Some of the most annoying problems Linux users encounter are boot issues, particularly when the system won’t start or shows insufficient boot options. The Boot Loader Specification (BLS) configuration is one often disregarded factor, although hardware issues and corrupted kernel images are frequent culprits.

Modern Linux systems rely heavily on the BLS flag to handle boot entries, arrange kernels, and interact with the bootloader. Misconfigurations in this area may result in failed updates, missing kernels, or an inability to boot at all.

This blog describes what BLS is, how it operates, and how setting up or fixing the BLS flag can swiftly fix a variety of boot-related issues.

Understanding Boot Loader Specification (BLS)

Systemd created a standardized framework called the Boot Loader Specification to standardize the way Linux systems handle boot entries. In the past, bootloaders like GRUB2 relied on a single, monolithic file. This big file had to be regenerated with every kernel update, which raised the possibility of mistakes, corrupted configurations, and overwrites.

BLS modernizes this process by separating each boot entry into an individual configuration file stored in:

/boot/loader/entries/

This modular approach makes it easier to maintain, troubleshoot, and verify boot entries. Kernel updates automatically generate new entry files without modifying the entire bootloader configuration. This leads to better reliability, especially on systems that regularly update or use multiple kernels.

Common Boot Issues Related to BLS

Several boot failures can be traced back to misconfigured or corrupted BLS entries:

Missing or Corrupted Boot Entries

When BLS is enabled but required entry files are missing or malformed, the system may fail to display kernel options, resulting in a stalled boot or black screen.

Conflicts Between GRUB2 and BLS

GRUB2 must be explicitly configured to use BLS. If this integration is incomplete or inconsistent, the bootloader may not locate kernel images or may display outdated kernel information.

Kernel Update Failures

If the BLS flag is misconfigured, kernel updates may not create new boot entries. As a result, the updated kernel does not appear in the boot menu, causing unexpected boot failures.

Checking Your Current BLS Configuration

Before modifying anything, verify whether BLS is currently enabled on your system. On RHEL, CentOS, and Fedora-based systems, use:

grubby –info=ALL | grep -i blsdir

  • You can confirm GRUB’s BLS setting with:

grep GRUB_ENABLE_BLSCFG /etc/default/grub

  • If the output shows GRUB_ENABLE_BLSCFG=true, BLS is enabled.
  • Check for the presence of boot entry files:

ls -l /boot/loader/entries/

  • This directory should contain .conf files representing your kernel entries. If files are missing or corrupted, boot problems are likely.

Enabling BLS to Fix Boot Issues

If BLS is disabled or configured incorrectly, enabling it may resolve many persistent boot failures.

Step 1: Edit the GRUB Configuration

  • Open the GRUB defaults file:

sudo nano /etc/default/grub

  • Add or modify the line:

GRUB_ENABLE_BLSCFG=true

Step 2: Regenerate the GRUB Configuration

  • For RHEL/CentOS/Fedora:

sudo grub2-mkconfig -o /boot/grub2/grub.cfg

  • For UEFI-based systems:

sudo grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg

Step 3: Create or Regenerate BLS Entries

  • If boot entries are missing:

sudo kernel-install add $(uname -r) /lib/modules/$(uname -r)/vmlinuz

  • Or convert BLS entries to standard GRUB format using:

sudo grubby –bls-to-grub

Disabling BLS When It Causes Problems

Not all systems behave well with BLS, especially older hardware or customized setups. Disabling BLS can sometimes restore stable boot functionality.

Step 1: Disable BLS in GRUB

Edit /etc/default/grub:

GRUB_ENABLE_BLSCFG=false

Step 2: Regenerate the GRUB Configuration

Use the same regeneration commands as earlier to rebuild your configuration.

Step 3: Verify Boot Entries

Ensure that GRUB recognizes all available kernel entries:

sudo grub2-editenv list

Repairing Corrupted BLS Entries

  • If BLS entries exist but contain errors, rebuilding them may solve the issue.
  • Backup the existing entries:

sudo cp -r /boot/loader/entries /boot/loader/entries.backup

  • Remove corrupted files:

sudo rm /boot/loader/entries/*.conf

  • Reinstall kernels to recreate entries:

sudo dnf reinstall kernel-core-<version>

  • This forces the system to generate fresh, valid BLS configuration files.

Troubleshooting Tips

Check SELinux Contexts

Incorrect SELinux labels can prevent BLS files from loading:

sudo restorecon -Rv /boot

Review Boot Logs

Inspect logs from the previous boot attempt:

sudo journalctl -b -1

This helps identify failed kernel loads, GRUB errors, or file permission issues.

Best Practices

To avoid future boot issues related to BLS:

  • Always back up boot-related directories before making configuration changes
  • Keep systems updated to ensure kernel and bootloader compatibility
  • Monitor available space in the /boot partition
  • Test major configuration changes in non-production environments
  • Document any custom kernel parameters included in BLS entry files

One of the best ways to fix persistent boot problems on contemporary Linux systems is to configure or fix the BLS flag. Knowing how BLS functions gives you more control over your system startup process, whether you decide to disable it to fix compatibility issues or enable it for better boot management. Stable kernel updates, predictable boot behavior, and fewer unexpected failures are all ensured by proper BLS entry management.

Advanced Linux server administration, including resolving complicated bootloader problems and repairing unreachable systems, is ServerAdminz’s area of expertise. For production workloads, their team guarantees precise diagnosis, quick repair, and long-term stability.

If you need any support on fixing Linux boot issues or configuring BLS settings for stable system performance, contact ServerAdminz for expert assistance.