you need to check that where overflow is occur. (not main function)

first i created a thread, then i checked the input ,which is at address 0x7ffff73fee40 and the canary is at 0x7ffff7ff6728 and then the offset is 0xbf78e8 .so on the sending this payload to overwrite the master canary. and then doing ret2get_shell in the leave_comment input.
payload1 = b'a'*0xbf78e8 payload1 += b'a'*8

pwntools crashes bcz its too big.So, i guess the offset is wrong .But where i am getting it wrong?