Re: debugging grub2

[daggs]

Greetings Pascal,

> Sent: Wednesday, September 05, 2018 at 8:48 PM
> From: "Pascal Hambourg" <address@hidden>
> To: address@hidden
> Subject: Re: debugging grub2
>
> Le 05/09/2018 à 08:26, daggs a écrit :
> > 
> > I have a setup that runs centos installer (CentOS-7-x86_64-DVD-1804.iso) 
> > from cd in uefi, thing is, when it get to grub, instead of showing the 
> > menu, I get grub rescue.
> > ls shows (cd0) but no partitions.
> 
> Why should it show any partitions ? A CD-ROM has no partitions.
el torito format has partitions, see;
$ fdisk -l /nfs_home/home/eial/CentOS-7-x86_64-DVD-1804.iso

Disk /nfs_home/home/dagg/CentOS-7-x86_64-DVD-1804.iso: 4470 MB, 4470079488 
bytes, 8730624 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk label type: dos
Disk identifier: 0x4c545524

                                           Device Boot      Start         End   
   Blocks   Id  System
/nfs_home/home/dagg/CentOS-7-x86_64-DVD-1804.iso1   *           0     8730623   
  4365312    0  Empty
/nfs_home/home/dagg/CentOS-7-x86_64-DVD-1804.iso2            2368       20343   
     8988   ef  EFI (FAT-12/16/32)

> Can ls show the contents of (cd0) ?
> 
no, I get this:
$ ls
 (cd0)
$ ls cd0
error: unknown filesystem
$ ls (cd0)
(cd0): Filesystem is unknown

> > I've verified the installer is ok on another system so this is not an 
> > installer issue.
> 
> In UEFI mode too ?
> 
of course.

> > I want to enable debugging in grub to see if I can understand what is the 
> > issue.
> > I've mounted the iso and entersed grub.cfg, there I've added this fourlines:
> > set pager=1
> > set debug=all
> > serial --unit=0
> > terminal_output serial
> 
> How did you manage to modify a file in a read-only filesystem (ISO9660) 
> on a read-only medium (CD-ROM) ?
simple, using the following commands:
sudo losetup --partscan --show /dev/loop1 CentOS-7-x86_64-DVD-1804.iso
mkdir root
sudo mount /dev/loop1p2 root
sudo vi root/EFI/BOOT/grub.cfg
sudo umount root

> 
> > after the first line (set default=1)
> > but I still don't get any debug prints in the serial.
> 
> Of course not. grub.cfg is read in normal mode, but dropping to the 
> rescue shell means that GRUB could not find the normal module and switch 
> to normal mode.
> 
I see, is there any way to debug this issue without recompiling the grub?

Thanks,

Dagg.