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

[Andrei Vagin]

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