libbpf: Postpone BTF ID finding for TRACING programs to load phase

Move BTF ID determination for BPF_PROG_TYPE_TRACING programs to a load phase.
Performing it at open step is inconvenient, because it prevents BPF skeleton
generation on older host kernel, which doesn't contain BTF_KIND_FUNCs
information in vmlinux BTF. This is a common set up, though, when, e.g.,
selftests are compiled on older host kernel, but the test program itself is
executed in qemu VM with bleeding edge kernel. Having this BTF searching
performed at load time allows to successfully use bpf_object__open() for
codegen and inspection of BPF object file.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20191214014341.3442258-11-andriin@fb.com

tools/lib/bpf/libbpf.c

@@ -3815,11 +3815,22 @@ load_program(struct bpf_program *prog, struct bpf_insn *insns, int insns_cnt,
 	return ret;
 }
 
-int
-bpf_program__load(struct bpf_program *prog,
-		  char *license, __u32 kern_version)
+static int libbpf_find_attach_btf_id(const char *name,
+				     enum bpf_attach_type attach_type,
+				     __u32 attach_prog_fd);
+
+int bpf_program__load(struct bpf_program *prog, char *license, __u32 kern_ver)
 {
-	int err = 0, fd, i;
+	int err = 0, fd, i, btf_id;
+
+	if (prog->type == BPF_PROG_TYPE_TRACING) {
+		btf_id = libbpf_find_attach_btf_id(prog->section_name,
+						   prog->expected_attach_type,
+						   prog->attach_prog_fd);
+		if (btf_id <= 0)
+			return btf_id;
+		prog->attach_btf_id = btf_id;
+	}
 
 	if (prog->instances.nr < 0 || !prog->instances.fds) {
 		if (prog->preprocessor) {
@@ -3843,7 +3854,7 @@ bpf_program__load(struct bpf_program *prog,
 				prog->section_name, prog->instances.nr);
 		}
 		err = load_program(prog, prog->insns, prog->insns_cnt,
-				   license, kern_version, &fd);
+				   license, kern_ver, &fd);
 		if (!err)
 			prog->instances.fds[0] = fd;
 		goto out;
@@ -3872,9 +3883,7 @@ bpf_program__load(struct bpf_program *prog,
 		}
 
 		err = load_program(prog, result.new_insn_ptr,
-				   result.new_insn_cnt,
-				   license, kern_version, &fd);
-
+				   result.new_insn_cnt, license, kern_ver, &fd);
 		if (err) {
 			pr_warn("Loading the %dth instance of program '%s' failed\n",
 				i, prog->section_name);
@@ -3918,9 +3927,6 @@ bpf_object__load_progs(struct bpf_object *obj, int log_level)
 	return 0;
 }
 
-static int libbpf_find_attach_btf_id(const char *name,
-				     enum bpf_attach_type attach_type,
-				     __u32 attach_prog_fd);
 static struct bpf_object *
 __bpf_object__open(const char *path, const void *obj_buf, size_t obj_buf_sz,
 		   const struct bpf_object_open_opts *opts)
@@ -3984,15 +3990,8 @@ __bpf_object__open(const char *path, const void *obj_buf, size_t obj_buf_sz,
 
 		bpf_program__set_type(prog, prog_type);
 		bpf_program__set_expected_attach_type(prog, attach_type);
-		if (prog_type == BPF_PROG_TYPE_TRACING) {
-			err = libbpf_find_attach_btf_id(prog->section_name,
-							attach_type,
-							attach_prog_fd);
-			if (err <= 0)
-				goto out;
-			prog->attach_btf_id = err;
+		if (prog_type == BPF_PROG_TYPE_TRACING)
 			prog->attach_prog_fd = attach_prog_fd;
-		}
 	}
 
 	return obj;