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

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Date:	Thu, 28 May 2015 16:50:37 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Kenton Varda <kenton@...dstorm.io>
Cc:	Alexander Larsson <alexl@...hat.com>, gnome-os-list@...me.org,
	Linux Containers <containers@...ts.linux-foundation.org>,
	"linux-kernel\@vger.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

Kenton Varda <kenton@...dstorm.io> writes:

> 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?

sizeof(struct user_namespace).

> Is there any system-wide limit on total namespaces, other than RAM? 

There is a system-wide maximum depth, but not count.

>  Is there
> (non-negligible) CPU overhead for each syscall seeking permissions in
> the namespace?

ns_capable(ns, X) in some cases can walk up the from a starting user
namespace to the initial user.  (The only non-constant operation I am
aware of).  However unless the user namespace depth is deep it should
still take a negligible amount of time.

Eric
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