On Fri, May 12, 2017 at 06:45:09PM -0700, Andrei Vagin wrote: > On Sat, May 13, 2017 at 01:53:40AM +0300, Cyrill Gorcunov wrote: > > On Sat, May 13, 2017 at 12:41:30AM +0200, Jann Horn wrote: > > > [resending as plaintext] > > > > > > I realize that the existing kcmp code has the same issue, but: > > > > > > Why are you not taking a reference to filp or filp_tgt? This can end up > > > performing a comparison between a pointer to a freed struct file and a > > > pointer to a struct file that was allocated afterwards, right? So it can > > > return a false "is equal" result when the two files aren't actually the same > > > if one of the target tasks is running? This looks like it unnecessarily > > > exposes information about whether an allocation reuses the memory of > > > a previously freed allocation. > > > > It work with unlocked data on purpose for speed sake. Moreover even > > if we grap a reference it is valid _only_ during comparision operation, > > next we drop ref and it can be easily freed by os. Thus it's up to > > a caller to keep references to files/task and other resources used. > > Looks like we can take rcu_read_lock() to guarantee that these objects > will not be freed, and rcu_read_lock() should not affect perfomance too much. Rather they should be get_file_rcu/fput. Still I'm not convinced we need it, but fine will update both: plain KCMP_FILE and KCMP_EPOLL_TFD since it won't hurt performance.
