Re: Debug grub scripts without rebooting into grub shell.

[Hongyi Zhao]

On Wed, Jul 12, 2023 at 2:15 PM Glenn Washburn
<development@efficientek.com> wrote:
>
> On Wed, 12 Jul 2023 09:25:15 +0800
> Hongyi Zhao <hongyi.zhao@gmail.com> wrote:
>
> > On Wed, Jul 12, 2023 at 5:26 AM Glenn Washburn
> > <development@efficientek.com> wrote:
> > >
> > > On Sat, 8 Jul 2023 19:58:29 +0800
> > > Hongyi Zhao <hongyi.zhao@gmail.com> wrote:
> > >
> > > > Hi here,
> > > >
> > > > Are there some convenient methods to debug grub scripts without
> > > > rebooting into grub shell?
> > >
> > > I would recommend running the GRUB EFI application in QEMU, as it will act
> > > more closely to when running on bare metal. Here's an example of a QEMU
> > > command that I use when doing development:
> > >
> > > qemu-system-x86_64 -name '[x86_64-efi] Test Grub' \
> > >  -m 2048 -bios OVMF.fd --enable-kvm -boot once=c,menu=on \
> > >  -nodefaults -rtc base=localtime -vga std -snapshot \
> > >  -drive file=/home/grub-tester/bootloader/efidisk.img,format=raw \
> > >  -device virtio-scsi-pci,id=scsi0,num_queues=4 \
> > >  -device 
> > > scsi-hd,drive=drive0,id=systema,bus=scsi0.0,scsi-id=0,lun=0,channel=0 \
> > >  -drive file=/dev/sdc,format=raw,if=none,id=drive0,readonly=on \
> > >  -device 
> > > scsi-hd,drive=drive1,id=systemb,bus=scsi0.0,scsi-id=0,lun=1,channel=0 \
> > >  -drive file=/home/user/disk.img,format=raw,if=none,id=drive1,readonly=on
> > >
> > > If you want to use a block device, then you'll need to run as root or
> > > otherwise have correct permissions to read from the device. But you can 
> > > have everything in a disk image file and not need to run as root.
> >
> > I tried to adapt your approach to my case, but it didn't work:
> >
> > werner@X10DAi:/media/werner/a5d0e46f-aea7-43f0-9a3e-cfb6d517d214/EFI/grub-mkstandalone$
> > sudo qemu-system-x86_64 -name '[x86_64-efi] Test Grub'  -m 2048 -bios
> > OVMF.fd --enable-kvm -boot once=c,menu=on  -nodefaults -rtc
> > base=localtime -vga std -snapshot  -drive
> > file=./grubx64.efi,format=raw  -device
> > virtio-scsi-pci,id=scsi0,num_queues=4  -device
> > scsi-hd,drive=drive0,id=systema,bus=scsi0.0,scsi-id=0,lun=0,channel=0
> > -drive file=/dev/sdc,format=raw,if=none,id=drive0,readonly=on  -device
> > scsi-hd,drive=drive1,id=systemb,bus=scsi0.0,scsi-id=0,lun=1,channel=0
> > format=raw,if=none,id=drive1,readonly=on
> > qemu-system-x86_64: -drive
> > file=/dev/sdc,format=raw,if=none,id=drive0,readonly=on: Could not open
> > '/dev/sdc': No such file or directory
>
> The above arguments emulate two disks. One coming from grubx64.efi and
> one /dev/sdc. I think for your use case, you only need one drive. But
> it needs to be an EFI bootable disk image. The file grubx64.efi is just
> the EFI application, not the disk image, which contains grubx64.efi.
> Remember, the disk image file will show up as a scsi hard drive, so it
> needs to look like a bootable drive (ie has a GPT label and EFI System
> Partition correctly setup). This link[1] shows how to do it on a block
> device, but you can create everything in a file too (though you might
> need to learn how to use losetup).
>
> Glenn
>
> [1]
> https://old.nixaid.com/creating-uefi-bootable-usb-flash-drive-with-several-linux-distributions/

I noticed the following description on the above Tech blog, which is
used to construct the linux & initrd lines based on the corresponding
grub configurations on the iso files:

To construct linux & initrd lines I have used the contents of
isolinux/menu.cfg file (in some cases it is different, but easy to
find using grep) that can be accessed if you simply mount/open the
.iso file


But I want to know whether this can be used as a general method to
construct the linux & initrd lines to boot the isos using grub.

Best,
Zhao

>
> >
> > > Also You can add the options "--nographic -serial stdio" to have the 
> > > interface
> > > be via serial, which allows text selection. In this case GRUB will need 
> > > to be
> > > configured to output to serial.
> > >
> > > Glenn
> >
> > Zhao