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From: Andy Lutomirski <luto@amacapital.net>
Date: Fri, 17 Feb 2017 08:52:59 -0800
Message-ID: <CALCETrUOaNaS6yAhLM9jLb8PD2eg2ZwXfu4v327aAOy82SbLwg@mail.gmail.com>
Subject: Re: [RFC 1/2] fs,eventpoll: Add ability to install target file by its number
To: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linuxfoundation.org>,
        Andrew Vagin <avagin@virtuozzo.com>,
        Pavel Emelyanov <xemul@virtuozzo.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Kirill Kolyshkin <kir@openvz.org>, Andrey Vagin <avagin@openvz.org>
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On Fri, Feb 17, 2017 at 12:30 AM, Cyrill Gorcunov <gorcunov@openvz.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.

From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>
Subject: Re: [RFC 1/2] fs,eventpoll: Add ability to install target file by its number
Date: Fri, 17 Feb 2017 08:52:59 -0800
Message-ID: <CALCETrUOaNaS6yAhLM9jLb8PD2eg2ZwXfu4v327aAOy82SbLwg@mail.gmail.com>
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On Fri, Feb 17, 2017 at 12:30 AM, Cyrill Gorcunov <gorcunov-GEFAQzZX7r8dnm+yROfE0A@public.gmane.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.