linux: add extra passthru

[eljamm]

Changes

Currently, kernel update scripts can't be triggered, given that passthru doesn't propagate to the final passthru.
This PR introduces extraPassthru (similar to extraMeta) for properly propagating the attribute.

Also, this PR switches the Zen and Libre kernels to use extraPassthru.updateScript, since they're the only ones that have update scripts, at the moment.

Test

Before

$ nix-shell maintainers/scripts/update.nix --argstr package linux_zen
error:
       … while evaluating 'strict' to select 'drvPath' on it
         at /builtin/derivation.nix:1:552:
       … while calling the 'derivationStrict' builtin
         at /builtin/derivation.nix:1:208:
       (stack trace truncated; use '--show-trace' to show the full trace)

       error: Package with an attribute name `linux_zen` does not have a `passthru.updateScript` attribute defined.

After

$ nix-shell maintainers/scripts/update.nix --argstr package linux_zen

Going to be running update for following packages:
 - linux-zen-6.14.7

Press Enter key to continue...

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• 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/)
• Nixpkgs 25.11 Release Notes (or backporting 24.11 and 25.05 Nixpkgs Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
• NixOS 25.11 Release Notes (or backporting 24.11 and 25.05 NixOS Release notes)
  • (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
• Fits CONTRIBUTING.md.

---

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[eljamm]

Manually pinging Zen and Libre kernel maintainers, given CI won't do it because of the zero rebuilds:
@thiagokokada @JerrySM64 @AxerTheAxe @alyssais

[ShamrockLee]

Do you know what the existing passthru variable is for? (I'll take a look tomorrow with an IDE. The phone screen is too small for a thorough review.)

BTW, I think passthru is a better name than extraPassthru.

[eljamm]

Do you know what the existing passthru variable is for?

I'm not very familiar with the inner workings of how kernels are composed, but I took this as a chance to take a deeper dive and here is what I understand so far:

• First, generic.nix creates an overridableKernel
  • An overridableKernel does what it says: it takes an attrset with generic kernel attributes (version, features, ...) and makes them overridable, but the basis of this is a kernel (which is also overridable)
  • A kernel recursively updates two passthrus: one inherits commonMakeFlags and another inherits some attributes.
  • So basically, a kernel is an override for manual-config.nix in order to set kernel-specific details (e.g. meta.maintainers), as opposed to the default ones.
• Second, we override the result, but for attributes and not arguments. Specifically, we compose a final passthru.
  • This combines the previously-created passthru with a bunch of other attributes
  • I suppose the redundancy here for some of the attributes is to ensure that kernel-specific values are applied. For example, if you remove isZen here, then you will have the wrong result when you evaluate:

    nix-repl> linux_zen.passthru.isZen
    false
    

    Whereas when it's set, you get:

    nix-repl> linux_zen.passthru.isZen
    true
    

• Finally, all of this is composed into a buildLinux function, which is then used to create kernel detivations.

To conclude, a kernel derivation's passthru is always internally composed, so setting something like passthru.updateScript is not propagated to the final passthru, which this PR essentially fixes.

BTW, I think passthru is a better name than extraPassthru.

I just tried to keep things consistent with extraMeta (which I don't know why it was called that way, either). I don't mind either name, but I'd like to hear what the other maintainers prefer as well and possibly agree on one of them.

[alyssais]

BTW, I think passthru is a better name than extraPassthru.

I just tried to keep things consistent with extraMeta (which I don't know why it was called that way, either). I don't mind either name, but I'd like to hear what the other maintainers prefer as well and possibly agree on one of them.

Yeah, I agree. I wouldn't have called it extraMeta either, but there's not much point changing it now, and it's good to be consistent with extraPassthru.

[ShamrockLee]

I'll take a look tomorrow with an IDE.

Sorry for missing this PR. I'm having too much on my plate.

I'll try to review this weekend.

[eljamm]

Friendly ping to @ShamrockLee

[thiagokokada]

Not related to this PR but kind strange that we are merging basicArgs to passthru.

[thiagokokada]

For example:

nix-repl> legacyPackages.x86_64-linux.linuxPackages_zen.kernel.passthru
{
  baseVersion = "6.14.7";
  commonMakeFlags = [ ... ];
  commonStructuredConfig = { ... };
  config = { ... };
  configEnv = «derivation /nix/store/ncilq58kr4pl2r10pqgnwnm0sgbwkggm-linux-zen-6.14.7.drv»;
  configfile = «derivation /nix/store/c1nfai5wq9x84wljlisqb2pri1r615s3-linux-config-6.14.7.drv»;
  extraMakeFlags = [ ... ];
  features = { ... };
  isHardened = false;
  isLibre = false;
  isXen = true;
  isZen = true;
  kernelAtLeast = «lambda versionAtLeast @ /nix/store/8ky1s7q96lz5n6gpyb6gdsiqhxin6dfr-source/lib/strings.nix:1944:24»;
  kernelOlder = «lambda versionOlder @ /nix/store/8ky1s7q96lz5n6gpyb6gdsiqhxin6dfr-source/lib/strings.nix:1910:22»;
  kernelPatches = [ ... ];
  modDirVersion = "6.14.7-zen1";
  moduleBuildDependencies = [ ... ];
  pname = "linux-zen";
  src = «derivation /nix/store/2by3j92cr187df98ijql39jj813az299-source.drv»;
  stdenv = «derivation /nix/store/ljjhsmahjyc0l49q4v21mkzrhn2p24ra-stdenv-linux.drv»;
  structuredExtraConfig = { ... };
  tests = { ... };
  updateScript = [ ... ];
  version = "6.14.7";
  withRust = true;
}

I don't think things like src or stdenv is supposed to be in passthru.

[eljamm]

I think this is why it was added: 6318529. Don't know if it's still relevant or not for our current infra.

[thiagokokada]

LGTM:

nix-repl> :p legacyPackages.x86_64-linux.linuxPackages_zen.kernel.passthru.updateScript
[
  /nix/store/8ky1s7q96lz5n6gpyb6gdsiqhxin6dfr-source/pkgs/os-specific/linux/kernel/update-zen.py
  "zen"
]