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Date: Thu, 10 Apr 2014 16:32:46 -0400
From: Theodore Ts'o <tytso@mit.edu>
Subject: Re: [PATCH 0/6] File Sealing & memfd_create()
Message-ID: <20140410203246.GB31614@thunk.org>
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To: Andy Lutomirski <luto@amacapital.net>
Cc: 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>

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.

Cheers,

					- Ted

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