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From: Andy Lutomirski <luto@amacapital.net>
Date: Thu, 10 Apr 2014 13:37:26 -0700
Message-ID: <CALCETrVmaGNCxo-L4-dPbUev3VXXEPR7xBzo3Fux6ny7yh_Gzw@mail.gmail.com>
Subject: Re: [PATCH 0/6] File Sealing & memfd_create()
To: "Theodore Ts'o" <tytso@mit.edu>, Andy Lutomirski <luto@amacapital.net>,
        David Herrmann <dh.herrmann@gmail.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Hugh Dickins <hughd@google.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Karol Lewandowski <k.lewandowsk@samsung.com>,
        Kay Sievers <kay@vrfy.org>, Daniel Mack <zonque@gmail.com>,
        Lennart Poettering <lennart@poettering.net>,
        John Stultz <john.stultz@linaro.org>,
        Greg Kroah-Hartman <greg@kroah.com>, Tejun Heo <tj@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        "dri-devel@lists.freedesktop.org" <dri-devel@lists.freedesktop.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ryan Lortie <desrt@desrt.ca>,
        "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
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On Thu, Apr 10, 2014 at 1:32 PM, Theodore Ts'o <tytso@mit.edu> wrote:
> On Thu, Apr 10, 2014 at 12:14:27PM -0700, Andy Lutomirski wrote:
>>
>> This is the second time in a week that someone has asked for a way to
>> have a struct file (or struct inode or whatever) that can't be reopened
>> through /proc/pid/fd.  This should be quite easy to implement as a
>> separate feature.
>
> What I suggested on a different thread was to add the following new
> file descriptor flags, to join FD_CLOEXEC, which would be maniuplated
> using the F_GETFD and F_SETFD fcntl commands:
>
> FD_NOPROCFS     disallow being able to open the inode via /proc/<pid>/fd
>
> FD_NOPASSFD     disallow being able to pass the fd via a unix domain socket
>
> FD_LOCKFLAGS    if this bit is set, disallow any further changes of FD_CLOEXEC,
>                 FD_NOPROCFS, FD_NOPASSFD, and FD_LOCKFLAGS flags.
>
> Regardless of what else we might need to meet the use case for the
> proposed File Sealing API, I think this is a useful feature that could
> be used in many other contexts besides just the proposed
> memfd_create() use case.

It occurs to me that, before going nuts with these kinds of flags, it
may pay to just try to fix the /proc/self/fd issue for real -- we
could just make open("/proc/self/fd/3", O_RDWR) fail if fd 3 is
read-only.  That may be enough for the file sealing thing.

--Andy
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