Using `kernelModuleMakeFlags` breaks use of `NIX_CFLAGS_COMPILE`

[samueldr-at-cyberus]

With e9b2edb, part of #402198, the CC variable in kernelModuleMakeFlags was forced to the unwrapped variant of the compiler.

• https://github.com/NixOS/nixpkgs/blame/f86e9fe549f551d6bb738a1d01efa7d61b2da3e7/pkgs/os-specific/linux/kernel/common-flags.nix#L10-L11

This breaks usage of NIX_CFLAGS_COMPILE when using kernelModuleMakeFlags.

This may have weird consequences for more complicated builds that would include a C++ compiler, as CXX comes from the -wrapped compiler still (see #484933 for an example, though no weirdness was observed AFAICT).

I'm pretty sure replacing CC in that way is likely a breakage in our "contract" of how the compilers are used withing Nixpkgs. Though I don't truly know what the stance here would be.

cc @RossComputerGuy, author of the PR. cc @blitz, original author of the commit.

[blitz]

This may have weird consequences for more complicated builds that would include a C++ compiler, as CXX comes from the -wrapped compiler still

For consistency, we should set CXX to the unwrapped compiler as well. I think I didn't do this, because Linux doesn't use it. But at that point I didn't think about weird out-of-tree modules... From my perspective, the compiler for the target architecture should be unwrapped, because what Linux does is sufficiently advanced that we should not try to interfere. (Tiny caveat here: This applies to compiling the kernel. It doesn't really apply to compiling any userspace components for the target, but the Linux kernel build system doesn't make this distinction, AFAIK.)

I also wonder whether there is any usecase for NIX_CFLAGS_COMPILE for kernel compilation besides disabling warnings.

I'm pretty sure replacing CC in that way is likely a breakage in our "contract" of how the compilers are used withing Nixpkgs. Though I don't truly know what the stance here would be.

I'm happy to change this to something more in line with what the rest of nixpkgs does. The current setup is the result of making the kernel build with LLVM and LTO (also thanks to @RossComputerGuy and @alyssais for having carried this over the finishing line!).

[alyssais]

For consistency, we should set CXX to the unwrapped compiler as well.

👍

I also wonder whether there is any usecase for NIX_CFLAGS_COMPILE for kernel compilation besides disabling warnings.

Doubt it, because the kernel's build system gives us plenty of ways to override flags without going behind its back. NIX_CFLAGS_COMPILE should be a last resort for build systems that don't.

[samueldr-at-cyberus]

the kernel's build system gives us plenty of ways to override flags without going behind its back

When considering the kernel build itself, that is totally fair.

With that said, I haven't taken the time to look deeply, but does that apply to using the make -C $KDIR M=$PWD out-of-tree modules build tooling? Especially thinking about how it has to compose on top of the flags that were used for the particular kernel build.

[alyssais]

I haven't tested, but I don't see why KBUILD_CFLAGS+=... or whatever wouldn't work.