linux-kernel - Re: [RFC PATCH 05/27] containers: Open a socket inside a container

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Date:   Fri, 27 Sep 2019 09:46:39 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Alun Evans <alun@...gerous.net>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 05/27] containers: Open a socket inside a container

Alun Evans <alun@...gerous.net> writes:

> Hi Eric,
>
>
> On Tue, 19 Feb 2019, Eric W. Biederman <ebiederm@...ssion.com> wrote:
>>
>> David Howells <dhowells@...hat.com> writes:
>>
>> > Provide a system call to open a socket inside of a container, using that
>> > container's network namespace.  This allows netlink to be used to manage
>> > the container.
>> >
>> > 	fd = container_socket(int container_fd,
>> > 			      int domain, int type, int protocol);
>> >
>>
>> Nacked-by: "Eric W. Biederman" <ebiederm@...ssion.com>
>>
>> Use a namespace file descriptor if you need this.  So far we have not
>> added this system call as it is just a performance optimization.  And it
>> has been too niche to matter.
>>
>> If this that has changed we can add this separately from everything else
>> you are doing here.
>
> I think I've found the niche.
>
>
> I'm trying to use network namespaces from Go.

Yes. Go sucks for this.

> Since setns is thread
> specific, I'm forced to use this pattern:
>
>     runtime.LockOSThread()
>     defer runtime.UnlockOSThread()
>     …
>     err = netns.Set(newns)
>
>
> This is only safe recently:
> https://github.com/vishvananda/netns/issues/17#issuecomment-367325770
>
> - but is still less than ideal performance wise, as it locks out other
>   socket operations.
>
> The socketat() / socketns() would be ideal:
>
>   https://lwn.net/Articles/406684/
>   https://lwn.net/Articles/407495/
>   https://lkml.org/lkml/2011/10/3/220
>
>
> One thing that is interesting, the LockOSThread works pretty well for
> receiving, since I can wrap it around the socket()/bind()/listen() at
> startup. Then accept() can run outside of the lock.
>
> It's creating new outbound tcp connections via socket()/connect() pairs
> that is the issue.

As I understand it you should be able to write socketat in go something like:

	runtime.LockOSThread()
        err = netns.Set(newns);
        fd = socket(...);
        err = netns.Set(defaultns);
	runtime.UnlockOSThread()

I have no real objections to a kernel system call doing that.  It has
just never risen to the level where it was necessary to optimize
userspace yet.

Eric