netdev - Re: [PATCH bpf-next 11/15] bpftool: add skeleton codegen command

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Date:   Tue, 10 Dec 2019 13:44:07 -0800
From:   Stanislav Fomichev <sdf@...ichev.me>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Andrii Nakryiko <andriin@...com>,
        LKML <linux-kernel@...r.kernel.org>, 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 11/15] bpftool: add skeleton codegen command

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.

(I assume most of the stuff you have at fb is also non-c and one of
c++/python/php/rust/go/whatver).