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Date:   Fri, 17 Feb 2017 08:52:59 -0800
From:   Andy Lutomirski <>
To:     Cyrill Gorcunov <>
Cc:     Linux FS Devel <>,
        "" <>,
        Linux API <>,
        Al Viro <>,
        Andrew Morton <>,
        Andrew Vagin <>,
        Pavel Emelyanov <>,
        Michael Kerrisk <>,
        Kirill Kolyshkin <>,
        Andrey Vagin <>
Subject: Re: [RFC 1/2] fs,eventpoll: Add ability to install target file by its number

On Fri, Feb 17, 2017 at 12:30 AM, Cyrill Gorcunov <> wrote:
> When we checkpoint a process we look into /proc/<pid>/fdinfo/<fd> of eventpoll
> file and parse target files list from there. In most situations this is fine
> because target file is present in the /proc/<pid>/fd/ list. But in case if file
> descriptor was dup'ed or transferred via unix socket and closed after,
> it might not be in the list and we can't figure out which file descriptor
> to pass into epoll_ctl call.
> To resolve this tie lets add EPOLL_CTL_DUP operation which simply takes
> target file descriptor number and installs it into a caller's file table,
> thus we can use kcmp() syscall and figure out which exactly file to be
> added into eventpoll on restore procedure.

This is a scary thing to let an unprivileged process do.

I'm wondering if there might be a nicer way to address this using a
better interface in /proc.

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