netdev - Re: [PATCH bpf-next] bpftool: Allow disabling features at compile time

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Date:   Thu, 30 Jun 2022 12:25:07 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Quentin Monnet <quentin@...valent.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next] bpftool: Allow disabling features at compile time

On Wed, Jun 29, 2022 at 7:40 AM Quentin Monnet <quentin@...valent.com> wrote:
>
> Some dependencies for bpftool are optional, and the associated features
> may be left aside at compilation time depending on the available
> components on the system (libraries, BTF, clang version, etc.).
> Sometimes, it is useful to explicitly leave some of those features aside
> when compiling, even though the system would support them. For example,
> this can be useful:
>
>     - for testing bpftool's behaviour when the feature is not present,
>     - for copmiling for a different system, where some libraries are
>       missing,
>     - for producing a lighter binary,
>     - for disabling features that do not compile correctly on older
>       systems - although this is not supposed to happen, this is
>       currently the case for skeletons support on Linux < 5.15, where
>       struct bpf_perf_link is not defined in kernel BTF.
>
> For such cases, we introduce, in the Makefile, some environment
> variables that can be used to disable those features: namely,
> BPFTOOL_FEATURE_NO_LIBBFD, BPFTOOL_FEATURE_NO_LIBCAP, and
> BPFTOOL_FEATURE_NO_SKELETONS.
>
> Signed-off-by: Quentin Monnet <quentin@...valent.com>
> ---
>  tools/bpf/bpftool/Makefile | 20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/tools/bpf/bpftool/Makefile b/tools/bpf/bpftool/Makefile
> index c19e0e4c41bd..b3dd6a1482f6 100644
> --- a/tools/bpf/bpftool/Makefile
> +++ b/tools/bpf/bpftool/Makefile
> @@ -93,8 +93,24 @@ INSTALL ?= install
>  RM ?= rm -f
>
>  FEATURE_USER = .bpftool
> -FEATURE_TESTS = libbfd disassembler-four-args zlib libcap \
> -       clang-bpf-co-re
> +FEATURE_TESTS := disassembler-four-args zlib

as an aside, zlib is not really optional, libbpf depends on it and
bpftool depends on libbpf, so... what's the point of a feature test?

> +
> +# Disable libbfd (for disassembling JIT-compiled programs) by setting
> +# BPFTOOL_FEATURE_NO_LIBBFD
> +ifeq ($(BPFTOOL_FEATURE_NO_LIBBFD),)
> +  FEATURE_TESTS += libbfd
> +endif
> +# Disable libcap (for probing features available to unprivileged users) by
> +# setting BPFTOOL_FEATURE_NO_LIBCAP
> +ifeq ($(BPFTOOL_FEATURE_NO_LIBCAP),)
> +  FEATURE_TESTS += libcap
> +endif
> +# Disable skeletons (e.g. for profiling programs or showing PIDs of processes
> +# associated to BPF objects) by setting BPFTOOL_FEATURE_NO_SKELETONS
> +ifeq ($(BPFTOOL_FEATURE_NO_SKELETONS),)
> +  FEATURE_TESTS += clang-bpf-co-re
> +endif
> +
>  FEATURE_DISPLAY = libbfd disassembler-four-args zlib libcap \
>         clang-bpf-co-re
>
> --
> 2.34.1
>