On Mon, Dec 16, 2019 at 6:16 AM Daniel Borkmann <daniel@xxxxxxxxxxxxx> 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
