Understanding the GRUB search --file boot method in the Fedora 42 Live ISO

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1

I’m new to Fedora and exploring its inner workings, so please forgive my inexperience.

While examining the boot configuration of the Fedora 42 Workstation Live ISO (Fedora-Workstation-Live-42-1.1.x86_64.iso), I noticed a specific method used to identify the root filesystem that I haven’t seen before.

I found the following files and configuration:

File: /boot/mbrid

Content: 0xc51564f6

File: /boot/0xc51564f6

Content: 0xc51564f6

File: /boot/grub2/grub.cfg

Relevant line: search --file --set=root /boot/0xc51564f6

It appears GRUB is configured to find a file with a unique name (0xc51564f6) to set the root device. I have a few questions about this approach:

  1. What is the technical reasoning behind this method? What is the specific role of the /boot/mbrid file and the uniquely named file (/boot/0xc51564f6)?

  2. How does this boot mechanism on the Live ISO relate to the final installation on a hard disk, especially if the user chooses full-disk encryption (LUKS)?

  3. Does using this static, predictable identifier in the Live ISO have any security implications for the final encrypted system? For instance, could it create a vulnerability that a man-in-the-middle attacker might exploit during or after installation?

I’m very curious about the logic here and want to better understand the potential security impact.

Thanks in advance for any information or insights you can share!

2

In the previous Fedora version the file system label of the iso 9660 file system was used to identify the root file system. The new way works even if the file system label is incorrect.

Not at all.

In the final installed system, the root file system is by default found using the file system UUID as usual, so nothing changed here.

Check the `/boot/grub2/grub.cfg and /boot/efi/EFI/fedora/grub.cfg for how the final installed system is configured.

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