$ cd /tmp/poc && gcc -O2 -pipe -o race_attach race_attach.c -lpthread
(first attempt failed: struct accept_filter_arg redefinition vs <sys/socket.h>;
 local definition removed -- it is __BSD_VISIBLE in the guest headers)
$ gcc -O2 -pipe -o race_attach race_attach.c -lpthread
(no output, exit 0)
BUILD_OK  (built as uid=1001(maxx) gid=1001(maxx) -- unprivileged)