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Date:   Fri, 12 May 2017 18:45:09 -0700
From:   Andrei Vagin <>
To:     Cyrill Gorcunov <>
CC:     Jann Horn <>, <>,
        kernel list <>,
        Linux API <>,
        Al Viro <>, <>,
        Michael Kerrisk-manpages <>,
        <>, <>,
        Andy Lutomirski <>
Subject: Re: [patch v4 resend 2/2] kcmp: Add KCMP_EPOLL_TFD mode to compare
 epoll target files

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

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