Merge branch 'set_attach_target'

Eelco Chaudron says:

====================
Currently when you want to attach a trace program to a bpf program
the section name needs to match the tracepoint/function semantics.

However the addition of the bpf_program__set_attach_target() API
allows you to specify the tracepoint/function dynamically.

The call flow would look something like this:

  xdp_fd = bpf_prog_get_fd_by_id(id);
  trace_obj = bpf_object__open_file("func.o", NULL);
  prog = bpf_object__find_program_by_title(trace_obj,
                                           "fentry/myfunc");
  bpf_program__set_expected_attach_type(prog, BPF_TRACE_FENTRY);
  bpf_program__set_attach_target(prog, xdp_fd,
                                 "xdpfilt_blk_all");
  bpf_object__load(trace_obj)

v1 -> v2: Remove requirement for attach type hint in API
v2 -> v3: Moved common warning to __find_vmlinux_btf_id, requested by Andrii
          Updated the xdp_bpf2bpf test to use this new API
v3 -> v4: Split up patch, update libbpf.map version
v4 -> v5: Fix return code, and prog assignment in test case
====================
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/lib/bpf/libbpf.c

@@ -4937,8 +4937,8 @@ int bpf_program__load(struct bpf_program *prog, char *license, __u32 kern_ver)
 {
 	int err = 0, fd, i, btf_id;
 
-	if (prog->type == BPF_PROG_TYPE_TRACING ||
-	    prog->type == BPF_PROG_TYPE_EXT) {
+	if ((prog->type == BPF_PROG_TYPE_TRACING ||
+	     prog->type == BPF_PROG_TYPE_EXT) && !prog->attach_btf_id) {
 		btf_id = libbpf_find_attach_btf_id(prog);
 		if (btf_id <= 0)
 			return btf_id;
@@ -6581,6 +6581,9 @@ static inline int __find_vmlinux_btf_id(struct btf *btf, const char *name,
 	else
 		err = btf__find_by_name_kind(btf, name, BTF_KIND_FUNC);
 
+	if (err <= 0)
+		pr_warn("%s is not found in vmlinux BTF\n", name);
+
 	return err;
 }
 
@@ -6653,8 +6656,6 @@ static int libbpf_find_attach_btf_id(struct bpf_program *prog)
 			err = __find_vmlinux_btf_id(prog->obj->btf_vmlinux,
 						    name + section_defs[i].len,
 						    attach_type);
-		if (err <= 0)
-			pr_warn("%s is not found in vmlinux BTF\n", name);
 		return err;
 	}
 	pr_warn("failed to identify btf_id based on ELF section name '%s'\n", name);
@@ -8130,6 +8131,31 @@ void bpf_program__bpil_offs_to_addr(struct bpf_prog_info_linear *info_linear)
 	}
 }
 
+int bpf_program__set_attach_target(struct bpf_program *prog,
+				   int attach_prog_fd,
+				   const char *attach_func_name)
+{
+	int btf_id;
+
+	if (!prog || attach_prog_fd < 0 || !attach_func_name)
+		return -EINVAL;
+
+	if (attach_prog_fd)
+		btf_id = libbpf_find_prog_btf_id(attach_func_name,
+						 attach_prog_fd);
+	else
+		btf_id = __find_vmlinux_btf_id(prog->obj->btf_vmlinux,
+					       attach_func_name,
+					       prog->expected_attach_type);
+
+	if (btf_id < 0)
+		return btf_id;
+
+	prog->attach_btf_id = btf_id;
+	prog->attach_prog_fd = attach_prog_fd;
+	return 0;
+}
+
 int parse_cpu_mask_str(const char *s, bool **mask, int *mask_sz)
 {
 	int err = 0, n, len, start, end = -1;

tools/lib/bpf/libbpf.h

@@ -334,6 +334,10 @@ LIBBPF_API void
 bpf_program__set_expected_attach_type(struct bpf_program *prog,
 				      enum bpf_attach_type type);
 
+LIBBPF_API int
+bpf_program__set_attach_target(struct bpf_program *prog, int attach_prog_fd,
+			       const char *attach_func_name);
+
 LIBBPF_API bool bpf_program__is_socket_filter(const struct bpf_program *prog);
 LIBBPF_API bool bpf_program__is_tracepoint(const struct bpf_program *prog);
 LIBBPF_API bool bpf_program__is_raw_tracepoint(const struct bpf_program *prog);

tools/lib/bpf/libbpf.map

@@ -235,3 +235,8 @@ LIBBPF_0.0.7 {
 		btf__align_of;
 		libbpf_find_kernel_btf;
 } LIBBPF_0.0.6;
+
+LIBBPF_0.0.8 {
+	global:
+		bpf_program__set_attach_target;
+} LIBBPF_0.0.7;

tools/testing/selftests/bpf/prog_tests/xdp_bpf2bpf.c

@@ -14,7 +14,7 @@ void test_xdp_bpf2bpf(void)
 	struct test_xdp *pkt_skel = NULL;
 	struct test_xdp_bpf2bpf *ftrace_skel = NULL;
 	struct vip key4 = {.protocol = 6, .family = AF_INET};
-	DECLARE_LIBBPF_OPTS(bpf_object_open_opts, opts);
+	struct bpf_program *prog;
 
 	/* Load XDP program to introspect */
 	pkt_skel = test_xdp__open_and_load();
@@ -27,11 +27,21 @@ void test_xdp_bpf2bpf(void)
 	bpf_map_update_elem(map_fd, &key4, &value4, 0);
 
 	/* Load trace program */
-	opts.attach_prog_fd = pkt_fd,
-	ftrace_skel = test_xdp_bpf2bpf__open_opts(&opts);
+	ftrace_skel = test_xdp_bpf2bpf__open();
 	if (CHECK(!ftrace_skel, "__open", "ftrace skeleton failed\n"))
 		goto out;
 
+	/* Demonstrate the bpf_program__set_attach_target() API rather than
+	 * the load with options, i.e. opts.attach_prog_fd.
+	 */
+	prog = ftrace_skel->progs.trace_on_entry;
+	bpf_program__set_expected_attach_type(prog, BPF_TRACE_FENTRY);
+	bpf_program__set_attach_target(prog, pkt_fd, "_xdp_tx_iptunnel");
+
+	prog = ftrace_skel->progs.trace_on_exit;
+	bpf_program__set_expected_attach_type(prog, BPF_TRACE_FEXIT);
+	bpf_program__set_attach_target(prog, pkt_fd, "_xdp_tx_iptunnel");
+
 	err = test_xdp_bpf2bpf__load(ftrace_skel);
 	if (CHECK(err, "__load", "ftrace skeleton failed\n"))
 		goto out;

tools/testing/selftests/bpf/progs/test_xdp_bpf2bpf.c

@@ -28,7 +28,7 @@ struct xdp_buff {
 } __attribute__((preserve_access_index));
 
 __u64 test_result_fentry = 0;
-SEC("fentry/_xdp_tx_iptunnel")
+SEC("fentry/FUNC")
 int BPF_PROG(trace_on_entry, struct xdp_buff *xdp)
 {
 	test_result_fentry = xdp->rxq->dev->ifindex;
@@ -36,7 +36,7 @@ int BPF_PROG(trace_on_entry, struct xdp_buff *xdp)
 }
 
 __u64 test_result_fexit = 0;
-SEC("fexit/_xdp_tx_iptunnel")
+SEC("fexit/FUNC")
 int BPF_PROG(trace_on_exit, struct xdp_buff *xdp, int ret)
 {
 	test_result_fexit = ret;