Merge branch 'Subskeleton support for BPF librariesThread-Topic: [PATCH bpf-next...

Merge branch 'Subskeleton support for BPF librariesThread-Topic: [PATCH bpf-next v4 0/5'

Delyan Kratunov says:

====================

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.

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 v3:
 - Re-add key/value type lookup for legacy user maps (fixing btf test)
 - Minor cleanups (missed sanitize_identifier call, error messages, formatting)

Changes since v2:
 - Reuse SEC_NAME strict mode flag
 - Init bpf_map->btf_value_type_id on open for internal maps *and* user BTF maps
 - Test custom section names (.data.foo) and overlapping kconfig externs between the final object and the library
 - Minor review comments in gen.c & libbpf.c

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__ */
====================

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>