Re: GRUB only offers a rescue shell

[Jordan Uggla]

On Wed, Jul 12, 2017 at 9:09 AM, Hao Lee <address@hidden> wrote:
> Hi,
>
> I encounter a problem when making a bootable disk image because GRUB
> only offers a rescue shell.
>
> I want to create a disk image and install GRUB 2 on it and then use
> Bochs to run it. The steps are as follows.
>
> System Environment:
> OS: Fedora 25 x86
> GRUB version: grub2-install (GRUB) 2.02~beta3
> Bochs version: 2.6.9
>
> 1. Create a empty disk image.
>
> dd if=/dev/zero of=hd.img bs=$((16*63*512)) count=100
>
> 2. Create a primary partition.
>
> fdisk -u -C100 -S63 -H16 hd.img
>
> The partition  table is:
> Device     Boot Start    End Sectors  Size Id Type
> hd.img1          2048 100799   98752 48.2M 83 Linux
>
> 3. Attach the whole image to /dev/loop0 and attach the first partition
> to /dev/loop1
>
> sudo losetup /dev/loop0 hd.img

You're fine up to here.

> sudo losetup -o $((2048*512)) /dev/loop1 hd.img

This is where your problem is. When you run grub-install it detects
two disks, /dev/loop0 and /dev/loop1 where /dev/loop0 appears to be a
partitioned disk with none of its partitions mounted and /dev/loop1
appears to be a disk without any partition table where the whole disk
is mounted to /mnt/. grub-install performs an installation that would
work if that were the case (try passing /dev/loop0 and /dev/loop1 as
disks to bochs and it should boot).

Instead, you should run "sudo kpartx -a /dev/loop0" which will probe
the partition table of /dev/loop0 and add device nodes in /dev/mapper/
for each of the partitions found. In this case, it would create a
device node /dev/mapper/loop0p1 . When grub-probe looks at
/dev/mapper/loop0p1 it will be able to detect that it is a partition
of the disk /dev/loop0 and thus grub-install will configure things
appropriately.

>
> 4. Format the first partition to ext4 and mount it to /mnt/
>
> sudo mkfs.ext4 /dev/loop1
> sudo mount -t ext4 /dev/loop1 /mnt

sudo mkfs.ext4 /dev/mapper/loop0p1
sudo mount -t ext4 /dev/mapper/loop0p1 /mnt/

>
> 5. Create GRUB directory.
>
> sudo mkdir -p /mnt/boot/grub2
>
> 6. Install GRUB 2
>
> sudo grub2-install --boot-directory=/mnt/boot /dev/loop0
>
> 7. Start Bochs.
>
> bochs -f test.bxrc
>
> These steps are straight and simple. However, GRUB only offers a
> rescue shell instead of entering normal mode.
> When I use the set and ls command to show more information, I can only
> see (hd0) and the first partition (hd0,1) is disappeared. The screen
> output is as follows:

That is because grub-install detected that /boot/grub/ was on a "disk"
with no partition table, and thus didn't include the part_msdos module
in the core.img. Again, the solution to that problem is described
above.

-- 
Jordan Uggla (Jordan_U on irc.freenode.net)

> ----------------------------------------------------
> Bochs BIOS - build: 02/16/17
> ata0 master: Generic 1234 ATA-6 Hard-Disk (  49 MBytes)
> Press F12 for boot menu.
> Booting from Hard Disk...
> .
> error: no such device: c5b2c483-70c2-4212-840b-da5f965d0555.
> error: unknown filesystem.
> Entering rescue mode...
> grub rescue> set
> prefix=(hd0)/boot/grub2
> root=hd0
> grub rescue> ls
> (hd0)
> grub rescue>
> ----------------------------------------------------
>
> Could someone help me out? Many thanks!
>
> Regards,
> Hao Lee
>
> _______________________________________________
> Help-grub mailing list
> address@hidden
> https://lists.gnu.org/mailman/listinfo/help-grub