Re: [PATCH bpf-next v2] selftests/bpf: use auto-dependencies for test objects

[Eduard Zingerman <eddyz87@gmail.com>]

On Fri, 2024-07-12 at 17:48 +0000, Ihor Solodrai wrote:

[...]

> I've made a mistake when I removed $(TRUNNER_BPF_OBJS) as a
> prerequisite for $(TRUNNER_TEST_OBJS:.o=.d)
> 
> I assumed it is covered by:
> 
>   $(TRUNNER_BPF_SKELS): %.skel.h: %.bpf.o $(BPFTOOL) | $(TRUNNER_OUTPUT)
> 
> Apparently there are .bpf.o files for which skels are not generated,
> yet they are used in tests.
> 
> Fixed in v3.

So, bear with me for a moment please.
We have 3 test_progs/smth.c files that depend on a few .bpf.o files at runtime,
but do not include skel files for those .bpf.o, namely:
- core_reloc.c
- verifier_bitfield_write.c
- pinning.c

And we fix this by adding a dependency:

    $(TRUNNER_TEST_OBJS:.o=.d): ... $(TRUNNER_BPF_OBJS)

Which makes all *.test.d files depend on .bpf.o files.
Thus, if progs/some.c file is changed and `make test_progs` is requested:
- because *.test.d files are included into current makefile [1],
  make invokes targets for *.test.d files;
- *.test.d targets depend on *.bpf.o, thus some.bpf.o is rebuilt
  (but only some.bpf.o, dependencies for other *.bpf.o are up to date);
- case A, skel for some.c is not included anywhere (CI failure for v2):
  - nothing happens further, as *.test.d files are unchanged *.test.o
    files are not rebuilt and test_progs is up to date
- case B, skel for some.c is included in prog_tests/other.c:
  - existing other.test.d lists some.skel.h as a dependency;
  - this dependency is not up to date, so other.test.o is rebuilt;
  - test_progs is rebuilt.

Do I understand the above correctly?

An alternative fix would be to specify additional dependencies for
core_reloc.test.o (and others) directly, e.g.:

    core_reloc.test.o: test_core_reloc_module.bpf.o ...

(with correct trunner prefix)

What are pros and cons between these two approaches?

[1] https://make.mad-scientist.net/constructed-include-files/