linux-kernel - Re: [PATCH] proc: fix mount -t proc -o AAA

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Message-Id: <20120326153738.aa728115.akpm@linux-foundation.org>
Date:	Mon, 26 Mar 2012 15:37:38 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Valdis.Kletnieks@...edu
Cc:	Vasiliy Kulikov <segoon@...nwall.com>,
	Arkadiusz Miśkiewicz 
	<a.miskiewicz@...il.com>, linux-kernel@...r.kernel.org,
	Alexey Dobriyan <adobriyan@...il.com>
Subject: Re: [PATCH] proc: fix mount -t proc -o AAA

On Sun, 25 Mar 2012 18:23:16 -0400
Valdis.Kletnieks@...edu wrote:

> On Sun, 25 Mar 2012 19:36:12 +0400, Vasiliy Kulikov said:
> > On Sun, Mar 25, 2012 at 09:24 +0200, Arkadiusz MiE.kiewicz wrote:
> > > but there is another problem - unmounting it and mounting without options
> > > causes old option to persist:
> > >
> > > # mount none /proc -t proc -o hidepid=2
> > > # umount /proc
> > > # mount none /proc -t proc
> > > # grep "/proc" /proc/mounts
> > > none /proc proc rw,relatime,hidepid=2 0 0
> > >
> > > There should be no hidepid=2 now.
> >
> > No, that's an expected behaviour.
> 
> "You keep using that word. I do not think it means what you think it means". ;)
> 
> > Procfs is a special filesystem.
> 
> And the fact it's "special" makes this *unexpected* behavior.  Are there
> any other filesystems where -o values will persist across an unmount
> and then take effect *even if no -o is given* on a subsequent mount?
> 
> Yes, it may be what the code actually *does*, but it certainly violates
> the Principle of Least Surprise...

It surprises me ;)  I never noticed that before.

It does seem pretty insane.  I wonder how much downstream damage would
result from fixing it.
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