Re: Fun with GRUB2, BLS and Multiboot USB

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On Wed, Apr 26, 2023, at 12:26 AM, Philip Rhoades via devel wrote:
> People,
>
> LILO and SysLinux were simple and GRUB1 was straightforward - but by the 
> time GRUB2 came around I was long past configuring my own kernels and 
> hacking around at a lower level . . now I think, finally, I need to know 
> more about how GRUB2 - works specifically with Fedoras 38+.
>
> This current interest was prompted when I upgraded my WS from F37 
> KDE(->Sway) via a beta F38 LiveSway and found that /boot/grub2/grub.cfg 
> did not contain a menuentry for the latest F38!? - much hacking and 
> experimenting followed . .

There are no GRUB native menu entries anymore since Fedora 30. There are Boot Loader Spec formatted "snippets" in /boot/loader/entries and Fedora's GRUB contains blscfg.mod which is a GRUB module that provides GRUB with the ability to read BLS snippets and create menu entries from them.
https://fedoraproject.org/wiki/Releases/30/ChangeSet#Make_BootLoaderSpec-style_configuration_files_the_default


> So, I have git cloned the grub2 stuff and had a look at it but I am too 
> old now to be digging around in that stuff just at the moment - so, has 
> any Fedora Guru produced a diagram or at least a point list, of what 
> Grub does from turning on the power to the display of the Grub menu on a 
> Fedora machine? - that would be nice as a more-detailed overview than I 
> have been able to find so far - but not so low-down I am amongst the 
> weeds . . that might come later . .

It's one of those cases of "no not that specification the other specification". There's no way you'd know this is the case without some digging.

Also since https://fedoraproject.org/wiki/Releases/34/ChangeSet#Unify_the_GRUB_configuration_files_location_across_all_supported_architectures there are two grub.cfg files. One is on the EFI System partition and merely points to (forwards) to the real one in /boot/grub2. This is consistent with BIOS and upstream GRUB's location for the grub.cfg. 

I guess hindsight being 20/20 we could have changed grubx64.efi to look for "grub.static" in the same directory as the EFI program instead of "grub.cfg" and then there'd only be one grub.cfg and it'd be the real one rather than the forwarding one.



-- 
Chris Murphy
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