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? 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. 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()? - 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. Thanks, Daniel