Re: [PATCH perf] perf: ignore deprecation warning when using libbpf's btf__get_from_id()

On Tue, Sep 14, 2021 at 11:21 AM Jiri Olsa [off-list ref] wrote:

On Tue, Sep 14, 2021 at 10:00:04AM -0700, Andrii Nakryiko wrote:

quoted

Perf code re-implements libbpf's btf__load_from_kernel_by_id() API as
a weak function, presumably to dynamically link against old version of
libbpf shared library. Unfortunately this causes compilation warning
when perf is compiled against libbpf v0.6+.

For now, just ignore deprecation warning, but there might be a better
solution, depending on perf's needs.

HI,
the problem we tried to solve is when perf is using symbols
which are not yet available in released libbpf.. but it all
linkes in default perf build because it's linked statically
libbpf.a in the tree

If you are always statically linking libbpf into perf, there is no
need to implement this __weak shim. Libbpf is never going to deprecate
an API if a new/replacement API hasn't been at least in a previous
released version. So in this case btf__load_from_kernel_by_id() was
added in libbpf 0.5, and btf__get_from_id() was marked deprecated in
libbpf 0.6 (not yet released, of course). So with that, do you still
think we need this __weak re-implementation?

I was wondering if this was done to make latest perf code compile
against some old libbpf source code or dynamically linked against old
libbpf. But if that's not the case, the fix should be a removal of
__weak btf__load_from_kernel_by_id().

so now we have weak function with that warning disabled locally,
which I guess could work?  also for future cases like that

jirka

quoted

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
 tools/perf/util/bpf-event.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tools/perf/util/bpf-event.c b/tools/perf/util/bpf-event.c
index 683f6d63560e..1a7112a87736 100644
--- a/tools/perf/util/bpf-event.c
+++ b/tools/perf/util/bpf-event.c

@@ -24,7 +24,10 @@
 struct btf * __weak btf__load_from_kernel_by_id(__u32 id)
 {
        struct btf *btf;
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
        int err = btf__get_from_id(id, &btf);
+#pragma GCC diagnostic pop

        return err ? ERR_PTR(err) : btf;
 }
--

2.30.2