Re: [PATCH bpf-next 3/5] selftests/bpf: fix test_cpp compilation failure with clang

[Sedat Dilek <sedat.dilek@gmail.com>]

On Sat, Apr 10, 2021 at 6:49 PM Yonghong Song <yhs@fb.com> wrote:
>
> With clang compiler:
>   make -j60 LLVM=1 LLVM_IAS=1  <=== compile kernel
>   make -j60 -C tools/testing/selftests/bpf LLVM=1 LLVM_IAS=1
> the test_cpp build failed due to the failure:
>   warning: treating 'c-header' input as 'c++-header' when in C++ mode, this behavior is deprecated [-Wdeprecated]
>   clang-13: warning: cannot specify -o when generating multiple output files
>
> test_cpp compilation flag looks like:
>   clang++ -g -Og -rdynamic -Wall -I<...> ... \
>   -Dbpf_prog_load=bpf_prog_test_load -Dbpf_load_program=bpf_test_load_program \
>   test_cpp.cpp <...>/test_core_extern.skel.h <...>/libbpf.a <...>/test_stub.o \
>   -lcap -lelf -lz -lrt -lpthread -o <...>/test_cpp
>
> The clang++ compiler complains the header file in the command line.
> Let us remove the header file from the command line which is not intended
> any way, and this fixed the problem.
>
> Signed-off-by: Yonghong Song <yhs@fb.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 6448c626498f..bbd61cc3889b 100644
> --- a/tools/testing/selftests/bpf/Makefile
> +++ b/tools/testing/selftests/bpf/Makefile
> @@ -481,7 +481,7 @@ $(OUTPUT)/test_verifier: test_verifier.c verifier/tests.h $(BPFOBJ) | $(OUTPUT)
>  # Make sure we are able to include and link libbpf against c++.
>  $(OUTPUT)/test_cpp: test_cpp.cpp $(OUTPUT)/test_core_extern.skel.h $(BPFOBJ)
>         $(call msg,CXX,,$@)
> -       $(Q)$(CXX) $(CFLAGS) $^ $(LDLIBS) -o $@
> +       $(Q)$(CXX) $(CFLAGS) test_cpp.cpp $(BPFOBJ) $(LDLIBS) -o $@
>
>  # Benchmark runner
>  $(OUTPUT)/bench_%.o: benchs/bench_%.c bench.h
> --
> 2.30.2
>

NOTE: bpf-next might be different from my build-environment.

Yonghong, can you please re-test by adding explicitly CXX=g++ to your make line?

Here I have:

$ grep test_cpp make-log_tools-testing-selftests-bpf_clang_clang++.txt
1907:clang++ -g -rdynamic -Wall -O2 -DHAVE_GENHDR
-I/home/dileks/src/linux-kernel/git/tools/testing/selftests/bpf
-I/home/dileks/src/linux-kernel/git/tools/testing/selftests/bpf/tools/include
-I/home/dileks/src/linux-kernel/git/include/generated
-I/home/dileks/src/linux-kernel/git/tools/lib
-I/home/dileks/src/linux-kernel/git/tools/include
-I/home/dileks/src/linux-kernel/git/tools/include/uapi
-I/home/dileks/src/linux-kernel/git/tools/testing/selftests/bpf
-Wno-unused-command-line-argument -Wno-format-security
-Dbpf_prog_load=bpf_prog_test_load
-Dbpf_load_program=bpf_test_load_program test_cpp.cpp
/home/dileks/src/linux-kernel/git/tools/testing/selftests/bpf/tools/build/libbpf/libbpf.a
-lcap -lelf -lz -lrt -lpthread -o
/home/dileks/src/linux-kernel/git/tools/testing/selftests/bpf/test_cpp

This clang++ line does not include <...>/test_core_extern.skel.h ^^^

$ grep test_core_extern.skel.h
make-log_tools-testing-selftests-bpf_clang_clang++.txt
704:/home/dileks/src/linux-kernel/git/tools/testing/selftests/bpf/tools/sbin/bpftool
gen skeleton /home/dileks/src/linux-kernel/git/tools/testing/selftests/bpf/test_co
re_extern.o > /home/dileks/src/linux-kernel/git/tools/testing/selftests/bpf/test_core_extern.skel.h
1592:/home/dileks/src/linux-kernel/git/tools/testing/selftests/bpf/tools/sbin/bpftool
gen skeleton /home/dileks/src/linux-kernel/git/tools/testing/selftests/bpf/no_alu
32/test_core_extern.o >
/home/dileks/src/linux-kernel/git/tools/testing/selftests/bpf/no_alu32/test_core_extern.skel.h

Checking test_cpp:

$ /opt/llvm-toolchain/bin/llvm-objdump-12 -Dr test_cpp | grep extern
0000000000417e50 <cmp_externs>:
 417e54: 75 22                         jne     0x417e78 <cmp_externs+0x28>
 417e59: 75 10                         jne     0x417e6b <cmp_externs+0x1b>
 417e61: 75 21                         jne     0x417e84 <cmp_externs+0x34>
 417e69: 75 1e                         jne     0x417e89 <cmp_externs+0x39>
 417e87: eb f2                         jmp     0x417e7b <cmp_externs+0x2b>
 417e8c: eb ed                         jmp     0x417e7b <cmp_externs+0x2b>

$ /opt/llvm-toolchain/bin/llvm-objdump-12 -Dr test_cpp | grep core_extern
[ EMPTY ]

When compiled with g++ in an earlier setup this contained "core_extern".

With this version of your patchser it breaks *here* when using CXX=g++
(and uses /usr/bin/ld as linker):

g++ -g -rdynamic -Wall -O2 -DHAVE_GENHDR
-I/home/dileks/src/linux-kernel/git/tools/testing/selftests/bpf
-I/home/dileks/src/linux-kernel/git/tools/testing/selftests/b
pf/tools/include -I/home/dileks/src/linux-kernel/git/include/generated
-I/home/dileks/src/linux-kernel/git/tools/lib
-I/home/dileks/src/linux-kernel/git/tools/include
-I/home/dileks/src/linux-kernel/git/tools/include/uapi
-I/home/dileks/src/linux-kernel/git/tools/testing/selftests/bpf
-Wno-unused-command-line-argument -Wno-format-security
-Dbpf_prog_load=bpf_prog_test_load
-Dbpf_load_program=bpf_test_load_program test_cpp.cpp
/home/dileks/src/linux-kernel/git/tools/testing/selftests/bpf/tools/build/libbpf/libbpf.a
-lcap -lelf -lz -lrt -lpthread -o
/home/dileks/src/linux-kernel/git/tools/testing/selftests/bpf/test_cpp

/usr/bin/ld: /home/dileks/src/linux-kernel/git/tools/testing/selftests/bpf/tools/build/libbpf/libbpf.a(libbpf-in.o):
relocation R_X86_64_32 against `.rodata.str1.1' ca
n not be used when making a PIE object; recompile with -fPIE
collect2: error: ld returned 1 exit status
make: *** [Makefile:457:
/home/dileks/src/linux-kernel/git/tools/testing/selftests/bpf/test_cpp]
Error 1

$ grep test_cpp ../make-log_tools-testing-selftests-bpf_clang_g++.txt
| grep test_core_extern.skel.h
[ EMPTY ]

As said I do NOT use bpf-next.

- Sedat -



- Sedat -