Date:   Wed, 9 Feb 2022 11:23:44 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Quentin Monnet <quentin@...valent.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 3/3] bpftool: Update versioning scheme, align
 on libbpf's version number

On Wed, Feb 9, 2022 at 11:15 AM Quentin Monnet <quentin@...valent.com> wrote:
>
> 2022-02-09 09:53 UTC-0800 ~ Andrii Nakryiko <andrii.nakryiko@...il.com>
> > On Wed, Feb 9, 2022 at 4:37 AM Quentin Monnet <quentin@...valent.com> wrote:
> >>
> >> 2022-02-08 16:39 UTC-0800 ~ Andrii Nakryiko <andrii.nakryiko@...il.com>
> >>> On Tue, Feb 8, 2022 at 4:07 AM Quentin Monnet <quentin@...valent.com> wrote:
> >>>>
> >>>> Since the notion of versions was introduced for bpftool, it has been
> >>>> following the version number of the kernel (using the version number
> >>>> corresponding to the tree in which bpftool's sources are located). The
> >>>> rationale was that bpftool's features are loosely tied to BPF features
> >>>> in the kernel, and that we could defer versioning to the kernel
> >>>> repository itself.
> >>>>
> >>>> But this versioning scheme is confusing today, because a bpftool binary
> >>>> should be able to work with both older and newer kernels, even if some
> >>>> of its recent features won't be available on older systems. Furthermore,
> >>>> if bpftool is ported to other systems in the future, keeping a
> >>>> Linux-based version number is not a good option.
> >>>>
> >>>> Looking at other options, we could either have a totally independent
> >>>> scheme for bpftool, or we could align it on libbpf's version number
> >>>> (with an offset on the major version number, to avoid going backwards).
> >>>> The latter comes with a few drawbacks:
> >>>>
> >>>> - We may want bpftool releases in-between two libbpf versions. We can
> >>>>   always append pre-release numbers to distinguish versions, although
> >>>>   those won't look as "official" as something with a proper release
> >>>>   number. But at the same time, having bpftool with version numbers that
> >>>>   look "official" hasn't really been an issue so far.
> >>>>
> >>>> - If no new feature lands in bpftool for some time, we may move from
> >>>>   e.g. 6.7.0 to 6.8.0 when libbpf levels up and have two different
> >>>>   versions which are in fact the same.
> >>>>
> >>>> - Following libbpf's versioning scheme sounds better than kernel's, but
> >>>>   ultimately it doesn't make too much sense either, because even though
> >>>>   bpftool uses the lib a lot, its behaviour is not that much conditioned
> >>>>   by the internal evolution of the library (or by new APIs that it may
> >>>>   not use).
> >>>>
> >>>> Having an independent versioning scheme solves the above, but at the
> >>>> cost of heavier maintenance. Developers will likely forget to increase
> >>>> the numbers when adding features or bug fixes, and we would take the
> >>>> risk of having to send occasional "catch-up" patches just to update the
> >>>> version number.
> >>>>
> >>>> Based on these considerations, this patch aligns bpftool's version
> >>>> number on libbpf's. This is not a perfect solution, but 1) it's
> >>>> certainly an improvement over the current scheme, 2) the issues raised
> >>>> above are all minor at the moment, and 3) we can still move to an
> >>>> independent scheme in the future if we realise we need it.
> >>>>
> >>>> Given that libbpf is currently at version 0.7.0, and bpftool, before
> >>>> this patch, was at 5.16, we use an offset of 6 for the major version,
> >>>> bumping bpftool to 6.7.0.
> >>>>
> >>>> It remains possible to manually override the version number by setting
> >>>> BPFTOOL_VERSION when calling make.
> >>>>
> >>>> Signed-off-by: Quentin Monnet <quentin@...valent.com>
> >>>> ---
> >>>> Contrarily to the previous discussion and to what the first patch of the
> >>>> set does, I chose not to use the libbpf_version_string() API from libbpf
> >>>> to compute the version for bpftool. There were three reasons for that:
> >>>>
> >>>> - I don't feel comfortable having bpftool's version number computed at
> >>>>   runtime. Somehow it really feels like we should now it when we compile
> >>>
> >>> Fair, but why not use LIBBPF_MAJOR_VERSION and LIBBPF_MINOR_VERSION to
> >>> define BPFTOOL_VERSION (unless it's overridden).
> >>
> >> I forgot the macros were exposed, which is silly, because I was the one
> >> to help expose them in the first place. Anyway.
> >>
> >>> Which all seems to be
> >>> doable at compilation time in C code, not in Makefile. This will work
> >>> with Github version of libbpf just as well with no extra Makefile
> >>> changes (and in general, the less stuff is done in Makefile the
> >>> better, IMO).
> >>>
> >>> Version string can also be "composed" at compile time with a bit of
> >>> helper macro, see libbpf_version_string() implementation in libbpf.
> >>
> >> Sounds good, I can do that.
>
> ... Except that you can only compose so much. The preprocessor won't
> allow me to sum libbpf's major version with the offset (6) before
> turning it into a string. I need to think about this a bit more.

Yeah, it sucks. Well, we can either go back to `make version` or
you'll have to do snprintf() to get string representation. 6 +
LIBBPF_MAJOR_VERSION should work in #if condition, it just doesn't
stringifies to 6, but rather "6 + 0", unfortunately.


>
> >>
> >> This won't give me the patch number, though, only major and minor
> >> version. We could add an additional LIBBPF_PATCH_VERSION to libbpf.
> >> Although thinking about it, I'm not sure we need a patch version for
> >> bpftool at the moment, because changes in libbpf's patch number is
> >> unlikely to reflect any change in bpftool, so it makes little sense to
> >> copy it. So I'm considering just leaving it at 0 in bpftool, and having
> >> updates on major/minor numbers only when libbpf releases a major/minor
> >> version. If we do want bugfix releases, it will still be possible to
> >> overwrite the version number with BPFTOOL_VERSION anyway. What do you think?
> >
> > So the patch version is not currently reflected in libbpf.map file. I
> > do patch version bumps only when we detect some small issue after
> > official release. It happened 2 or 3 times so far. I'm hesitant to
> > expose that as LIBBPF_PATCH_VERSION, because I'll need to remember to
> > bump it manually (and coordinating this between kernel sources and
> > Github is a slow nightmare). Let's not rely on patch version, too much
> > burden.
>
> Agreed, thanks.
> Quentin

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