linux-kernel - Re: Variant symlink filesystem

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Message-ID: <CACsf_wxdF-BkddKbu_Yke6Sid1NVLTpW+JTRnoCwhBbc3CHpVw@mail.gmail.com>
Date:	Fri, 11 Mar 2016 22:52:52 +0200
From:	Cole <cole@...eqint.net>
To:	"Austin S. Hemmelgarn" <ahferroin7@...il.com>
Cc:	Richard Weinberger <richard@....at>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Variant symlink filesystem

On 11 March 2016 at 22:38, Austin S. Hemmelgarn <ahferroin7@...il.com> wrote:
> On 2016-03-11 15:24, Richard Weinberger wrote:
>>
>> Am 11.03.2016 um 21:22 schrieb Cole:
>>>
>>> If I remember correctly, when we were testing the fuse version, we hard
>>> coded
>>> the path to see if that solved the problem, and the difference between
>>> the env lookup
>>> code and the hard coded path was almost the same, but substantially
>>> slower than
>>> the native file system.
>>
>>
>> And where exactly as the performance problem?
>>
>> Anyway, if you submit your filesystem also provide a decent use case for
>> it. :-)
>>
> I don't know that this qualifies as a use case, but I've seen a number of
> capability based systems that have a similar concept they usually refer to
> as 'context dependent symbolic links'.  In such cases, the resolution is
> usually based on what capabilities you posses, and is more of a mapping than
> a value expansion most of the time, but such usage could be emulated (albeit
> much less securely) with this.  If this could be extended to expand other
> values (for example, process bit width, or SELinux context, or even what
> namespace the process is in), it could provide the same functionality almost
> as securely.


The implementation doesn't necessarily have to continue to work with
env variables. On FreeBSD, the variant symlinks function by using
variables stored in kernel memory, and have a hierarchical lookup,
starting with user defined values and terminating with global entries.
I am not aware of such functionality existing on linux, but if someone
could point me at something similar to that, I would much prefer to
use that, as there are issues with variables that are exported or
modified during process execution.

Regards
/Cole