libbpf: Add support for attaching BPF programs to other BPF programs

Extend libbpf api to pass attach_prog_fd into bpf_object__open.
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20191114185720.1641606-19-ast@kernel.org

tools/include/uapi/linux/bpf.h

@@ -425,6 +425,7 @@ union bpf_attr {
 		__aligned_u64	line_info;	/* line info */
 		__u32		line_info_cnt;	/* number of bpf_line_info records */
 		__u32		attach_btf_id;	/* in-kernel BTF type id to attach to */
+		__u32		attach_prog_fd; /* 0 to attach to vmlinux */
 	};
 
 	struct { /* anonymous struct used by BPF_OBJ_* commands */

tools/lib/bpf/bpf.c

@@ -228,10 +228,13 @@ int bpf_load_program_xattr(const struct bpf_load_program_attr *load_attr,
 	memset(&attr, 0, sizeof(attr));
 	attr.prog_type = load_attr->prog_type;
 	attr.expected_attach_type = load_attr->expected_attach_type;
-	if (attr.prog_type == BPF_PROG_TYPE_TRACING)
+	if (attr.prog_type == BPF_PROG_TYPE_TRACING) {
 		attr.attach_btf_id = load_attr->attach_btf_id;
-	else
+		attr.attach_prog_fd = load_attr->attach_prog_fd;
+	} else {
 		attr.prog_ifindex = load_attr->prog_ifindex;
+		attr.kern_version = load_attr->kern_version;
+	}
 	attr.insn_cnt = (__u32)load_attr->insns_cnt;
 	attr.insns = ptr_to_u64(load_attr->insns);
 	attr.license = ptr_to_u64(load_attr->license);
@@ -245,7 +248,6 @@ int bpf_load_program_xattr(const struct bpf_load_program_attr *load_attr,
 		attr.log_size = 0;
 	}
 
-	attr.kern_version = load_attr->kern_version;
 	attr.prog_btf_fd = load_attr->prog_btf_fd;
 	attr.func_info_rec_size = load_attr->func_info_rec_size;
 	attr.func_info_cnt = load_attr->func_info_cnt;

tools/lib/bpf/bpf.h

@@ -77,7 +77,10 @@ struct bpf_load_program_attr {
 	const struct bpf_insn *insns;
 	size_t insns_cnt;
 	const char *license;
+	union {
 		__u32 kern_version;
+		__u32 attach_prog_fd;
+	};
 	union {
 		__u32 prog_ifindex;
 		__u32 attach_btf_id;

tools/lib/bpf/libbpf.c

@@ -189,6 +189,7 @@ struct bpf_program {
 
 	enum bpf_attach_type expected_attach_type;
 	__u32 attach_btf_id;
+	__u32 attach_prog_fd;
 	void *func_info;
 	__u32 func_info_rec_size;
 	__u32 func_info_cnt;
@@ -3683,8 +3684,13 @@ load_program(struct bpf_program *prog, struct bpf_insn *insns, int insns_cnt,
 	load_attr.insns = insns;
 	load_attr.insns_cnt = insns_cnt;
 	load_attr.license = license;
+	if (prog->type == BPF_PROG_TYPE_TRACING) {
+		load_attr.attach_prog_fd = prog->attach_prog_fd;
+		load_attr.attach_btf_id = prog->attach_btf_id;
+	} else {
 		load_attr.kern_version = kern_version;
 		load_attr.prog_ifindex = prog->prog_ifindex;
+	}
 	/* if .BTF.ext was loaded, kernel supports associated BTF for prog */
 	if (prog->obj->btf_ext)
 		btf_fd = bpf_object__btf_fd(prog->obj);
@@ -3699,7 +3705,6 @@ load_program(struct bpf_program *prog, struct bpf_insn *insns, int insns_cnt,
 	load_attr.line_info_cnt = prog->line_info_cnt;
 	load_attr.log_level = prog->log_level;
 	load_attr.prog_flags = prog->prog_flags;
-	load_attr.attach_btf_id = prog->attach_btf_id;
 
 retry_load:
 	log_buf = malloc(log_buf_size);
@@ -3856,9 +3861,9 @@ bpf_object__load_progs(struct bpf_object *obj, int log_level)
 	return 0;
 }
 
-static int libbpf_attach_btf_id_by_name(const char *name,
-					enum bpf_attach_type attach_type);
-
+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,
 		   struct bpf_object_open_opts *opts)
@@ -3869,6 +3874,7 @@ __bpf_object__open(const char *path, const void *obj_buf, size_t obj_buf_sz,
 	const char *obj_name;
 	char tmp_name[64];
 	bool relaxed_maps;
+	__u32 attach_prog_fd;
 	int err;
 
 	if (elf_version(EV_CURRENT) == EV_NONE) {
@@ -3899,6 +3905,7 @@ __bpf_object__open(const char *path, const void *obj_buf, size_t obj_buf_sz,
 	obj->relaxed_core_relocs = OPTS_GET(opts, relaxed_core_relocs, false);
 	relaxed_maps = OPTS_GET(opts, relaxed_maps, false);
 	pin_root_path = OPTS_GET(opts, pin_root_path, NULL);
+	attach_prog_fd = OPTS_GET(opts, attach_prog_fd, 0);
 
 	CHECK_ERR(bpf_object__elf_init(obj), err, out);
 	CHECK_ERR(bpf_object__check_endianness(obj), err, out);
@@ -3923,11 +3930,13 @@ __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_attach_btf_id_by_name(prog->section_name,
-							   attach_type);
+			err = libbpf_find_attach_btf_id(prog->section_name,
+							attach_type,
+							attach_prog_fd);
 			if (err <= 0)
 				goto out;
 			prog->attach_btf_id = err;
+			prog->attach_prog_fd = attach_prog_fd;
 		}
 	}
 
@@ -5086,8 +5095,42 @@ int libbpf_find_vmlinux_btf_id(const char *name,
 	return err;
 }
 
-static int libbpf_attach_btf_id_by_name(const char *name,
-					enum bpf_attach_type attach_type)
+static int libbpf_find_prog_btf_id(const char *name, __u32 attach_prog_fd)
+{
+	struct bpf_prog_info_linear *info_linear;
+	struct bpf_prog_info *info;
+	struct btf *btf = NULL;
+	int err = -EINVAL;
+
+	info_linear = bpf_program__get_prog_info_linear(attach_prog_fd, 0);
+	if (IS_ERR_OR_NULL(info_linear)) {
+		pr_warn("failed get_prog_info_linear for FD %d\n",
+			attach_prog_fd);
+		return -EINVAL;
+	}
+	info = &info_linear->info;
+	if (!info->btf_id) {
+		pr_warn("The target program doesn't have BTF\n");
+		goto out;
+	}
+	if (btf__get_from_id(info->btf_id, &btf)) {
+		pr_warn("Failed to get BTF of the program\n");
+		goto out;
+	}
+	err = btf__find_by_name_kind(btf, name, BTF_KIND_FUNC);
+	btf__free(btf);
+	if (err <= 0) {
+		pr_warn("%s is not found in prog's BTF\n", name);
+		goto out;
+	}
+out:
+	free(info_linear);
+	return err;
+}
+
+static int libbpf_find_attach_btf_id(const char *name,
+				     enum bpf_attach_type attach_type,
+				     __u32 attach_prog_fd)
 {
 	int i, err;
 
@@ -5099,6 +5142,10 @@ static int libbpf_attach_btf_id_by_name(const char *name,
 			continue;
 		if (strncmp(name, section_names[i].sec, section_names[i].len))
 			continue;
+		if (attach_prog_fd)
+			err = libbpf_find_prog_btf_id(name + section_names[i].len,
+						      attach_prog_fd);
+		else
 			err = libbpf_find_vmlinux_btf_id(name + section_names[i].len,
 							 attach_type);
 		if (err <= 0)

tools/lib/bpf/libbpf.h

@@ -108,8 +108,9 @@ struct bpf_object_open_opts {
 	 * auto-pinned to that path on load; defaults to "/sys/fs/bpf".
 	 */
 	const char *pin_root_path;
+	__u32 attach_prog_fd;
 };
-#define bpf_object_open_opts__last_field pin_root_path
+#define bpf_object_open_opts__last_field attach_prog_fd
 
 LIBBPF_API struct bpf_object *bpf_object__open(const char *path);
 LIBBPF_API struct bpf_object *