nixos/memtest86: Does not function with non-default ESP mount point.

Nixpkgs version

• Stable (25.05)

Describe the bug

The menu entry added by boot.loader.grub.memtest86.enable does not function when boot.loader.efi.efiSysMountPoint is set to a non-default location, such as /boot/efi.

Steps to reproduce

• Set these options:

boot.loader.grub.enable = true;
boot.loader.efi.efiSysMountPoint = "/boot/efi";
boot.loader.grub.memtest86.enable = true;

• Do a nixos-rebuild build-vm-with-bootloader on the above configuration.
• Select the memtest86+ entry in the boot menu.
• Observe error: file '/boot/memtest86.bin' not found.

Expected behaviour

The menu entry should successfully run memtest86.

Screenshots

No response

Relevant log output

Additional context

No response

System metadata

• system: "x86_64-linux"
• host os: Linux 6.12.34, NixOS, 25.05 (Warbler), 25.05.20250625.7284e2d
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.28.3
• nixpkgs: /nix/store/rf07lvgmjakj2mnwbd99n2bdd76sjvps-source

Notify maintainers

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I assert that this issue is relevant for Nixpkgs

• I assert that this is a bug and not a support request.
• I assert that this is not a duplicate of an existing issue.
• I assert that I have read the NixOS Code of Conduct and agree to abide by it.

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Start by tracing the implementation of boot.loader.grub.memtest86.enable and how it uses boot.loader.efi.efiSysMountPoint. Reproduce the issue with nixos-rebuild build-vm-with-bootloader and the listed options; done means selecting the memtest86+ entry successfully runs memtest86 when the ESP is mounted at /boot/efi.