kselftests try to build with gcc

[nickdesaulniers]

$ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang HOSTCC=clang -j46 quicktest=1 kselftest
...
make[3]: aarch64-linux-gnu-gcc: Command not found
...

[nickdesaulniers]

They should be using $(HOSTCC), assuming it was exported correctly from the top level Makefile.

[bulwahn]

I plan to give this to my student as a good exercise to touch some build scripts and submit his first kernel patch.

[bulwahn]

To reproduce the issue, we actually do not need the cross-compile setup. It suffices to run make CC=clang HOSTCC=clang kselftest and observe that gcc is used for compiling.

[nickdesaulniers]

In particular, the author of the rseq system call mentioned that their kselftests fail to build because they reset KBUILD_CFLAGS, thus dropping -no-integrated-as. They should be using ${CLANG_FLAGS}.

So as part of this issue, it's not just that the kselftests build with GCC, but also that they may fail to build with Clang (and we should fix that, too).

[nickdesaulniers]

cc @gctucker

[gctucker]

Thanks for the heads, up. We're trying to fix all existing build errors we have in our environment with gcc first, then we'll take a look at building kselftests with clang.

[nathanchance]

Relevant patch/thread: https://lore.kernel.org/r/20201203221005.2813159-1-adelg@google.com/

[gctucker]

@nickdesaulniers BTW we've enabled clang builds with kselftest on staging, does this look good?
https://storage.staging.kernelci.org/kernelci/staging-next/staging-next-20210211.0/arm64/defconfig+kselftest/clang-11/build.log

[nathanchance]

I do see quite a few errors in there but it does look like clang is being used.

[nickdesaulniers]

yeah cool those are clang warnings all right!

[gctucker]

Nice, we're working on splitting out the kselftest build log, in fact having a build log for each step of the build job, and then that will get turned on in production. It's working on the command line already, hopefully it'll be there by the end of the month.

Then we could include a separate section in the build email reports and on the web dashboard for kselftest errors, or maybe have a "kselftest special" report with both build and test results. We'll be discussing that once we get there.

[nathanchance]

$ make -skj"$(nproc)" LLVM=1 LLVM_IAS=1 distclean defconfig quicktest=1 kselftest

shows that clang is being used so closing this issue. We can open individual issues for the errors that we see.

[gctucker]

FYI we now have separate build steps working on staging, here's a sample kselftest log:
https://storage.staging.kernelci.org/kernelci/staging-mainline/staging-mainline-20210423.0/x86_64/x86_64_defconfig+kselftest/clang-10/logs/kselftest.log

This should get enabled in production by the end of the month. We could then look into generating a kselftest special email report with both build and runtime issues.

[nickdesaulniers]

@gctucker nice! Looks like we have a good list of warnings+errors to start fixing in the kselftests!