Overriding nativeBuildInputs on buildLinux

Development Nixpkgs Architecture
1

I need to build an older kernel, namely 5.16.9, on NixOS 22.11 in order to find a bug as mentioned in another thread here: Building kernel 5.16.9 from source fails, how to downgrade dwarves/pahole

However due to some problem with pahole version 1.24 these kernels no longer build on NixOS 22.11. Downgrading to pahole version 1.23 should avoid the issue as mentioned here: pahole v1.24: FAILED: load BTF from vmlinux: Invalid argument

Question How to downgrade pahole in building the Linux kernel short of downgrading NixOS as a whole? Since pahole is in the nativeBuildInputs in this file here https://github.com/NixOS/nixpkgs/blob/fbf79d73ec78596a7e9f0449619412ef13cdcbf8/pkgs/os-specific/linux/kernel/manual-config.nix I’d assume that is what I’d have to override.

Hence the more general question: How to override nativeBuildInputs on buildLinux?

(p.s. I decided to make another thread for this here in the development section as it is a rather specific question concerning the build process for the Linux kernel on NixOS. And it might concern others when formulating it this way.)

2

It looks like pahole isn’t used by anything except the Linux kernel, so you should just be able to git checkout $OLD_REVISION pkgs/development/tools/misc/pahole/ and then nix-build your kernel.

Well yes, but that pahole is passed as a function argument (the whole file is one gigantic function) declared here:

The callPackage ../os-specific/linux/kernel/manual-config.nix {} here is what passes that argument (callPackage is a bit of voodoo wizardry that fills in any missing arguments, using the top-level package set):

So, basically,

  1. You’re unlucky, the kernel-building parts of nixpkgs are… well… a bit tangled. If you’re doing anything out of the ordinary they can be really hard to keep under control. I eventually gave up and wrote my own kernel build expression when I put together ownerboot.

but,

  1. You’re in luck, you can (probably) just roll back the pahole/default.nix expression and everything should just work.
1 Like
3

@amjoseph

Thanks for the detailed reply!

So since I am building my system from a flake, I’d assume I should just use a package overlay for pahole in the nixpkgs.overlays = [ ]; for my system and override the attribute set to take version 1.23? Then buildLinux will just pick it up. At least I will try that later today.

4

I’m afraid I have no experience with flakes, but I think you’re on the right track there.

5

Alright, it appears I can now build and boot kernel 5.16.9 again. :grin:

For anyone who might be interested, my overlay solution is here:

overlay-custom-kernel = final: prev: {
        pahole = prev.pahole.overrideAttrs (oldAttrs: {
          version = "1.23";
          src = prev.fetchgit {
            url = "https://git.kernel.org/pub/scm/devel/pahole/pahole.git";
            rev = "v1.23";
            sha256 = "sha256-Dt3ZcUfjwdtTTv6qRFRgwK5GFWXdpN7fvb9KhpS1O94=";
          };

          buildInputs = [ prev.elfutils prev.zlib final.libbpf-0-7-0 ];
        });

        libbpf-0-7-0 = prev.libbpf.overrideAttrs (oldAttrs: {
          version = "0.7.0";

          src = prev.fetchFromGitHub {
            owner = "libbpf";
            repo = "libbpf";
            rev = "v0.7.0";
            sha256 = "sha256-NFVJ8JquWVzu+QoaaOMzhnu6/IqdP1FPhtJFidXA4L4=";
          };

          buildInputs = [ prev.libelf prev.zlib ];
        });
      };

As you can see, besides downgrading pahole itself, I also needed to downgrade libbpf on which pahole version 1.23 depends. So I created another override for this one and made the pahole override depend on its result.

Later when defining my system in my flake the overlay is used like so:

nixpkgs.overlays = [ overlay-custom-kernel ];