Re: [PATCH bpf-next v2 5/5] selftests/bpf: test subskeleton functionality

[Delyan Kratunov <delyank@fb.com>]

On Fri, 2022-03-11 at 15:40 -0800, Andrii Nakryiko wrote:
> 
> we shouldn't need or use name for subskeleton (in real life you won't
> know the name of the final bpf_object)

Let's have this discussion in the bpftool email thread. Happy to remove the name
in the Makefile and fall back on the filename though.

> > 
> >  $(TRUNNER_BPF_LSKELS): %.lskel.h: %.o $(BPFTOOL) | $(TRUNNER_OUTPUT)
> >         $$(call msg,GEN-SKEL,$(TRUNNER_BINARY),$$@)
> > @@ -421,6 +428,7 @@ $(TRUNNER_BPF_SKELS_LINKED): $(TRUNNER_BPF_OBJS) $(BPFTOOL) | $(TRUNNER_OUTPUT)
> >         $(Q)diff $$(@:.skel.h=.linked2.o) $$(@:.skel.h=.linked3.o)
> >         $$(call msg,GEN-SKEL,$(TRUNNER_BINARY),$$@)
> >         $(Q)$$(BPFTOOL) gen skeleton $$(@:.skel.h=.linked3.o) name $$(notdir $$(@:.skel.h=)) > $$@
> > +       $(Q)$$(BPFTOOL) gen subskeleton $$(@:.skel.h=.linked3.o) name $$(notdir $$(@:.skel.h=)) > $$(@:.skel.h=.subskel.h)
> 
> probably don't need subskel for LSKELS (and it just adds race when we
> generate both skeleton and light skeleton for the same object file)

We're not generating subskels for LSKELS, that's just confusing diff output.
This is under the $(TRUNNER_BPF_SKELS_LINKED) outputs.

> 
> can you please add CONFIG_BPF_SYSCALL here as well, to check that
> externs are properly "merged" and found, even if they overlap between
> library and app BPF code

Sure.

> 
> libbpf supports .data.my_custom_name and .rodata.my_custom_whatever,
> let's have a variable to test this also works?

Sure.

> 
> let's move this into progs/test_subskeleton.c instead. It will
> simulate a bit more complicated scenario, where library expects
> application to define and provide a map, but the library itself
> doesn't define it. It should work just fine right now (I think), but
> just in case let's double check that having only "extern map" in the
> library works.

This fails to even open in bpftool:

libbpf: map 'map2': unsupported map linkage extern.
Error: failed to open BPF object file: Operation not supported

If we think this is valuable enough to support, let's tackle it separately after
the bulk of this functionality is merged?


-- Delyan