Re: Debug grub scripts without rebooting into grub shell.

[Glenn Washburn]

On Wed, 12 Jul 2023 15:26:18 +0800
Hongyi Zhao <hongyi.zhao@gmail.com> wrote:

> 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.

No, it won't work in general, but it might work for most of your isos
depending on which ones you use. Checkout supergrubdisk[1] as Andrei
mentioned. Also here are a couple other links that might be
helpful[2][3].

Glenn

[1]
https://github.com/supergrub/supergrub/blob/master/menus/sgd/autoiso.cfg
[2] https://github.com/jim945/autoiso-multiboot
[3] https://www.supergrubdisk.org/wiki/Loopback.cfg

> 
> 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