netdev - Re: [PATCH 2/7] ns: Introduce the setns syscall

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Message-ID: <m1bozepqa5.fsf@fess.ebiederm.org>
Date:	Sat, 07 May 2011 06:57:06 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Rémi Denis-Courmont <remi@...lab.net>
Cc:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	jamal <hadi@...erus.ca>, Daniel Lezcano <daniel.lezcano@...e.fr>,
	Linux Containers <containers@...ts.osdl.org>,
	Renato Westphal <renatowestphal@...il.com>
Subject: Re: [PATCH 2/7] ns: Introduce the setns syscall

"Rémi Denis-Courmont" <remi@...lab.net> writes:

> Le samedi 7 mai 2011 05:24:56 Eric W. Biederman, vous avez écrit :
>> Pieces of this puzzle can also be solved by instead of
>> coming up with a general purpose system call coming up
>> with targed system calls perhaps socketat that solve
>> a subset of the larger problem.  Overall that appears
>> to be more work for less reward.
>
> socketat() is still required for multithreaded namespace-aware userspace, I 
> believe.

The network namespace is a per task property so there are no problems
with multithreaded network namespace aware userspace applications.  The
implementation of a userspace socketat will still need to disable signal
handling around the network namespace switch to be signal safe.  Which
means that ultimately a kernel version of socketat may be desirable,
for performance reasons but I know of know correctness reasons to need
it.

For the time being I have simply removed socketat from what I plan to
merge because it is not strictly needed, I don't yet have a test case
for socketat, and I don't have as much time to work on this as I
would like.

There is one bug a multi-threaded network namespace aware user space
application might run into, and that is /proc/net is a symlink to
/proc/self.  Which means that if you open /proc/net/foo from a task with
a different network namespace than your the task whose tid equals your
tgid, the /proc/net will return the wrong file.  Still you can
avoid even that silliness by opening /proc/<tid>/net.

Eric
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