to_user_ptr
unsafe_put_user(to_user_ptr(mctx), &old_ctx->uc_regs, failed);
unsafe_put_user(to_user_ptr(&frame->uc.uc_mcontext), &frame->uc.uc_regs, failed);
unsafe_put_user(to_user_ptr(ksig->ka.sa.sa_handler), &sc->handler, failed);
unsafe_put_user(to_user_ptr(mctx), &sc->regs, failed);