linux-kernel - Re: [PATCH 01/11] SYSCTL: export root and set handling routines

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Date:	Wed, 11 Jan 2012 09:21:43 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Stanislav Kinsbursky <skinsbursky@...allels.com>
Cc:	"Trond.Myklebust\@netapp.com" <Trond.Myklebust@...app.com>,
	"linux-nfs\@vger.kernel.org" <linux-nfs@...r.kernel.org>,
	Pavel Emelianov <xemul@...allels.com>,
	"neilb\@suse.de" <neilb@...e.de>,
	"netdev\@vger.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	James Bottomley <jbottomley@...allels.com>,
	"bfields\@fieldses.org" <bfields@...ldses.org>,
	"davem\@davemloft.net" <davem@...emloft.net>,
	"devel\@openvz.org" <devel@...nvz.org>
Subject: Re: [PATCH 01/11] SYSCTL: export root and set handling routines

Stanislav Kinsbursky <skinsbursky@...allels.com> writes:

> 11.01.2012 02:39, Eric W. Biederman пишет:
>> Stanislav Kinsbursky<skinsbursky@...allels.com>  writes:
>>
>>> 03.01.2012 07:49, Eric W. Biederman пишет:
>>>> Stanislav Kinsbursky<skinsbursky@...allels.com>   writes:
>>>>
>>>>> 19.12.2011 20:37, Eric W. Biederman пишет:
>>>>>> Stanislav Kinsbursky<skinsbursky@...allels.com>    writes:
>>>>>>
>>>>>> Doing that independently of the rest of the sysctls is pretty horrible
>>>>>> and confusing to users.   What I am planning might suit your needs and
>>>>>> if not we need to talk some more about how to get the vfs to do
>>>>>> something reasonable.
>>>>>>
>>>>>
>>>>> Ok, Eric. Would be glad to discuss your sysctls plans.
>>>>> But actually you already know my needs: I would like to make sysctls work in the
>>>>> way like sysfs does: i.e. content of files depends on mount maker -
>>>>> not viewer.
>>>>
>>>> What drives the desire to have sysctls depend on the mount maker?
>>>
>>> Because we can (will, actually) have nested fs root's for containers. IOW,
>>> container's root will be accessible from it's creator context. And I want to
>>> tune container's fs from creators context.
>>
>> Tuning the child context from the parent context is an entirely
>> reasonable thing to do.  To affect a namespace that is not yours
>> the requirement is simply that we don't use current to lookup the
>> sysctl.  So what I am proposing should work for your case.
>>
>
> Could you explain, what are you proposing?
> I still don't know any details about it.

I am proposing treating /proc/sys like /proc/net is currently treated.
See below.

>>>> Especially what drives that desire not to have it have a /proc/<pid>/sys
>>>> directory that reflects the sysctls for a given process.
>>>>
>>>
>>> This is not so important for me, where to access sysctl's. But I'm worrying
>>> about backward compatibility. IOW, I'm afraid of changing path
>>> "/proc/sys/sunprc/*" to "/proc/<pid>/sys/sunrpc". This would break a lot of
>>> user-space programs.
>>
>> The part that keeps it all working is by adding a symlink from /proc/sys
>> to /proc/self/sys.  That technique has worked well for /proc/net, and I
>> don't expect there will be any problems with /proc/sys either.  It is
>> possible but is very rare for the introduction of a symlink in a path
>> to cause problems.
>>
>
> Probably I don't understand you, but as I see it now, symlink to "/proc/self/"
> is unacceptable because of the following:
> 1) will be used current context (any) instead of desired one
(Using the current context is the desirable outcome for existing tools).
> 1) if CT has other pid namespace - then we just have broken link.

Assuming the process in question is not in the pid namespace available
to proc then yes you will indeed have a broken link.  But a broken
link is only a problem for new applications that are doing something strange.

I am proposing treating /proc/sys like /proc/net has already been
treated.  Aka move have the version of /proc/sys that relative to a
process be visible at: /proc/<pid>/sys, and with a compat symlink 
from /proc/sys -> /proc/self/sys.

Just like has already been done with /proc/net.

Semantically this should be easy to understand, and about as backwards
compatible as it gets.

Eric
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