Bug#964859: grub-common: Debian background image not shown

[Heinrich Schuchardt]

Package: grub-common
Version: 2.04-8
Severity: minor

Dear Maintainer,

/etc/grub.d/05_debian_theme executes

    /usr/share/desktop-base/grub_background.sh

/usr/share/desktop-base/grub_background.sh sets:

    WALLPAPER=/usr/share/images/desktop-base/desktop-grub.png

Now 05_debian_theme calls

    is_path_readable_by_grub()

which is defined in file /usr/share/grub/grub-mkconfig_lib to determine
if the wallpaper file is readable.

is_path_readable_by_grub() returns 0 though GRUB does not have any
credentials to decrypt the dm-crypt partition which holds the LVM volume.

nvme0n1             259:0    0   477G  0 disk
├─nvme0n1p1         259:1    0   953M  0 part
├─nvme0n1p2         259:2    0   1.9G  0 part  /boot
└─nvme0n1p3         259:3    0 474.1G  0 part
  └─nvme0n1p3_crypt 253:0    0 474.1G  0 crypt
    ├─LV1-home      253:1    0   381G  0 lvm   /home
    ├─LV1-root      253:2    0  74.5G  0 lvm   /
    └─LV1-swap      253:3    0  18.6G  0 lvm   [SWAP]

The symbolic links are expanded and in grub.cfg we end up with

    if background_image \
    /usr/share/desktop-base/futureprototype-theme/grub/grub-16x9.png

This of cause fails because /usr/share is not accessible without
decrypting the root partition and we end up without a background image.

I suggest to never assume that GRUB can read the background image if it
is not in path /boot/grub.

Best regards

Heinrich