找到对应的文件：

system/extras/su/su.c

在main函数中，开头我们可以看下以下代码：

 uid_t current_uid = getuid();
 if (current_uid != AID_ROOT && current_uid != AID_SHELL) error(1, 0, "not allowed");

将其注释掉重新编译系统即可开放安卓系统权限。
int main源码如下:

int main(int argc, char** argv) {
	/*
    uid_t current_uid = getuid();
	//改变安卓系统权限问题2016.12.5
    if (current_uid != AID_ROOT && current_uid != AID_SHELL) error(1, 0, "not allowed");
	*/
    // Handle -h and --help.
    ++argv;
    if (*argv && (strcmp(*argv, "--help") == 0 || strcmp(*argv, "-h") == 0)) {
        fprintf(stderr,
                "usage: su [UID[,GID[,GID2]...]] [COMMAND [ARG...]]\n"
                "\n"
                "Switch to WHO (default 'root') and run the given command (default sh).\n"
                "\n"
                "where WHO is a comma-separated list of user, group,\n"
                "and supplementary groups in that order.\n"
                "\n");
        return 0;
    }

    // The default user is root.
    uid_t uid = 0;
    gid_t gid = 0;

    // If there are any arguments, the first argument is the uid/gid/supplementary groups.
    if (*argv) {
        gid_t gids[10];
        int gids_count = sizeof(gids)/sizeof(gids[0]);
        extract_uidgids(*argv, &uid, &gid, gids, &gids_count);
        if (gids_count) {
            if (setgroups(gids_count, gids)) {
                error(1, errno, "setgroups failed");
            }
        }
        ++argv;
    }

    if (setgid(gid)) error(1, errno, "setgid failed");
    if (setuid(uid)) error(1, errno, "setuid failed");

    // Reset parts of the environment.
    setenv("PATH", _PATH_DEFPATH, 1);
    unsetenv("IFS");
    struct passwd* pw = getpwuid(uid);
    setenv("LOGNAME", pw->pw_name, 1);
    setenv("USER", pw->pw_name, 1);

    // Set up the arguments for exec.
    char* exec_args[argc + 1];  // Having too much space is fine.
    size_t i = 0;
    for (; *argv != NULL; ++i) {
      exec_args[i] = *argv++;
    }
    // Default to the standard shell.
    if (i == 0) exec_args[i++] = "/system/bin/sh";
    exec_args[i] = NULL;

    execvp(exec_args[0], exec_args);
    error(1, errno, "failed to exec %s", exec_args[0]);
}