nixos/hidepid: remove module, it's broken

[xaverdh]

Motivation for this change

hidepid is currently broken.
cf. #73800 and #111629

Things done

Built the manual.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[xaverdh]

How would I go about adding a trace to the module / is there some official infrastructure for doing this?
Or should I just add builtins.trace in config after mkIf config.security.hideProcessInformation (hopefully this is lazy enough then)?

[joachifm]

If it cannot be fixed/is broken for all kernels, I propose removing the module & adding an entry to https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/rename.nix

[joachifm]

If the problem is only with wayland/graphical configs, I suggest adding a warning via the module assert mechanism.

[flokli]

hidepid is sufficiently broken, doesn't work with cgroupsv2 (so is a dead end on that front already).

It breaks userspace, like gdm. Having it in the hardened profile is yet another footgun.

Removing it from the module system entirely seems to be the right course of action - if people want to bring it back once these bugs are fixed, there's always the possibility to re-introduce, but we should probably ship less footguns.

@xaverdh, can you update the PR, and add a small release note entry?

[flokli]

Please also add a line to nixos/doc/manual/release-notes/rl-2105.xml.

[xaverdh]

I think this should be good now

[flokli]

@GrahamcOfBorg eval

[nixos-discourse]

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

https://discourse.nixos.org/t/whats-the-state-of-hidepid/51886/1