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Date:   Sat, 13 May 2017 01:53:40 +0300
From:   Cyrill Gorcunov <>
To:     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 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.


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