libbpf: Support opting out from autoloading BPF programs declaratively

Establish SEC("?abc") naming convention (i.e., adding question mark in
front of otherwise normal section name) that allows to set corresponding
program's autoload property to false. This is effectively just
a declarative way to do bpf_program__set_autoload(prog, false).

Having a way to do this declaratively in BPF code itself is useful and
convenient for various scenarios. E.g., for testing, when BPF object
consists of multiple independent BPF programs that each needs to be
tested separately. Opting out all of them by default and then setting
autoload to true for just one of them at a time simplifies testing code
(see next patch for few conversions in BPF selftests taking advantage of
this new feature).

Another real-world use case is in libbpf-tools for cases when different
BPF programs have to be picked depending on particulars of the host
kernel due to various incompatible changes (like kernel function renames
or signature change, or to pick kprobe vs fentry depending on
corresponding kernel support for the latter). Marking all the different
BPF program candidates as non-autoloaded declaratively makes this more
obvious in BPF source code and allows simpler code in user-space code.

When BPF program marked as SEC("?abc") it is otherwise treated just like
SEC("abc") and bpf_program__section_name() reported will be "abc".

Signed-off-by: Andrii Nakryiko <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/lib/bpf/libbpf.c

···
 	void *priv;
 	bpf_program_clear_priv_t clear_priv;
 
-	bool load;
+	bool autoload;
 	bool mark_btf_static;
 	enum bpf_prog_type type;
 	enum bpf_attach_type expected_attach_type;
···
 	prog->insns_cnt = prog->sec_insn_cnt;
 
 	prog->type = BPF_PROG_TYPE_UNSPEC;
-	prog->load = true;
+
+	/* libbpf's convention for SEC("?abc...") is that it's just like
+	 * SEC("abc...") but the corresponding bpf_program starts out with
+	 * autoload set to false.
+	 */
+	if (sec_name[0] == '?') {
+		prog->autoload = false;
+		/* from now on forget there was ? in section name */
+		sec_name++;
+	} else {
+		prog->autoload = true;
+	}
 
 	prog->instances.fds = NULL;
 	prog->instances.nr = -1;
···
 	}
 
 	bpf_object__for_each_program(prog, obj) {
-		if (!prog->load)
+		if (!prog->autoload)
 			continue;
 		if (prog_needs_vmlinux_btf(prog))
 			return true;
···
 			/* no need to apply CO-RE relocation if the program is
 			 * not going to be loaded
 			 */
-			if (!prog->load)
+			if (!prog->autoload)
 				continue;
 
 			/* adjust insn_idx from section frame of reference to the local
···
 		 */
 		if (prog_is_subprog(obj, prog))
 			continue;
-		if (!prog->load)
+		if (!prog->autoload)
 			continue;
 
 		err = bpf_object__relocate_calls(obj, prog);
···
 		prog = &obj->programs[i];
 		if (prog_is_subprog(obj, prog))
 			continue;
-		if (!prog->load)
+		if (!prog->autoload)
 			continue;
 		err = bpf_object__relocate_data(obj, prog);
 		if (err) {
···
 		prog = &obj->programs[i];
 		if (prog_is_subprog(obj, prog))
 			continue;
-		if (!prog->load) {
+		if (!prog->autoload) {
 			pr_debug("prog '%s': skipped loading\n", prog->name);
 			continue;
 		}
···
 
 bool bpf_program__autoload(const struct bpf_program *prog)
 {
-	return prog->load;
+	return prog->autoload;
 }
 
 int bpf_program__set_autoload(struct bpf_program *prog, bool autoload)
···
 	if (prog->obj->loaded)
 		return libbpf_err(-EINVAL);
 
-	prog->load = autoload;
+	prog->autoload = autoload;
 	return 0;
 }
 
···
 		struct bpf_program *prog = *s->progs[i].prog;
 		struct bpf_link **link = s->progs[i].link;
 
-		if (!prog->load)
+		if (!prog->autoload)
 			continue;
 
 		/* auto-attaching not supported for this program */