linux-kernel - Re: [RFC 1/2] fs,eventpoll: Add ability to install target file by its number

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Date:   Fri, 17 Feb 2017 08:52:59 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Cyrill Gorcunov <gorcunov@...nvz.org>
Cc:     Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...uxfoundation.org>,
        Andrew Vagin <avagin@...tuozzo.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Kirill Kolyshkin <kir@...nvz.org>,
        Andrey Vagin <avagin@...nvz.org>
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 <gorcunov@...nvz.org> 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.