linux-kernel - Re: RFC: named anonymous vmas

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Date:	Mon, 24 Jun 2013 16:45:12 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Colin Cross <ccross@...gle.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Alex Elsayed <eternaleye@...il.com>,
	Linux-MM <linux-mm@...ck.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: RFC: named anonymous vmas

On Mon, Jun 24, 2013 at 10:26 AM, Colin Cross <ccross@...gle.com> wrote:
> On Mon, Jun 24, 2013 at 4:48 AM, Christoph Hellwig <hch@...radead.org> wrote:
>> On Sat, Jun 22, 2013 at 12:47:29PM -0700, Alex Elsayed wrote:
>>> Couldn't this be done by having a root-only tmpfs, and having a userspace
>>> component that creates per-app directories with restrictive permissions on
>>> startup/app install? Then each app creates files in its own directory, and
>>> can pass the fds around.
>
> If each app gets its own writable directory that's not really
> different than a world writable tmpfs.  It requires something that
> watches for apps to exit for any reason and cleans up their
> directories, and it requires each app to come up with an unused name
> when it wants to create a file, and the kernel can give you both very
> cleanly.

Though, I believe having a daemon that has exclusive access to tmpfs,
and creates, unlinks and passes the fd to the requesting application
would provide a userspace only implementation of the second feature
requirement ("without having a world-writable tmpfs that untrusted
apps could fill with files").  Though I'm not sure what the
proc/<pid>/maps naming would look like on the unlinked file, so it
might not solve the third naming issue.

thanks
-john
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