Re: Debug grub scripts without rebooting into grub shell.

[Glenn Washburn]

On Wed, 12 Jul 2023 14:45:36 +0800
Hongyi Zhao <hongyi.zhao@gmail.com> wrote:

> On Wed, Jul 12, 2023 at 2:23 PM Glenn Washburn
> <development@efficientek.com> wrote:
> >
> > 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.
> 
> Maybe my version is too old, so I cannot see this option:
> 
> werner@X10DAi:~$ grub-emu -V
> grub-emu (GRUB) 2.06-2ubuntu12.1

Yes, you need a build from master git repo.

> werner@X10DAi:~$ grub-emu --help
> Usage: grub-emu [OPTION...]
> GRUB emulator.
> 
>   -d, --directory=DIR        use GRUB files in the directory DIR
>                              [default=/boot/grub]
>   -H, --hold[=SECS]          wait until a debugger will attach
>   -m, --device-map=FILE      use FILE as the device map
>                              [default=/boot/grub/device.map]
>       --memdisk=FILE         use FILE as memdisk
>   -r, --root=DEVICE_NAME     Set root device.
>   -v, --verbose              print verbose messages.
>   -?, --help                 give this help list
>       --usage                give a short usage message
>   -V, --version              print program version
> 
> Mandatory or optional arguments to long options are also mandatory or optional
> for any corresponding short options.
> 
> Report bugs to <bug-grub@gnu.org>.
> 
> 
> > 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.
> 
> Yes. BTW, the *reboot* behavior is what you indicated by *indirectly*
> as you described previously:
> 
> > > > Point of clarification. If I'm not mistaken, grub emu can now indirectly
> > > > load and start a kernel via kexec.

Not quite. Technically its not a reboot or boot, but from a user
perspective it looks like one. To the machines firmware (UEFI) it will
not look like a reboot, ie in UEFI the boot services state will not be
entered. By indirectly, I meant that grub-emu will use systemctl or
kexec to do the job of loading the kernel, not the grub-emu process
itself (in contrast to GRUB running on bare metal in some
configurations).

Glenn

> 
> Best,
> Zhao
> 
> > [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
> > > > >