linux-kernel - Re: [PATCH 1/2] perf tools: Add more weak libbpf functions

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Date:   Sat, 13 Nov 2021 10:40:23 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Ian Rogers <irogers@...gle.com>
Cc:     Jiri Olsa <jolsa@...hat.com>, lkml <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Ingo Molnar <mingo@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Michael Petlan <mpetlan@...hat.com>,
        linux-perf-users@...r.kernel.org,
        Andrii Nakryiko <andrii@...nel.org>
Subject: Re: [PATCH 1/2] perf tools: Add more weak libbpf functions

Em Tue, Nov 09, 2021 at 10:49:53AM -0800, Ian Rogers escreveu:
> On Tue, Nov 9, 2021 at 6:07 AM Jiri Olsa <jolsa@...hat.com> wrote:
> >
> > We hit the window where perf uses libbpf functions, that did not
> > make it to the official libbpf release yet and it's breaking perf
> > build with dynamicly linked libbpf.
> >
> > Fixing this by providing the new interface as weak functions which
> > calls the original libbpf functions. Fortunatelly the changes were
> > just renames.
> 
> Could we just provide these functions behind a libbpf version #if ?
> Weak symbols break things in subtle ways, under certain circumstances
> the weak symbol is preferred over the strong due to lazy object file
> resolution:
> https://maskray.me/blog/2021-06-20-symbol-processing#archive-processing
> This bit me last week, but in general you get away with it as the lazy
> object file will get processed in an archive exposing the strong
> symbol. With an #if you either get a linker error for 2 definitions or
> 0 definitions, and it's clear what is broken.
> 
> In the past we had problems due to constant propagation from weak
> const variables, where #if was the solution:
> https://lore.kernel.org/lkml/20191001003623.255186-1-irogers@google.com/
> 
> There was some recent conversation on libbpf version for pahole here:
> https://lore.kernel.org/bpf/CAP-5=fUc3LtU0WYg-Py9Jf+9picaWHJdSw=sdOMA54uY3p1pdw@mail.gmail.com/T/
> https://lore.kernel.org/bpf/20211021183330.460681-1-irogers@google.com/

Valid concerns and we should improve the situation, but I'm going with
Jiri's stopgap solution so as to have everything buildind for v5.16,
which window is closing.

We can improve this in followup patches.

Thanks,

- Arnaldo