Re: [patch v4 resend 2/2] kcmp: Add KCMP_EPOLL_TFD mode to compare epoll target files

[Cyrill Gorcunov <gorcunov-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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.