Re: [PATCH v3 1/2] perf build: Properly guard libbpf includes

[Ian Rogers <irogers@google.com>]

On Mon, Jan 9, 2023 at 10:10 AM Jiri Olsa <olsajiri@gmail.com> wrote:
>
> On Mon, Jan 09, 2023 at 12:12:15PM -0300, Arnaldo Carvalho de Melo wrote:
> > Em Fri, Jan 06, 2023 at 11:06:46AM -0800, Ian Rogers escreveu:
> > > So trying to get build-test working on my Debian derived distro is a
> > > PITA with broken feature detection for options I don't normally use.
> >
> > Its really difficult to have perf building with so many dependent
> > libraries, mowing out some should be in order.
> >
> > > I'll try to fix this.
> >
> > Thanks.
> >
> > > In any case I think I've spotted what is really happening here and it
> > > isn't a failure but a feature :-D The build is specifying
> >
> > I get it.
> >
> > > LIBBPF_DYNAMIC=1 which means you get the libbpf headers from
> > > /usr/include. I think the build is trying to do this on a system with
> > > an old libbpf and hence getting the failures above. Previously, even
> > > though we wanted the dynamic headers we still had a -I, this time for
> > > the install_headers version. Now you really are using the system
> > > version and it is broken. This means a few things:
> > > - the libbpf feature test should fail if code like above is going to fail,
> >
> > Agreed.
> >
> > > - we may want to contemplate supporting older libbpfs (I'd rather not),
> >
> > I'd rather require everybody to be up to the latest trends, but I really
> > don't think that is a reasonable expectation.
> >
> > > - does build-test have a way to skip known issues like this?
> >
> > Unsure, Jiri?
>
> I don't think so it just triggers the build, it's up to the features check
> to disable the feature if the library is not compatible with perf code
>
> could we add that specific libbpf call to the libbpf feature check?

Looking at the failure closer, the failing code is code inside a
feature check trying to workaround the feature not being present. We
need to do something like:

```
diff --git a/tools/perf/util/bpf-loader.c b/tools/perf/util/bpf-loader.c
index 6e9b06cf06ee..a1c3cc230273 100644
--- a/tools/perf/util/bpf-loader.c
+++ b/tools/perf/util/bpf-loader.c
@@ -33,17 +33,18 @@
#include <internal/xyarray.h>

#ifndef HAVE_LIBBPF_BPF_PROGRAM__SET_INSNS
-int bpf_program__set_insns(struct bpf_program *prog __maybe_unused,
-                          struct bpf_insn *new_insns __maybe_unused,
size_t new_insn_cnt __maybe_un
used)
+static int bpf_program__set_insns(struct bpf_program *prog __maybe_unused,
+                                 struct bpf_insn *new_insns __maybe_unused,
+                                 size_t new_insn_cnt __maybe_unused)
{
       pr_err("%s: not support, update libbpf\n", __func__);
       return -ENOTSUP;
}

-int libbpf_register_prog_handler(const char *sec __maybe_unused,
-                                 enum bpf_prog_type prog_type __maybe_unused,
-                                 enum bpf_attach_type exp_attach_type
__maybe_unused,
-                                 const struct
libbpf_prog_handler_opts *opts __maybe_unused)
+static int libbpf_register_prog_handler(const char *sec __maybe_unused,
+                                       enum bpf_prog_type prog_type
__maybe_unused,
+                                       enum bpf_attach_type
exp_attach_type __maybe_unused,
+                                       const void *opts __maybe_unused)
{
       pr_err("%s: not support, update libbpf\n", __func__);
       return -ENOTSUP;
```

There are some other fixes necessary too. I'll try to write the fuller
patch but I have no means for testing except for undefining
HAVE_LIBBPF_BPF_PROGRAM__SET_INSNS.

Thanks,
Ian

> jirka
>
> >
> > But yeah, previous experiences with Andrii were that we can do not too
> > costly feature checks, not using .c programs that would fail if some
> > required feature wasn't present but instead would just do some grep on a
> > header and if some "smell" wasn't scent, just fail the cap query.
> >
> > - Arnaldo