SECCON 2017 Online CTF: Theory of Relativity

,

https://ctftime.org/event/512/

problem

An interpreter of a simple language and the web-interface are given. Submit a code which runs more than $100$sec (but the execution time limit is $20$sec).

solution

Print user 100.001s to the stderr, using error messages. The server executes bash -c "time timeout 20 python %s %s" 1>%s 2>%s and retrieve the elapsed time using the regex ^user\t([0-9]+m)?([0-9]+\.[0-9]{3}s)$.

implementation

ls r0, '\nuser\t0m100.001s\n'
li r1, 42
add r0, r0, r1