libbpf: Add bpf_obj_get_opts()

Add an extensible variant of bpf_obj_get() capable of setting the
`file_flags` parameter.

This parameter is needed to enable unprivileged access to BPF maps.
Without a method like this, users must manually make the syscall.

Signed-off-by: Joe Burton <jevburton@google.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220729202727.3311806-1-jevburton.kernel@gmail.com

src/bpf.c

@@ -578,12 +578,21 @@ int bpf_obj_pin(int fd, const char *pathname)
 }
 
 int bpf_obj_get(const char *pathname)
+{
+	return bpf_obj_get_opts(pathname, NULL);
+}
+
+int bpf_obj_get_opts(const char *pathname, const struct bpf_obj_get_opts *opts)
 {
 	union bpf_attr attr;
 	int fd;
 
+	if (!OPTS_VALID(opts, bpf_obj_get_opts))
+		return libbpf_err(-EINVAL);
+
 	memset(&attr, 0, sizeof(attr));
 	attr.pathname = ptr_to_u64((void *)pathname);
+	attr.file_flags = OPTS_GET(opts, file_flags, 0);
 
 	fd = sys_bpf_fd(BPF_OBJ_GET, &attr, sizeof(attr));
 	return libbpf_err_errno(fd);

src/bpf.h

@@ -270,8 +270,19 @@ LIBBPF_API int bpf_map_update_batch(int fd, const void *keys, const void *values
 				    __u32 *count,
 				    const struct bpf_map_batch_opts *opts);
 
+struct bpf_obj_get_opts {
+	size_t sz; /* size of this struct for forward/backward compatibility */
+
+	__u32 file_flags;
+
+	size_t :0;
+};
+#define bpf_obj_get_opts__last_field file_flags
+
 LIBBPF_API int bpf_obj_pin(int fd, const char *pathname);
 LIBBPF_API int bpf_obj_get(const char *pathname);
+LIBBPF_API int bpf_obj_get_opts(const char *pathname,
+				const struct bpf_obj_get_opts *opts);
 
 struct bpf_prog_attach_opts {
 	size_t sz; /* size of this struct for forward/backward compatibility */

src/libbpf.map

@@ -355,6 +355,7 @@ LIBBPF_0.8.0 {
 
 LIBBPF_1.0.0 {
 	global:
+		bpf_obj_get_opts;
 		bpf_prog_query_opts;
 		bpf_program__attach_ksyscall;
 		btf__add_enum64;