What is GRUB_ENABLE_BLSCFG in /etc/default/grub?

Environment

• Red Hat Enterprise Linux 8
• Red Hat Enterprise Linux 9
• Red Hat Enterprise Linux 10
• GRUB 2

Issue

• After upgrading to RHEL 8 from RHEL 7, GRUB_ENABLE_BLSCFG=true was added in /etc/default/grub file.
• What if I delete GRUB_ENABLE_BLSCFG=true from /etc/default/grub file?

Resolution

• With GRUB_ENABLE_BLSCFG=true in /etc/default/grub, GRUB2 uses blscfg files and entries in /boot/loader instead of menuentry entries in grub.cfg. It's the standard configuration since RHEL 8.
• On RHEL 8, if GRUB_ENABLE_BLSCFG is removed or set to "false", grub.cfg generated by grub2-mkconfig will define the menu entries just as RHEL 7.
• The default value of GRUB_ENABLE_BLSCFG is "true" on RHEL 9. Deleting GRUB_ENABLE_BLSCFG=true from /etc/default/grub won't disable BLS. User has to set GRUB_ENABLE_BLSCFG=false to disable BLS.
• Note On RHEL 8, grub2-mkconfig is never run on kernel upgrade even if GRUB_ENABLE_BLSCFG=false, user has to run grub2-mkconfig after upgrading kernel. The issue is fixed in RHSA-2022:2110.
• Note On RHEL 8, when GRUB_ENABLE_BLSCFG is not set in /etc/default/grub, grub2-mkconfig is never run on kernel upgrade. User has to run grub2-mkconfig after upgrading kernel.

Diagnostic Steps

Disable BLS:

# grep BLS /etc/default/grub
GRUB_ENABLE_BLSCFG=false
# grub2-mkconfig -o /boot/grub2/grub.cfg
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-5.14.0-42.el9.x86_64
Found initrd image: /boot/initramfs-5.14.0-42.el9.x86_64.img
Found linux image: /boot/vmlinuz-0-rescue-6c50dcebbd4843e89cd5bfb715c156a4
Found initrd image: /boot/initramfs-0-rescue-6c50dcebbd4843e89cd5bfb715c156a4.img
Adding boot menu entry for UEFI Firmware Settings ...
done
# grep ^menuent /boot/grub2/grub.cfg | awk -F\' '{ print $2 }'
Red Hat Enterprise Linux (5.14.0-42.el9.x86_64) 9.0 (Plow)     
Red Hat Enterprise Linux (0-rescue-6c50dcebbd4843e89cd5bfb715c156a4) 9.0 (Plow)

Enable BLS:

# grep BLS /etc/default/grub
GRUB_ENABLE_BLSCFG=true
# grub2-mkconfig -o /boot/grub2/grub.cfg
Generating grub configuration file ...
Adding boot menu entry for UEFI Firmware Settings ...
done
# grep ^menuent /boot/grub2/grub.cfg | awk -F\' '{ print $2 }'
#                                       <== nothing output

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