perf tools: Options being passed to clang when building the BPF part of skels

  Hi,

	On gentoo I'm noticing this:

clang-16clang-16: : clang-16error: : error: clang-16clang-16: clang-16clang-16clang-16: clang-16error: clang-16: : ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]: : : clang-16error: ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]error: error: : 
error: 
error: error: error: ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]error: ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]

ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]

ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]

And while just doing:



"Fixes" it, I'm curious as where is that we're setting up the options
that are being passed to clang at:

tools/perf/Makefile.perf

$(SKEL_TMP_OUT)/%.bpf.o: util/bpf_skel/%.bpf.c $(LIBBPF) $(SKEL_OUT)/vmlinux.h | $(SKEL_TMP_OUT)
        $(QUIET_CLANG)$(CLANG) -g -O2 --target=bpf -Wall -Werror $(BPF_INCLUDE) $(TOOLS_UAPI_INCLUDE) \
          -c $(filter util/bpf_skel/%.bpf.c,$^) -o $@

Perhaps it is plain just using CFLAGS? I.e. maybe the native build
CFLAGS is being passed to the BPF target clang calls?

Ideas?

- Arnaldo
  

Hi Arnaldo,

On Tue, Aug 22, 2023 at 09:33:16AM -0300, Arnaldo Carvalho de Melo wrote:
> Hi,
> 
> 	On gentoo I'm noticing this:
> 
> clang-16clang-16: : clang-16error: : error: clang-16clang-16: clang-16clang-16clang-16: clang-16error: clang-16: : ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]: : : clang-16error: ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]error: error: : 
> error: 
> error: error: error: ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]error: ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]
> 
> ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]
> 
> ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]
> 
> And while just doing:
> 
> diff --git a/tools/perf/Makefile.perf b/tools/perf/Makefile.perf
> index a5dd1ba..7726b57 100644
> --- a/tools/perf/Makefile.perf
> +++ b/tools/perf/Makefile.perf
> @@ -1101,7 +1101,7 @@ else
>  endif
>  
>  $(SKEL_TMP_OUT)/%.bpf.o: util/bpf_skel/%.bpf.c $(LIBBPF) $(SKEL_OUT)/vmlinux.h | $(SKEL_TMP_OUT)
> -	$(QUIET_CLANG)$(CLANG) -g -O2 --target=bpf -Wall -Werror $(BPF_INCLUDE) $(TOOLS_UAPI_INCLUDE) \
> +	$(QUIET_CLANG)$(CLANG) -g -O2 --target=bpf -Wall -Werror -Wno-option-ignored $(BPF_INCLUDE) $(TOOLS_UAPI_INCLUDE) \
>  	  -c $(filter util/bpf_skel/%.bpf.c,$^) -o $@
>  
>  $(SKEL_OUT)/%.skel.h: $(SKEL_TMP_OUT)/%.bpf.o | $(BPFTOOL)
> 
> 
> "Fixes" it, I'm curious as where is that we're setting up the options
> that are being passed to clang at:
> 
> tools/perf/Makefile.perf
> 
> $(SKEL_TMP_OUT)/%.bpf.o: util/bpf_skel/%.bpf.c $(LIBBPF) $(SKEL_OUT)/vmlinux.h | $(SKEL_TMP_OUT)
>         $(QUIET_CLANG)$(CLANG) -g -O2 --target=bpf -Wall -Werror $(BPF_INCLUDE) $(TOOLS_UAPI_INCLUDE) \
>           -c $(filter util/bpf_skel/%.bpf.c,$^) -o $@
> 
> Perhaps it is plain just using CFLAGS? I.e. maybe the native build
> CFLAGS is being passed to the BPF target clang calls?
> 
> Ideas?

In the patches I sent previously, I had a commit to address this. In
summary, the extra flags you see are hardening flags added by default.
Have also a look at /usr/include/gentoo/fortify.h. I suppose other 
distributions may add such flags as well. It's a pity the hardening
flags are added indiscriminately to all target architectures, but just
making the compiler not error out should be enough to work around the
issue. In my patch, I put -Werror under the control of the WERROR build
option and disabled it when configuring perf in the ebuild. Since other
warnings may be triggered in the future, having an easy way to disable
-Werror when building the package would be nice to have, otherwise when
a new compiler comes out users may hit build failures, since we are a
source based distribution.

Best regards,
-Guilherme
  

Em Tue, Aug 22, 2023 at 08:58:09PM +0200, Guilherme Amadio escreveu:
> On Tue, Aug 22, 2023 at 09:33:16AM -0300, Arnaldo Carvalho de Melo wrote:
> > 	On gentoo I'm noticing this:

> > clang-16clang-16: : clang-16error: : error: clang-16clang-16: clang-16clang-16clang-16: clang-16error: clang-16: : ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]: : : clang-16error: ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]error: error: : 
> > error: 
> > error: error: error: ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]error: ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]
> > 
> > ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]

> > ignoring '-fstack-protector-strong' option as it is not currently supported for target 'bpf' [-Werror,-Woption-ignored]

> > And while just doing:
> > 
> > diff --git a/tools/perf/Makefile.perf b/tools/perf/Makefile.perf
> > index a5dd1ba..7726b57 100644
> > --- a/tools/perf/Makefile.perf
> > +++ b/tools/perf/Makefile.perf
> > @@ -1101,7 +1101,7 @@ else
> >  endif

> >  $(SKEL_TMP_OUT)/%.bpf.o: util/bpf_skel/%.bpf.c $(LIBBPF) $(SKEL_OUT)/vmlinux.h | $(SKEL_TMP_OUT)
> > -	$(QUIET_CLANG)$(CLANG) -g -O2 --target=bpf -Wall -Werror $(BPF_INCLUDE) $(TOOLS_UAPI_INCLUDE) \
> > +	$(QUIET_CLANG)$(CLANG) -g -O2 --target=bpf -Wall -Werror -Wno-option-ignored $(BPF_INCLUDE) $(TOOLS_UAPI_INCLUDE) \
> >  	  -c $(filter util/bpf_skel/%.bpf.c,$^) -o $@

> >  $(SKEL_OUT)/%.skel.h: $(SKEL_TMP_OUT)/%.bpf.o | $(BPFTOOL)

> > "Fixes" it, I'm curious as where is that we're setting up the options
> > that are being passed to clang at:

> > tools/perf/Makefile.perf

> > $(SKEL_TMP_OUT)/%.bpf.o: util/bpf_skel/%.bpf.c $(LIBBPF) $(SKEL_OUT)/vmlinux.h | $(SKEL_TMP_OUT)
> >         $(QUIET_CLANG)$(CLANG) -g -O2 --target=bpf -Wall -Werror $(BPF_INCLUDE) $(TOOLS_UAPI_INCLUDE) \
> >           -c $(filter util/bpf_skel/%.bpf.c,$^) -o $@

> > Perhaps it is plain just using CFLAGS? I.e. maybe the native build
> > CFLAGS is being passed to the BPF target clang calls?

> > Ideas?

> In the patches I sent previously, I had a commit to address this. In

I tried the one that introduced CLANG_OPTIONS and checked WERROR= to set
Werror, read more further down this message.

I think we need to have it but for this problem I want to have finer
grained control on what goes into the CFLAGS for CLANG _when building
BPF bytecode_.

> summary, the extra flags you see are hardening flags added by default.
> Have also a look at /usr/include/gentoo/fortify.h. I suppose other 

> distributions may add such flags as well. It's a pity the hardening
> flags are added indiscriminately to all target architectures, but just

Oh, I see, so we need to overcome that in the perf Makefiles by
_removing_ those unsupported options for the BPF target, something like
what is done in tools/perf/util/setup.py, because some of the options
may not be supported for one version of clang but be supported in a
newer one :-\

> making the compiler not error out should be enough to work around the
> issue. In my patch, I put -Werror under the control of the WERROR build
> option and disabled it when configuring perf in the ebuild. Since other

Right, so it is the one I saw.

> warnings may be triggered in the future, having an easy way to disable
> -Werror when building the package would be nice to have, otherwise when

Right, I'll apply that patch, I tested it, it just didn't some the
problem at hand, i.e. after applying it one has to use WERROR=0.

- Arnaldo

> a new compiler comes out users may hit build failures, since we are a
> source based distribution.
> 
> Best regards,
> -Guilherme