Instead of snprintf(pathbuf, sizeof(pathbuf), "/proc/self/fd/%u", fd); execve(pathbuf, argv, envp); you do execveat(fd, "", argv, envp, AT_EMPTY_

Instead of snprintf(pathbuf, sizeof(pathbuf), "/proc/self/fd/%u", fd); execve(pathbuf, argv, envp); you do execveat(fd, "", argv, envp, AT_EMPTY_PATH); and that's it.