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Re: How does the grub efi know where to look for the grub.cfg?

From: Andrei Borzenkov
Subject: Re: How does the grub efi know where to look for the grub.cfg?
Date: Mon, 7 Mar 2016 21:10:01 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1

07.03.2016 19:05, Arjun Krishnan пишет:
> On Sun, Mar 6, 2016 at 10:33 PM, Andrei Borzenkov <address@hidden>
> wrote:
>> 06.03.2016 18:34, Arjun Krishnan пишет:
>>> On Sat, Mar 5, 2016 at 10:35 PM, Andrei Borzenkov <address@hidden>
>> wrote:
>>>> 05.03.2016 20:27, Arjun Krishnan пишет:
>>>>> I installed grub-efi using
>>>>>     grub-install --efi-directory=/mnt/efi --boot-directory=/boot
>>>>> --target=x86_64-efi
>>>>> I did not specify a root partition anywhere (like you used to have to,
>>>>> when installing it to the mbr), so how does grub set its $root and
>>>>> $prefix?
>>>> grub-install configures image so that it will look for disk where
>>>> /boot/grub is located at boot time and set $root and $prefix
>> accordingly.
>>>>> So I looked at a dual booting system that has two efi grub
>>>>> installations, ubuntu and arch. It has a /efi/ubuntu directory that
>>>>> has a grubx64.efi file and a grub.cfg. This grub.cfg simply sets the
>>>> On EFI binary can query firmware for path it was loaded from. GRUB image
>>>> can be configured so that it will use this information as value of
>> $prefix.
>>> This explains how GRUB finds the path it was loaded from, and hence
>>> the grub.cfg if it's in the path.
>>> But it doesn't explain the archlinux setup, where there is no
>>> grub.cfg.
>> I answered this in previous sentence.
>> In that case, let me clarify my question so I understand it better. The
> grubx64.efi is in the EFI partition under (hd0,1)/EFI/grub for example. On
> my hard disk there are several possible root partitions, say (hd0,2) and
> (hd0,3) both of which have boot/grub/grub.cfg.

grub image is configured by grub-install to look for filesystem which is
set by --boot-directory and defaults to where /boot is located (at the
time grub-install is run).

> The efi binary is loaded from the EFI partition, and it queries the
> firmware for the path it was loaded from. It was loaded from
> (hd0,1)/EFI/grub. So it sets its prefix=(hd0,1)/EFI/grub. However, there is

No, image created by grub-install does not do it. It looks for
filesystem where /boot/grub was present at the time grub-install was

> no grub.cfg inside ${prefix}/grub/.

Loaded image path is pretty much irrelevant for grub-install. It is made
available for use in grub.cfg, but is not used otherwise.

> There is, however, a grub.cfg on (hd0,2)/boot/grub/grub.cfg and
> (hd0,3)/boot/grub/grub.cfg . But only one of the two is the correct one,
> and of course, grub knows where to look for this since it was installed
> with --boot-directory=(hd0,2)/boot/grub. Where is this information stored?

Information how to find /boot/grub is stored in created image. This
information uniquely identifies one of existing partitions.

>>> Is it stored in the efi file?
>> No. It would make reconfiguring GRUB too difficult. When GRUB starts it
>> reads $prefix/grub.cfg.
>>>>> correct prefix and root. I suppose the --efi-directory is used as a
>>>>> system root partition. However, inside /efi/arch I simply have a
>>>>> grubx64.efi. The root and prefix are nowhere to be found. I even when
>>>>> I tried to read the efi file in hex.
>>>>> How does the grubx64.efi on the arch side figure out the correct
>>>>> system root partition?
>>>>> Arjun
>>>>> _______________________________________________
>>>>> Help-grub mailing list
>>>>> address@hidden

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