Re: Unexpected prefix value

[Pascal Hambourg]

On 28/11/2023 at 08:11, Andrei Borzenkov wrote:
> On Tue, Nov 28, 2023 at 9:45 AM Pascal Hambourg <pascal@plouf.fr.eu.org> wrote:
> > On 28/11/2023 at 04:44, Andrei Borzenkov wrote:
> > > On 27.11.2023 23:06, Pascal Hambourg wrote:
> > > > But if I build an EFI GRUB core image with
> > > >
> > > >     grub-mkimage --prefix='(hd0,gpt4)' --format=x86_64-efi \
> > > >      --output=/boot/efi/EFI/azerty/core.efi part_gpt ext2
> > > >
> > > > (no / at the end of prefix), then when booting it, the $prefix value
> > > > seems to be the combination of the --prefix device and the $cmdpath path:
> > > >
> > > >     cmdpath=(hd0,gpt1)/EFI/azerty
> > > >     prefix=(hd0,gpt4)/EFI/azerty
> > > >
> > > > Is this expected behaviour ?
> > >
> > > Yes. $prefix consists of two parts - device and the directory path. If
> > > one of them is missing during initialization, grub attempts to guess and
> > > fill it in.
> >
> > Guessing a missing or incomplete device makes sense, but what is the use
> > case for guessing a missing path when the device is present ? AFAICS if
> > the devices in $cmdpath and --prefix do not match, the resulting $prefix
> > is doomed to failure.
>
> I do not think it is done on purpose, it is more - garbage in, garbage
> out. The file name without the path component is invalid, grub cannot
> use it anyway.

In my experience, when I set prefix=(device) either by config file or 
manually at the grub prompt, GRUB can use it just fine so it seems to be 
perfectly valid. It is a prefix, not a full path, and as far as I can 
see grub uses it by prepending it to some file path starting with /. So 
as long as the resulting path is valid it should be fine.

> So it should either drop the value passed in --prefix
> or do something to make it usable. I suppose, ignoring the value in
> --prefix in this case would be more logical

Wouldn't it be more logical to do something like this:

if prefix is empty
  set prefix = cmdpath
else if device part is empty or incomplete
  set missing device part from cmdpath

and do not touch the path part ?