On Thu, 30 Jun 2022 at 20:25, Andrii Nakryiko [off-list ref] wrote:

On Wed, Jun 29, 2022 at 7:40 AM Quentin Monnet [off-list ref] wrote:

quoted

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 <redacted>
---
 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?

I'm not sure either, it looks like it's mostly a way to print that the
lib is missing (when it's the case) before attempting to compile [0].
Probably something we can look into removing, I agree the feature test
doesn't bring much here. We'll soon need a new test for the latest
libbfd changes though [1].

[0] https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/commit/?id=d66fa3c70e598746a907e5db5ed024035e01817a
[1] https://lore.kernel.org/bpf/20220622181918.ykrs5rsnmx3og4sv@alap3.anarazel.de/ (local)