[PATCH bpf-next] libbpf: Add support for dynamic program attach target

[Eelco Chaudron <echaudro@redhat.com>]

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_attach_target(prog, xdp_fd,
                                 "fentry/xdpfilt_blk_all");
  bpf_object__load(trace_obj)

Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
---
 tools/lib/bpf/libbpf.c   |   41 +++++++++++++++++++++++++++++++++++------
 tools/lib/bpf/libbpf.h   |    4 ++++
 tools/lib/bpf/libbpf.map |    1 +
 3 files changed, 40 insertions(+), 6 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 514b1a524abb..2ce879c301bb 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -4933,15 +4933,16 @@ load_program(struct bpf_program *prog, struct bpf_insn *insns, int insns_cnt,
 	return ret;
 }
 
-static int libbpf_find_attach_btf_id(struct bpf_program *prog);
+static int libbpf_find_attach_btf_id(struct bpf_program *prog,
+				     const char *name);
 
 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) {
-		btf_id = libbpf_find_attach_btf_id(prog);
+	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, NULL);
 		if (btf_id <= 0)
 			return btf_id;
 		prog->attach_btf_id = btf_id;
@@ -6202,6 +6203,31 @@ void bpf_program__set_expected_attach_type(struct bpf_program *prog,
 	prog->expected_attach_type = type;
 }
 
+int bpf_program__set_attach_target(struct bpf_program *prog,
+				   int attach_prog_fd,
+				   const char *attach_func_name)
+{
+	__u32 org_attach_prog_fd;
+	int btf_id;
+
+	if (!prog || attach_prog_fd < 0 || !attach_func_name)
+		return -EINVAL;
+
+	org_attach_prog_fd = prog->attach_prog_fd;
+	prog->attach_prog_fd = attach_prog_fd;
+
+	btf_id = libbpf_find_attach_btf_id(prog,
+					   attach_func_name);
+
+	if (btf_id < 0) {
+		prog->attach_prog_fd = org_attach_prog_fd;
+		return btf_id;
+	}
+
+	prog->attach_btf_id = btf_id;
+	return 0;
+}
+
 #define BPF_PROG_SEC_IMPL(string, ptype, eatype, is_attachable, btf, atype) \
 	{ string, sizeof(string) - 1, ptype, eatype, is_attachable, btf, atype }
 
@@ -6633,13 +6659,16 @@ static int libbpf_find_prog_btf_id(const char *name, __u32 attach_prog_fd)
 	return err;
 }
 
-static int libbpf_find_attach_btf_id(struct bpf_program *prog)
+static int libbpf_find_attach_btf_id(struct bpf_program *prog,
+				     const char *name)
 {
 	enum bpf_attach_type attach_type = prog->expected_attach_type;
 	__u32 attach_prog_fd = prog->attach_prog_fd;
-	const char *name = prog->section_name;
 	int i, err;
 
+	if (!name)
+		name = prog->section_name;
+
 	if (!name)
 		return -EINVAL;
 
diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 3fe12c9d1f92..02fc58a21a7f 100644
--- 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);
diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
index b035122142bb..8aba5438a3f0 100644
--- a/tools/lib/bpf/libbpf.map
+++ b/tools/lib/bpf/libbpf.map
@@ -230,6 +230,7 @@ LIBBPF_0.0.7 {
 		bpf_program__name;
 		bpf_program__is_extension;
 		bpf_program__is_struct_ops;
+		bpf_program__set_attach_target;
 		bpf_program__set_extension;
 		bpf_program__set_struct_ops;
 		btf__align_of;