netboot: Support cmdline variable from netboot.xyz

[Enzime]

Motivation for this change

Allows the kernel params to be specified from netboot.xyz's utilities menu.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.05 Release Notes (or backporting 21.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[nshalman]

The idea looks good to me. I'm getting confused by that ''
To my eye that looks like it would end the string in the wrong place rather than just adding the raw ${cmdline} in the right spot. Am I missing something?

[Enzime]

${cmdline} gets expanded by Nix, you need the the double single quotes prefix to escape it

[nshalman]

${cmdline} gets expanded by Nix, you need the the double single quotes prefix to escape it

A quick test with the repl shows me that you're correct (I find that syntax confusing all the same...):

nix-repl> foo = "bar"

nix-repl> ''${foo} ''${foo} ${foo}''
"bar ${foo} bar"

[nshalman]

Thank you!

[Enzime]

@GrahamcOfBorg test boot.uefiNetboot boot.biosNetboot

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/prs-already-reviewed/2617/424

[nshalman]

@Enzime noted:

Although I'm not sure if I should default cmdline to config.boot.kernelParams if cmdline is not set, or if it should remain as it is now.

For clarity, it looks like currently config.boot.kernelParams just contains loglevel=4 if that helps with decision making.

Maybe @antonym can quickly weigh in with a suggestion? I don't have a strong preference either way.

[Enzime]

After thinking about it, I think just having cmdline be separate is better, that way you'll always get the kernel params you compiled with unless you explicitly override it in cmdline, otherwise you would have to retype all your kernel params in cmdline if you ever set it.

[nshalman]

After thinking about it, I think just having cmdline be separate is better, that way you'll always get the kernel params you compiled with unless you explicitly override it in cmdline, otherwise you would have to retype all your kernel params in cmdline if you ever set it.

I agree.

[github-actions]

Successfully created backport PR #155562 for release-21.11.