libbpf: Allows disabling auto attach

Adds libbpf APIs for disabling auto-attach for individual functions.
This is motivated by the use case of cgroup iter [1]. Some iter
types require their parameters to be non-zero, therefore applying
auto-attach on them will fail. With these two new APIs, users who
want to use auto-attach and these types of iters can disable
auto-attach on the program and perform manual attach.

[1] https://lore.kernel.org/bpf/CAEf4BzZ+a2uDo_t6kGBziqdz--m2gh2_EUwkGLDtMd65uwxUjA@mail.gmail.com/

Signed-off-by: Hao Luo <haoluo@google.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220816234012.910255-1-haoluo@google.com

tools/lib/bpf/libbpf.c

@@ -417,6 +417,7 @@ struct bpf_program {
 
 	int fd;
 	bool autoload;
+	bool autoattach;
 	bool mark_btf_static;
 	enum bpf_prog_type type;
 	enum bpf_attach_type expected_attach_type;
@@ -755,6 +756,8 @@ bpf_object__init_prog(struct bpf_object *obj, struct bpf_program *prog,
 		prog->autoload = true;
 	}
 
+	prog->autoattach = true;
+
 	/* inherit object's log_level */
 	prog->log_level = obj->log_level;
 
@@ -8314,6 +8317,16 @@ int bpf_program__set_autoload(struct bpf_program *prog, bool autoload)
 	return 0;
 }
 
+bool bpf_program__autoattach(const struct bpf_program *prog)
+{
+	return prog->autoattach;
+}
+
+void bpf_program__set_autoattach(struct bpf_program *prog, bool autoattach)
+{
+	prog->autoattach = autoattach;
+}
+
 const struct bpf_insn *bpf_program__insns(const struct bpf_program *prog)
 {
 	return prog->insns;
@@ -12346,7 +12359,7 @@ int bpf_object__attach_skeleton(struct bpf_object_skeleton *s)
 		struct bpf_program *prog = *s->progs[i].prog;
 		struct bpf_link **link = s->progs[i].link;
 
-		if (!prog->autoload)
+		if (!prog->autoload || !prog->autoattach)
 			continue;
 
 		/* auto-attaching not supported for this program */

tools/lib/bpf/libbpf.h

@@ -260,6 +260,8 @@ LIBBPF_API const char *bpf_program__name(const struct bpf_program *prog);
 LIBBPF_API const char *bpf_program__section_name(const struct bpf_program *prog);
 LIBBPF_API bool bpf_program__autoload(const struct bpf_program *prog);
 LIBBPF_API int bpf_program__set_autoload(struct bpf_program *prog, bool autoload);
+LIBBPF_API bool bpf_program__autoattach(const struct bpf_program *prog);
+LIBBPF_API void bpf_program__set_autoattach(struct bpf_program *prog, bool autoattach);
 
 struct bpf_insn;
 

tools/lib/bpf/libbpf.map

@@ -358,6 +358,8 @@ LIBBPF_1.0.0 {
 		bpf_obj_get_opts;
 		bpf_prog_query_opts;
 		bpf_program__attach_ksyscall;
+		bpf_program__autoattach;
+		bpf_program__set_autoattach;
 		btf__add_enum64;
 		btf__add_enum64_value;
 		libbpf_bpf_attach_type_str;