From: Eelco Chaudron <echaudro@redhat.com>
Date: Thu, 20 Feb 2020 13:26:35 +0000
Subject: libbpf: Add support for dynamic program attach target
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Patch-mainline: v5.7-rc1
Git-commit: ff26ce5cd7680ebc9c5446cda013e2087decc60f
References: bsc#1177028

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)

Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/158220519486.127661.7964708960649051384.stgit@xdp-tutorial
Acked-by: Gary Lin <glin@suse.com>
---
 tools/lib/bpf/libbpf.c   |   34 ++++++++++++++++++++++++++++++----
 tools/lib/bpf/libbpf.h   |    4 ++++
 tools/lib/bpf/libbpf.map |    2 ++
 3 files changed, 36 insertions(+), 4 deletions(-)

--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -4945,8 +4945,8 @@ int bpf_program__load(struct bpf_program
 {
 	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;
@@ -6590,6 +6590,9 @@ static inline int __find_vmlinux_btf_id(
 	else
 		err = btf__find_by_name_kind(btf, name, BTF_KIND_FUNC);
 
+	if (err <= 0)
+		pr_warning("%s is not found in vmlinux BTF\n", name);
+
 	return err;
 }
 
@@ -6662,8 +6665,6 @@ static int libbpf_find_attach_btf_id(str
 			err = __find_vmlinux_btf_id(prog->obj->btf_vmlinux,
 						    name + section_defs[i].len,
 						    attach_type);
-		if (err <= 0)
-			pr_warning("%s is not found in vmlinux BTF\n", name);
 		return err;
 	}
 	pr_warning("failed to identify btf_id based on ELF section name '%s'\n", name);
@@ -8140,6 +8141,31 @@ void bpf_program__bpil_offs_to_addr(stru
 	}
 }
 
+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;
--- a/tools/lib/bpf/libbpf.h
+++ b/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);
--- a/tools/lib/bpf/libbpf.map
+++ b/tools/lib/bpf/libbpf.map
@@ -237,4 +237,6 @@ LIBBPF_0.0.7 {
 } LIBBPF_0.0.6;
 
 LIBBPF_0.0.8 {
+	global:
+		bpf_program__set_attach_target;
 } LIBBPF_0.0.7;