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.
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20191214014341.3442258-14-andriin@fb.com