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Date:   Tue, 10 Dec 2019 14:59:00 -0800
From:   Stanislav Fomichev <sdf@fomichev.me>
To:     Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc:     Jakub Kicinski <jakub.kicinski@netronome.com>,
        Andrii Nakryiko <andriin@fb.com>,
        LKML <linux-kernel@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
        Networking <netdev@vger.kernel.org>,
        Alexei Starovoitov <ast@fb.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Kernel Team <kernel-team@fb.com>
Subject: Re: [PATCH bpf-next 11/15] bpftool: add skeleton codegen command
Message-ID: <20191210225900.GB3105713@mini-arch>
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On 12/10, Andrii Nakryiko wrote:
> On Tue, Dec 10, 2019 at 1:44 PM Stanislav Fomichev <sdf@fomichev.me> wrote:
> >
> > On 12/10, Jakub Kicinski wrote:
> > > On Tue, 10 Dec 2019 09:11:31 -0800, Andrii Nakryiko wrote:
> > > > On Mon, Dec 9, 2019 at 5:57 PM Jakub Kicinski wrote:
> > > > > On Mon, 9 Dec 2019 17:14:34 -0800, Andrii Nakryiko wrote:
> > > > > > struct <object-name> {
> > > > > >       /* used by libbpf's skeleton API */
> > > > > >       struct bpf_object_skeleton *skeleton;
> > > > > >       /* bpf_object for libbpf APIs */
> > > > > >       struct bpf_object *obj;
> > > > > >       struct {
> > > > > >               /* for every defined map in BPF object: */
> > > > > >               struct bpf_map *<map-name>;
> > > > > >       } maps;
> > > > > >       struct {
> > > > > >               /* for every program in BPF object: */
> > > > > >               struct bpf_program *<program-name>;
> > > > > >       } progs;
> > > > > >       struct {
> > > > > >               /* for every program in BPF object: */
> > > > > >               struct bpf_link *<program-name>;
> > > > > >       } links;
> > > > > >       /* for every present global data section: */
> > > > > >       struct <object-name>__<one of bss, data, or rodata> {
> > > > > >               /* memory layout of corresponding data section,
> > > > > >                * with every defined variable represented as a struct field
> > > > > >                * with exactly the same type, but without const/volatile
> > > > > >                * modifiers, e.g.:
> > > > > >                */
> > > > > >                int *my_var_1;
> > > > > >                ...
> > > > > >       } *<one of bss, data, or rodata>;
> > > > > > };
> > > > >
> > > > > I think I understand how this is useful, but perhaps the problem here
> > > > > is that we're using C for everything, and simple programs for which
> > > > > loading the ELF is majority of the code would be better of being
> > > > > written in a dynamic language like python?  Would it perhaps be a
> > > > > better idea to work on some high-level language bindings than spend
> > > > > time writing code gens and working around limitations of C?
> > > >
> > > > None of this work prevents Python bindings and other improvements, is
> > > > it? Patches, as always, are greatly appreciated ;)
> > >
> > > This "do it yourself" shit is not really funny :/
> > >
> > > I'll stop providing feedback on BPF patches if you guy keep saying
> > > that :/ Maybe that's what you want.
> > >
> > > > This skeleton stuff is not just to save code, but in general to
> > > > simplify and streamline working with BPF program from userspace side.
> > > > Fortunately or not, but there are a lot of real-world applications
> > > > written in C and C++ that could benefit from this, so this is still
> > > > immensely useful. selftests/bpf themselves benefit a lot from this
> > > > work, see few of the last patches in this series.
> > >
> > > Maybe those applications are written in C and C++ _because_ there
> > > are no bindings for high level languages. I just wish BPF programming
> > > was less weird and adding some funky codegen is not getting us closer
> > > to that goal.
> > >
> > > In my experience code gen is nothing more than a hack to work around
> > > bad APIs, but experiences differ so that's not a solid argument.
> > *nod*
> >
> > We have a nice set of C++ wrappers around libbpf internally, so we can do
> > something like BpfMap<key type, value type> and get a much better interface
> > with type checking. Maybe we should focus on higher level languages instead?
> > We are open to open-sourcing our C++ bits if you want to collaborate.
> 
> Python/C++ bindings and API wrappers are an orthogonal concerns here.
> I personally think it would be great to have both Python and C++
> specific API that uses libbpf under the cover. The only debatable
> thing is the logistics: where the source code lives, how it's kept in
> sync with libbpf, how we avoid crippling libbpf itself because
> something is hard or inconvenient to adapt w/ Python, etc.

[..]
> The problem I'm trying to solve here is not really C-specific. I don't
> think you can solve it without code generation for C++. How do you
> "generate" BPF program-specific layout of .data, .bss, .rodata, etc
> data sections in such a way, where it's type safe (to the degree that
> language allows that, of course) and is not "stringly-based" API? This
> skeleton stuff provides a natural, convenient and type-safe way to
> work with global data from userspace pretty much at the same level of
> performance and convenience, as from BPF side. How can you achieve
> that w/ C++ without code generation? As for Python, sure you can do
> dynamic lookups based on just the name of property/method, but amount
> of overheads is not acceptable for all applications (and Python itself
> is not acceptable for those applications). In addition to that, C is
> the best way for other less popular languages (e.g., Rust) to leverage
> libbpf without investing lots of effort in re-implementing libbpf in
> Rust.
I'd say that a libbpf API similar to dlopen/dlsym is a more
straightforward thing to do. Have a way to "open" a section and
a way to find a symbol in it. Yes, it's a string-based API,
but there is nothing wrong with it. IMO, this is easier to
use/understand and I suppose Python/C++ wrappers are trivial.

As for type-safety: it's C, forget about it :-)