netdev - Re: [PATCH bpf-next 4/9] libbpf: add libbpf_swap

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Message-ID: <CAEf4BzaoFOyUnaNE2k3_E6f6Ozf0V4X3TGC5nrEMD9wbxuM9LQ@mail.gmail.com>
Date:   Sat, 27 Jul 2019 11:49:20 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Stanislav Fomichev <sdf@...ichev.me>,
        Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next 4/9] libbpf: add libbpf_swap_print to get
 previous print func

On Fri, Jul 26, 2019 at 5:30 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Fri, Jul 26, 2019 at 02:47:28PM -0700, Andrii Nakryiko wrote:
> > On Fri, Jul 26, 2019 at 2:28 PM Stanislav Fomichev <sdf@...ichev.me> wrote:
> > >
> > > On 07/26, Andrii Nakryiko wrote:
> > > > libbpf_swap_print allows to restore previously set print function.
> > > > This is useful when running many independent test with one default print
> > > > function, but overriding log verbosity for particular subset of tests.
> > > Can we change the return type of libbpf_set_print instead and return
> > > the old function from it? Will it break ABI?
> >
> > Yeah, thought about that, but I wasn't sure about ABI breakage. It
> > seems like it shouldn't, so I'll just change libbpf_set_print
> > signature instead.
>
> I think it's ok to change return value of libbpf_set_print() from void
> to useful pointer.

Some googling gave inconclusive results. StackOverflow answers claim
it is compatible ABI change ([0]), but I also found some guidelines
for Android that designate any return type change as incompatible
([1]). [2] wasn't very helpful about defining compatibility rules,
unfortunately. I'm going with [0], though, and changing return type.

  [0] https://stackoverflow.com/questions/15626579/c-abi-is-changing-a-void-function-to-return-an-int-a-breaking-change
  [1] https://source.android.com/devices/architecture/vndk/abi-stability
  [2] https://www.cs.dartmouth.edu/~sergey/cs258/ABI/UlrichDrepper-How-To-Write-Shared-Libraries.pdf

> This function is not marked as __attribute__((__warn_unused_result__)),
> so there should be no abi issues.
>
> Please double check by compiler perf with different gcc-s as Arnaldo's setup does.
>

Compiled (make -C tools/perf) with GCC 4.8.5, GCC 7, and Clang 8. None
of them produced any warning, so I'm going forward with just return
type change.