libbpf: Introduce bpf_get_fd_by_id_opts and bpf_map_get_fd_by_id_opts()

Define a new data structure called bpf_get_fd_by_id_opts, with the member
open_flags, to be used by callers of the _opts variants of
bpf_*_get_fd_by_id() to specify the permissions needed for the file
descriptor to be obtained.

Also, introduce bpf_map_get_fd_by_id_opts(), to let the caller pass a
bpf_get_fd_by_id_opts structure.

Finally, keep the existing bpf_map_get_fd_by_id(), and call
bpf_map_get_fd_by_id_opts() with NULL as opts argument, to request
read-write permissions (current behavior).
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221006110736.84253-3-roberto.sassu@huaweicloud.com

tools/lib/bpf/bpf.c

@@ -948,19 +948,29 @@ int bpf_prog_get_fd_by_id(__u32 id)
 	return libbpf_err_errno(fd);
 }
 
-int bpf_map_get_fd_by_id(__u32 id)
+int bpf_map_get_fd_by_id_opts(__u32 id,
+			      const struct bpf_get_fd_by_id_opts *opts)
 {
 	const size_t attr_sz = offsetofend(union bpf_attr, open_flags);
 	union bpf_attr attr;
 	int fd;
 
+	if (!OPTS_VALID(opts, bpf_get_fd_by_id_opts))
+		return libbpf_err(-EINVAL);
+
 	memset(&attr, 0, attr_sz);
 	attr.map_id = id;
+	attr.open_flags = OPTS_GET(opts, open_flags, 0);
 
 	fd = sys_bpf_fd(BPF_MAP_GET_FD_BY_ID, &attr, attr_sz);
 	return libbpf_err_errno(fd);
 }
 
+int bpf_map_get_fd_by_id(__u32 id)
+{
+	return bpf_map_get_fd_by_id_opts(id, NULL);
+}
+
 int bpf_btf_get_fd_by_id(__u32 id)
 {
 	const size_t attr_sz = offsetofend(union bpf_attr, open_flags);

tools/lib/bpf/bpf.h

@@ -365,8 +365,18 @@ LIBBPF_API int bpf_prog_get_next_id(__u32 start_id, __u32 *next_id);
 LIBBPF_API int bpf_map_get_next_id(__u32 start_id, __u32 *next_id);
 LIBBPF_API int bpf_btf_get_next_id(__u32 start_id, __u32 *next_id);
 LIBBPF_API int bpf_link_get_next_id(__u32 start_id, __u32 *next_id);
+
+struct bpf_get_fd_by_id_opts {
+	size_t sz; /* size of this struct for forward/backward compatibility */
+	__u32 open_flags; /* permissions requested for the operation on fd */
+	size_t :0;
+};
+#define bpf_get_fd_by_id_opts__last_field open_flags
+
 LIBBPF_API int bpf_prog_get_fd_by_id(__u32 id);
 LIBBPF_API int bpf_map_get_fd_by_id(__u32 id);
+LIBBPF_API int bpf_map_get_fd_by_id_opts(__u32 id,
+				const struct bpf_get_fd_by_id_opts *opts);
 LIBBPF_API int bpf_btf_get_fd_by_id(__u32 id);
 LIBBPF_API int bpf_link_get_fd_by_id(__u32 id);
 LIBBPF_API int bpf_obj_get_info_by_fd(int bpf_fd, void *info, __u32 *info_len);

tools/lib/bpf/libbpf.map

@@ -371,6 +371,7 @@ LIBBPF_1.0.0 {
 
 LIBBPF_1.1.0 {
 	global:
+		bpf_map_get_fd_by_id_opts;
 		user_ring_buffer__discard;
 		user_ring_buffer__free;
 		user_ring_buffer__new;