linux-kernel - Re: [PATCH] devpts: Add ptmx_uid and ptmx

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Message-ID: <CAOP=4wggpXOC4qLWgNAdw7Ws4vtYR=hscNYzDCfby+-VUxhoQg@mail.gmail.com>
Date:	Thu, 28 May 2015 13:17:30 -0700
From:	Kenton Varda <kenton@...dstorm.io>
To:	Alexander Larsson <alexl@...hat.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	gnome-os-list@...me.org,
	Linux Containers <containers@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	mclasen@...hat.com, Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] devpts: Add ptmx_uid and ptmx_gid options

On Thu, May 28, 2015 at 1:06 PM, Alexander Larsson <alexl@...hat.com> wrote:
> On Thu, 2015-05-28 at 12:14 -0500, Eric W. Biederman wrote:
>>
>> > Where does the second namespace enter into this?
>>
>> Step a.  Create create a user namespace where uid 0 is mapped to your
>> real uid, and set up your sandbox (aka mount /dev/pts and everything
>> else).
>>
>> Step b.  Create a nested user namespace where your uid is identity
>> mapped and run your desktop application.  You can even drop all caps
>> in
>> your namespace.
>
> Just tried this. Its not the nicest, and it doubles the number of
> namespaces in action for each sandbox, but it does work.

How much overhead is involved in each user namespace? Is there any
system-wide limit on total namespaces, other than RAM? Is there
(non-negligible) CPU overhead for each syscall seeking permissions in
the namespace?

-Kenton
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