[PATCH bpf-next v2 0/5] Subskeleton support for BPF libraries

[Delyan Kratunov <delyank@fb.com>]

In the quest for ever more modularity, a new need has arisen - the ability to
access data associated with a BPF library from a corresponding userspace library.
The catch is that we don't want the userspace library to know about the structure of the
final BPF object that the BPF library is linked into.

In pursuit of this modularity, this patch series introduces *subskeletons.*
Subskeletons are similar in use and design to skeletons with a couple of differences:

1. The generated storage types do not rely on contiguous storage for the library's
variables because they may be interspersed randomly throughout the final BPF object's sections.

2. Subskeletons do not own objects and instead require a loaded bpf_object* to
be passed at runtime in order to be initialized. By extension, symbols are resolved at
runtime by parsing the final object's BTF. This has the interesting effect that the same
userspace code can interoperate with the library BPF code *linked into different final objects.*

3. Subskeletons allow access to all global variables, programs, and custom maps. They also expose
the internal maps *of the final object*. This allows bpf_var_skeleton objects to contain a bpf_map**
instead of a section name.

Changes since v1:
 - Introduced new strict mode knob for single-routine-in-.text compatibility behavior, which
   disproportionately affects library objects. bpftool works in 1.0 mode so subskeleton generation
   doesn't have to worry about this now.
 - Made bpf_map_btf_value_type_id available earlier and used it wherever applicable.
 - Refactoring in bpftool gen.c per review comments.
 - Subskels now use typeof() for array and func proto globals to avoid the need for runtime split btf.
 - Expanded the subskeleton test to include arrays, custom maps, extern maps, weak symbols, and kconfigs.
 - selftests/bpf/Makefile now generates a subskel.h for every skel.h it would make.

For reference, here is a shortened subskeleton header:

#ifndef __TEST_SUBSKELETON_LIB_SUBSKEL_H__
#define __TEST_SUBSKELETON_LIB_SUBSKEL_H__

struct test_subskeleton_lib {
	struct bpf_object *obj;
	struct bpf_object_subskeleton *subskel;
	struct {
		struct bpf_map *map2;
		struct bpf_map *map1;
		struct bpf_map *data;
		struct bpf_map *rodata;
		struct bpf_map *bss;
		struct bpf_map *kconfig;
	} maps;
	struct {
		struct bpf_program *lib_perf_handler;
	} progs;
	struct test_subskeleton_lib__data {
		int *var6;
		int *var2;
		int *var5;
	} data;
	struct test_subskeleton_lib__rodata {
		int *var1;
	} rodata;
	struct test_subskeleton_lib__bss {
		struct {
			int var3_1;
			__s64 var3_2;
		} *var3;
		int *libout1;
		typeof(int[4]) *var4;
		typeof(int (*)()) *fn_ptr;
	} bss;
	struct test_subskeleton_lib__kconfig {
		_Bool *CONFIG_BPF_SYSCALL;
	} kconfig;

static inline struct test_subskeleton_lib *
test_subskeleton_lib__open(const struct bpf_object *src)
{
	struct test_subskeleton_lib *obj;
	struct bpf_object_subskeleton *s;
	int err;

	...
	s = (struct bpf_object_subskeleton *)calloc(1, sizeof(*s));
	...

	s->var_cnt = 9;
	...

	s->vars[0].name = "var6";
	s->vars[0].map = &obj->maps.data;
	s->vars[0].addr = (void**) &obj->data.var6;
  ...

	/* maps */
	...

	/* programs */
	s->prog_cnt = 1;
	...

	err = bpf_object__open_subskeleton(s);
  ...
	return obj;
}
#endif /* __TEST_SUBSKELETON_LIB_SUBSKEL_H__ */

Delyan Kratunov (5):
  libbpf: add new strict flag for .text subprograms
  libbpf: init btf_{key,value}_type_id on internal map open
  libbpf: add subskeleton scaffolding
  bpftool: add support for subskeletons
  selftests/bpf: test subskeleton functionality

 .../bpf/bpftool/Documentation/bpftool-gen.rst |  25 +
 tools/bpf/bpftool/bash-completion/bpftool     |  14 +-
 tools/bpf/bpftool/gen.c                       | 589 ++++++++++++++++--
 tools/lib/bpf/libbpf.c                        | 151 ++++-
 tools/lib/bpf/libbpf.h                        |  29 +
 tools/lib/bpf/libbpf.map                      |   2 +
 tools/lib/bpf/libbpf_legacy.h                 |   6 +
 tools/testing/selftests/bpf/.gitignore        |   1 +
 tools/testing/selftests/bpf/Makefile          |  10 +-
 .../selftests/bpf/prog_tests/subskeleton.c    |  83 +++
 .../selftests/bpf/progs/test_subskeleton.c    |  23 +
 .../bpf/progs/test_subskeleton_lib.c          |  56 ++
 .../bpf/progs/test_subskeleton_lib2.c         |  16 +
 13 files changed, 919 insertions(+), 86 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/subskeleton.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_subskeleton.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_subskeleton_lib.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_subskeleton_lib2.c

--
2.34.1