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

[Andrii Nakryiko]

On Tue, Dec 10, 2019 at 1:44 PM Stanislav Fomichev <sdf@xxxxxxxxxxx> 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.

So while having nice high-level language-specific APIs is good, it's not enough.

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

Yes, C++ using libbpf directly or through very thin wrappers. For
BCC-based stuff, obviously, we rely on C++ parts of BCC. This struct
I'm generating is extremely useful for C++ as well, as it gives very
natural way to access *and initialize* global variables.