
I already managed to chainload SYSLINUX from GRUB, but that changed nothing. Same problem happens, just like when booting directly from GRUB loopback: BATOCERA’slinux starts but it then cannot find the root blockdev (label=BATOCERA).

Unfortunately… There’s no solution for my needs 😭

Atari

wormz Well you did say “not suitable for booting linux” so I was just warning you against loading the linux file itself. It’s an understandable mistake to make.

You are loading syslinux, so that’s all right. But that’s weird it’s not working from there. What program are you using to create the GRUB config?

wormz

Atari Every tests I’ve done inside GRUB's console: grub>.
I entered each command manually.
I think it’s some sort “obscure” (at least, to me) steps I’m missing when messing with loopback capability.

There are no differences in booting BATOCERA’s linux directly via GRUB or via SYSLINUX (chainloaded or not).
Both are bootloaders and they are both suitable in handling the boot process of Linux kernels on BIOS and UEFI x86_64 machines (at least this is the hardware for what I can speak for).

Some tests I made months ago within a VirtualMachine: I got rid of all SYSLINUX files by nuking the whole EFI and boot/syslinux folders. I was still able to boot BATOCERA via GRUB perfectly fine, because it was working from within the dedicated BATOCERA’s own partition.
That’s why I’m pretty sure the problem it’s not about GRUB itself, but rather related to the loopback mechanism.

Atari

wormz You’re doing so with an EFI directly instead of using a program on your main OS to create it?

tbh I don’t think I’ve seen that anywhere else, users are usually recommended to run grub-config or their equivalent tools from their main OS and it sorts it out for them.

Anyway this is over my head at this point, hopefully someone else is able to come in and help out with it. Sorry that I can’t be of more help.

wormz

Atari Yes! This is not too hard, because BATOCERA’s wiki provides a lot of very detailed and useful information!! 😎

Simply set the GRUB's root pointing to BATOCERA’s root partition and then boot with:

grub> linux /boot/linux video=1920x1080@120 label=BATOCERA mitigations=off
grub> initrd /boot/initrd.gz
grub> boot

Don’t be sorry @Atari, thanks for you interest! 😊