netdev - Re: [PATCH bpf-next] selftests/bpf: suppress readelf stderr when probing for BTF support

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Date:   Wed, 23 Jan 2019 19:47:31 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Stanislav Fomichev <sdf@...ichev.me>
Cc:     Stanislav Fomichev <sdf@...gle.com>, netdev@...r.kernel.org,
        davem@...emloft.net, ast@...nel.org, daniel@...earbox.net
Subject: Re: [PATCH bpf-next] selftests/bpf: suppress readelf stderr when
 probing for BTF support

On Wed, Jan 23, 2019 at 07:15:07PM -0800, Stanislav Fomichev wrote:
> On 01/23, Alexei Starovoitov wrote:
> > On Wed, Jan 23, 2019 at 09:24:35AM -0800, Stanislav Fomichev wrote:
> > > Before:
> > > $ make -s -C tools/testing/selftests/bpf
> > > readelf: Error: Missing knowledge of 32-bit reloc types used in DWARF
> > > sections of machine number 247
> > > readelf: Warning: unable to apply unsupported reloc type 10 to section
> > > .debug_info
> > > readelf: Warning: unable to apply unsupported reloc type 1 to section
> > > .debug_info
> > > readelf: Warning: unable to apply unsupported reloc type 10 to section
> > > .debug_info
> > > 
> > > After:
> > > $ make -s -C tools/testing/selftests/bpf
> > > 
> > > Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
> > > ---
> > >  tools/testing/selftests/bpf/Makefile | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
> > > index 70229de510f5..ff3cab5525c1 100644
> > > --- a/tools/testing/selftests/bpf/Makefile
> > > +++ b/tools/testing/selftests/bpf/Makefile
> > > @@ -131,7 +131,7 @@ BTF_PAHOLE_PROBE := $(shell $(BTF_PAHOLE) --help 2>&1 | grep BTF)
> > >  BTF_OBJCOPY_PROBE := $(shell $(LLVM_OBJCOPY) --help 2>&1 | grep -i 'usage.*llvm')
> > >  BTF_LLVM_PROBE := $(shell echo "int main() { return 0; }" | \
> > >  			  $(CLANG) -target bpf -O2 -g -c -x c - -o ./llvm_btf_verify.o; \
> > > -			  readelf -S ./llvm_btf_verify.o | grep BTF; \
> > > +			  readelf -S ./llvm_btf_verify.o 2>/dev/null | grep BTF; \
> > >  			  /bin/rm -f ./llvm_btf_verify.o)
> > 
> > Silencing the warning may hide real issue later.
> > I'd rather fix whatever is necessary in readelf.
> > Does it use bfd? if so bpf support for bfd was practically complete.
> Are you concerned about the situation where llvm has btf support, but
> readelf (binutils) is unaware of it? In this case, we should probably
> use llvm's readelf (llvm-readelf binary). We already have a precedent of
> using llvm's tools with LLVM_OBJCOPY. Let me try it out tomorrow.

llvm-readelf or llvm-objdump are fine options too.