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. > > Cyrill
