Message-ID: <CAEf4Bzb2=R0+D0XXrH0N1n1X+7i6aFkACS2gb0xAQFwcBHjVQA@mail.gmail.com>
Date:   Mon, 16 Dec 2019 10:53:58 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next 11/15] bpftool: add skeleton codegen command

On Mon, Dec 16, 2019 at 6:16 AM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On Mon, Dec 09, 2019 at 05:14:34PM -0800, Andrii Nakryiko wrote:
> > Add `bpftool gen skeleton` command, which takes in compiled BPF .o object file
> > and dumps a BPF skeleton struct and related code to work with that skeleton.
> > Skeleton itself is tailored to a specific structure of provided BPF object
> > file, containing accessors (just plain struct fields) for every map and
> > program, as well as dedicated space for bpf_links. If BPF program is using
> > global variables, corresponding structure definitions of compatible memory
> > layout are emitted as well, making it possible to initialize and subsequently
> > read/update global variables values using simple and clear C syntax for
> > accessing fields. This skeleton majorly improves usability of
> > opening/loading/attaching of BPF object, as well as interacting with it
> > throughout the lifetime of loaded BPF object.
> >
> > Generated skeleton struct has the following structure:
> >
> > 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>;
> > };
> >
> > This provides great usability improvements:
> > - no need to look up maps and programs by name, instead just
> >   my_obj->maps.my_map or my_obj->progs.my_prog would give necessary
> >   bpf_map/bpf_program pointers, which user can pass to existing libbpf APIs;
> > - pre-defined places for bpf_links, which will be automatically populated for
> >   program types that libbpf knows how to attach automatically (currently
> >   tracepoints, kprobe/kretprobe, raw tracepoint and tracing programs). On
> >   tearing down skeleton, all active bpf_links will be destroyed (meaning BPF
> >   programs will be detached, if they are attached). For cases in which libbpf
> >   doesn't know how to auto-attach BPF program, user can manually create link
> >   after loading skeleton and they will be auto-detached on skeleton
> >   destruction:
> >
> >       my_obj->links.my_fancy_prog = bpf_program__attach_cgroup_whatever(
> >               my_obj->progs.my_fancy_prog, <whatever extra param);
> >
> > - it's extremely easy and convenient to work with global data from userspace
> >   now. Both for read-only and read/write variables, it's possible to
> >   pre-initialize them before skeleton is loaded:
> >
> >       skel = my_obj__open(raw_embed_data);
> >       my_obj->rodata->my_var = 123;
> >       my_obj__load(skel); /* 123 will be initialization value for my_var */
> >
> >   After load, if kernel supports mmap() for BPF arrays, user can still read
> >   (and write for .bss and .data) variables values, but at that point it will
> >   be directly mmap()-ed to BPF array, backing global variables. This allows to
> >   seamlessly exchange data with BPF side. From userspace program's POV, all
> >   the pointers and memory contents stay the same, but mapped kernel memory
> >   changes to point to created map.
> >   If kernel doesn't yet support mmap() for BPF arrays, it's still possible to
> >   use those data section structs to pre-initialize .bss, .data, and .rodata,
> >   but after load their pointers will be reset to NULL, allowing user code to
> >   gracefully handle this condition, if necessary.
> >
> > Given a big surface area, skeleton is kept as an experimental non-public
> > API for now, until more feedback and real-world experience is collected.
>
> Can you elaborate on the plan here? This is until v5.6 is out and hence a new
> bpftool release implicitly where this becomes frozen / non-experimental?

Yes, I've exposed all those interfaces as public, thus they are going
to stabilize with new release of libbpf/bpftool. I've received some
good usability feedback from Alexei after he tried it out locally, so
I'm going to adjust auto-generated part a bit. Libbpf APIs were
designed with extensibility built in, so we can extend them as any
other APIs, if need be.

>
> There is also tools/bpf/bpftool/Documentation/bpftool-gen.rst missing. Given
> you aim to collect more feedback (?), it would be appropriate to document
> everything in there so users have a clue how to use it for getting started.

sure, will add it

>
> Also, I think at least some more clarification is needed in such document on
> the following topics:
>
> - libbpf and bpftool is both 'GPL-2.0-only' or 'BSD-2-Clause'. Given this
>   is a code generator, what license is the `bpftool gen skeleton` result?
>   In any case, should there also be a header comment emitted via do_skeleton()?

Not a lawyer here, I assumed auto-generated code isn't copyrighted,
but how about I just emit SPDX header with the same license as libbpf
itself:

SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)


>
> - Clear statement that this codegen is an alternative to regular libbpf
>   API usage but that both are always kept feature-complete and hence not
>   disadvantaged in one way or another (to rule out any uncertainties for
>   users e.g. whether they now need to start rewriting their existing code
>   etc); with purpose of the former (codgen) to simplify loader interaction.

ok, will add that as well

>
> Thanks,
> Daniel

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