linux: disable KUnit

[yu-re-ka]

fixes a crash when loading the vc4 driver

Description of changes

Fixes parts of NixOS/nixos-hardware#631
The downstream vc4 driver uses kunit tests: https://github.com/raspberrypi/linux/blob/9d516c9f10eb7c6950131488e9893d3c1530c0dc/Documentation/gpu/vc4.rst#kunit-tests

When it is detected that the driver is running in a kunit test, it frees an allocation that is later needed to work with real hardware.

Let's free the dlist allocation right away if
we're running under kunit, we won't risk a dlist corruption anyway.

vc4_hvs.c

For some reason, when our config has CONFIG_KUNIT=m, it believes it is running in a kunit test and this leads to a crash such as NixOS/nixos-hardware#631 (comment).

Raspbian does not have AUTO_MODULES like we have, so there CONFIG_KUNIT is always 'n'.

I'm not entirely sure if this is the correct place to add it, but seemingly there is no other place either.

Upstream bug report: raspberrypi/linux#5568

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/)
• 23.11 Release Notes (or backporting 23.05 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
• Fits CONTRIBUTING.md.

[purcell]

The method for this fix seems reasonable, and the fix itself is vital for this kernel to work correctly.

[kloenk]

KUNIT is not designed for production use. LGTM

[yu-re-ka]

Changed from a linux_rpi* specific patch to generic KUNIT = no in common-config.nix

[kloenk]

Disabling globally sounds more reasonable. LGTM

[lheckemann]

@yu-re-ka wrote to me as a member of the security team since she suspected that the option previously broke KASLR (thanks for your vigilance!), but it seems not to be the case based on this test:

with import ./. {};
nixosTest {
  name = "kaslr";
  nodes.machine = {
    # uncommenting this breaks the test, so I guess kaslr works
    #boot.kernelParams = ["nokaslr"];
  };
  testScript = ''
    start_all()
    a = machine.succeed("grep pvh_start_xen /proc/kallsyms").strip()
    machine.shutdown()
    start_all()
    b = machine.succeed("grep pvh_start_xen /proc/kallsyms").strip()
    assert a != b
  '';
}

[vcunat]

This broke configuration of some older kernels, e.g. 5.4:
https://hydra.nixos.org/build/231004748/nixlog/1/tail

[trofi]

I think KUNIT appeared first in 5.5. Proposed the tweak as #248663