bpf: libbpf: Add STRUCT_OPS support

This patch adds BPF STRUCT_OPS support to libbpf.

The only sec_name convention is SEC(".struct_ops") to identify the
struct_ops implemented in BPF,
e.g. To implement a tcp_congestion_ops:

SEC(".struct_ops")
struct tcp_congestion_ops dctcp = {
	.init           = (void *)dctcp_init,  /* <-- a bpf_prog */
	/* ... some more func prts ... */
	.name           = "bpf_dctcp",
};

Each struct_ops is defined as a global variable under SEC(".struct_ops")
as above.  libbpf creates a map for each variable and the variable name
is the map's name.  Multiple struct_ops is supported under
SEC(".struct_ops").

In the bpf_object__open phase, libbpf will look for the SEC(".struct_ops")
section and find out what is the btf-type the struct_ops is
implementing.  Note that the btf-type here is referring to
a type in the bpf_prog.o's btf.  A "struct bpf_map" is added
by bpf_object__add_map() as other maps do.  It will then
collect (through SHT_REL) where are the bpf progs that the
func ptrs are referring to.  No btf_vmlinux is needed in
the open phase.

In the bpf_object__load phase, the map-fields, which depend
on the btf_vmlinux, are initialized (in bpf_map__init_kern_struct_ops()).
It will also set the prog->type, prog->attach_btf_id, and
prog->expected_attach_type.  Thus, the prog's properties do
not rely on its section name.
[ Currently, the bpf_prog's btf-type ==> btf_vmlinux's btf-type matching
  process is as simple as: member-name match + btf-kind match + size match.
  If these matching conditions fail, libbpf will reject.
  The current targeting support is "struct tcp_congestion_ops" which
  most of its members are function pointers.
  The member ordering of the bpf_prog's btf-type can be different from
  the btf_vmlinux's btf-type. ]

Then, all obj->maps are created as usual (in bpf_object__create_maps()).

Once the maps are created and prog's properties are all set,
the libbpf will proceed to load all the progs.

bpf_map__attach_struct_ops() is added to register a struct_ops
map to a kernel subsystem.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200109003514.3856730-1-kafai@fb.com

1 files changed, 4 insertions, 1 deletions

diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 26bf539f1b3c..01639f9a1062 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -239,6 +239,8 @@ bpf_program__attach_raw_tracepoint(struct bpf_program *prog,
 
 LIBBPF_API struct bpf_link *
 bpf_program__attach_trace(struct bpf_program *prog);
+struct bpf_map;
+LIBBPF_API struct bpf_link *bpf_map__attach_struct_ops(struct bpf_map *map);
 struct bpf_insn;
 
 /*
@@ -315,6 +317,7 @@ LIBBPF_API int bpf_program__set_sched_act(struct bpf_program *prog);
 LIBBPF_API int bpf_program__set_xdp(struct bpf_program *prog);
 LIBBPF_API int bpf_program__set_perf_event(struct bpf_program *prog);
 LIBBPF_API int bpf_program__set_tracing(struct bpf_program *prog);
+LIBBPF_API int bpf_program__set_struct_ops(struct bpf_program *prog);
 
 LIBBPF_API enum bpf_prog_type bpf_program__get_type(struct bpf_program *prog);
 LIBBPF_API void bpf_program__set_type(struct bpf_program *prog,
@@ -335,6 +338,7 @@ LIBBPF_API bool bpf_program__is_sched_act(const struct bpf_program *prog);
 LIBBPF_API bool bpf_program__is_xdp(const struct bpf_program *prog);
 LIBBPF_API bool bpf_program__is_perf_event(const struct bpf_program *prog);
 LIBBPF_API bool bpf_program__is_tracing(const struct bpf_program *prog);
+LIBBPF_API bool bpf_program__is_struct_ops(const struct bpf_program *prog);
 
 /*
  * No need for __attribute__((packed)), all members of 'bpf_map_def'
@@ -354,7 +358,6 @@ struct bpf_map_def {
  * The 'struct bpf_map' in include/linux/bpf.h is internal to the kernel,
  * so no need to worry about a name clash.
  */
-struct bpf_map;
 LIBBPF_API struct bpf_map *
 bpf_object__find_map_by_name(const struct bpf_object *obj, const char *name);