Re: [PATCH bpf-next v2 0/5] bpf: tools: support build selftests/bpf with clang

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

On Mon, Apr 12, 2021 at 4:29 PM Yonghong Song <yhs@fb.com> wrote:
>
> To build kernel with clang, people typically use
>   make -j60 LLVM=1 LLVM_IAS=1
> LLVM_IAS=1 is not required for non-LTO build but
> is required for LTO build. In my environment,
> I am always having LLVM_IAS=1 regardless of
> whether LTO is enabled or not.
>
> After kernel is build with clang, the following command
> can be used to build selftests with clang:
>   make -j60 -C tools/testing/selftests/bpf LLVM=1 LLVM_IAS=1
>
> But currently, using the above command, some compilations
> still use gcc and there are also compilation errors and warnings.
> This patch set intends to fix these issues.
> Patch #1 and #2 fixed the issue so clang/clang++ is
> used instead of gcc/g++. Patch #3 fixed a compilation
> failure. Patch #4 and #5 fixed various compiler warnings.
>

Might be good to add some hints like when the build stops or errors:

Like in my case when I had no CONFIG_DEBUG_INFO_BTF set.

Of course in combination with Clang-LTO a pointer "you need pahole
version 1.21." and CONFIG_DEBUG_INFO_BTF=y.

Finally, a hint for missing xxx-dev(el) packages (see Nick's report).

The tools directory has its own build rules thus I cannot say how to
check for specific Kconfigs.
I have not looked.

NOTE: I have not checked without setting CONFIG_DEBUG_INFO.

- Sedat -

> Changelog:
>   v1 -> v2:
>     . add -Wno-unused-command-line-argument and -Wno-format-security
>       for clang only as (1). gcc does not exhibit those
>       warnings, and (2). -Wno-unused-command-line-argument is
>       only supported by clang. (Sedat)
>
> Yonghong Song (5):
>   selftests: set CC to clang in lib.mk if LLVM is set
>   tools: allow proper CC/CXX/... override with LLVM=1 in
>     Makefile.include
>   selftests/bpf: fix test_cpp compilation failure with clang
>   selftests/bpf: silence clang compilation warnings
>   bpftool: fix a clang compilation warning
>
>  tools/bpf/bpftool/net.c              |  2 +-
>  tools/scripts/Makefile.include       | 12 ++++++++++--
>  tools/testing/selftests/bpf/Makefile |  7 ++++++-
>  tools/testing/selftests/lib.mk       |  4 ++++
>  4 files changed, 21 insertions(+), 4 deletions(-)
>
> --
> 2.30.2
>