Re: [PATCH bpf-next 3/3] selftests/bpf: Allow ignoring some flags for Clang builds

[Eduard Zingerman <eddyz87@gmail.com>]

On Wed, 2024-10-16 at 13:37 -0700, Andrii Nakryiko wrote:
> On Mon, Oct 14, 2024 at 11:55 PM Viktor Malik <vmalik@redhat.com> wrote:
> > 
> > There exist compiler flags supported by GCC but not supported by Clang
> > (e.g. -specs=...). Currently, these cannot be passed to BPF selftests
> > builds, even when building with GCC, as some binaries (urandom_read and
> > liburandom_read.so) are always built with Clang and the unsupported
> > flags make the compilation fail (as -Werror is turned on).
> > 
> > Add new Makefile variable CLANG_FILTEROUT_FLAGS which can be used by
> > users to specify which flags (from the user-provided CFLAGS or LDFLAGS)
> > should be filtered out for Clang invocations.
> > 
> > This allows to do things like:
> > 
> >     $ CFLAGS="-specs=/usr/lib/rpm/redhat/redhat-hardened-cc1" \
> >       CLANG_FILTEROUT_FLAGS="-specs=%" \
> >       make -C tools/testing/selftests/bpf
> > 
> > Without this patch, the compilation would fail with:
> > 
> >     [...]
> >     clang: error: argument unused during compilation: '-specs=/usr/lib/rpm/redhat/redhat-hardened-cc1' [-Werror,-Wunused-command-line-argument]
> 
> maybe we should just not error out (i.e., enable
> -Wno-unused-command-line-argument)?

I agree with Andrii, grepping for FILTEROUT in kernel source code does
not show anything similar to this. Are such filter-out variables some
kind of convention?

Another option might be to remove `-Werror` and add it on CI via EXTRA_CFLAGS.

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