Re: Debug grub scripts without rebooting into grub shell.

[Glenn Washburn]

On Wed, 12 Jul 2023 09:27:59 +0800
Hongyi Zhao <hongyi.zhao@gmail.com> wrote:

> On Wed, Jul 12, 2023 at 5:08 AM Glenn Washburn
> <development@efficientek.com> wrote:
> >
> > On Sun, 9 Jul 2023 08:13:40 +0300
> > Andrei Borzenkov <arvidjaar@gmail.com> wrote:
> >
> > > On 09.07.2023 03:21, Hongyi Zhao wrote:
> > > > On Sat, Jul 8, 2023 at 9:53 PM Andrei Borzenkov <arvidjaar@gmail.com> 
> > > > wrote:
> > > >>
> > > >> On 08.07.2023 14:58, Hongyi Zhao wrote:
> > > >>> Hi here,
> > > >>>
> > > >>> Are there some convenient methods to debug grub scripts without
> > > >>> rebooting into grub shell?
> > > >>>
> > > >>
> > > >> You may try grub-emu which emulates grub.
> > > >
> > > > I tried, but it seems that grub-emu doesn't meet my requirement as
> > > > described below.
> > > >
> > > > I want to debug the grubx64.efi created by the following command:
> > > >
> > >
> > > If you want to debug grubx64.efi, then you of course need to load it
> > > either on real hardware or in VM. But you can debug scripts used by
> > > grubx64.efi using grub-emu by setting up suitable simulated environment.
> > > There are clear limitations (scripts cannot load and start any kernel).
> >
> > Point of clarification. If I'm not mistaken, grub emu can now indirectly
> > load and start a kernel via kexec.
> 
> What's the concrete steps?

I've never used it, so I can't really say. Here's the patch[1] that got
commited, but there have been further changes. Looks like there's a -X
or --kexec option to grub-emu. If used once, it will use systemctl to
do the kexec. If used twice, it will do the kexec itself. If you're not
familiar with kexec, read about it before trying it. If successful, it
will reboot your system, which I believe is not what you want.

[1] https://mail.gnu.org/archive/html/grub-devel/2022-08/msg00284.html

> > Glenn
> 
> Zhao
> 
> > > > $ grub-mkstandalone -O x86_64-efi -o grubx64.efi --modules='lvm fat
> > > > ntfs part_msdos part_gpt ext2 btrfs probe regexp search configfile'
> > > > boot/grub/grub.cfg=./grub.cfg
> > > >
> > > > The content of the ./grub.cfg is as follows:
> > > >
> > > > $ egrep -v '^[ ]*(#|$)' grub.cfg
> > > > regexp -s __cmdpath_efi '^\(([^,]+)' "$cmdpath"
> > > > export __cmdpath_efi
> > > > for file in ($__cmdpath_efi,*)/multibootusb.git/grub.cfg; do
> > > >    if regexp -s __root '^(\([^*]+\))' "$file"; then
> > > >      set __prefix=$__root/multibootusb.git
> > > >      export __root
> > > >      export __prefix
> > > >      configfile $__prefix/grub.cfg
> > > >      break
> > > >    fi
> > > > done
> > > >
> > > > The files' layout is as follows:
> > > >
> > > > $ tree .
> > > > .
> > > > ├── grub.cfg
> > > > ├── grub-mkstandalone.sh
> > > > ├── grubx64.efi
> > > > └── iso
> > > >      ├── deepin-desktop-community-23-Beta-amd64.iso
> > > >      ├── rescatux-0.74.iso
> > > >      ├── supergrub2-2.06s1-beta2-multiarch-CD.iso
> > > >      ├── systemrescue-10.01-amd64.iso
> > > >      ├── ubuntu-23.04-desktop-amd64.iso
> > > >      └── ubuntukylin-23.04-desktop-amd64.iso
> > > >
> > > > 1 directory, 9 files
> > > >
> > > > Regards,
> > > > Zhao
> > >