linux-kernel - Re: [PATCH 2.6.24-rc3] Fix /proc/net breakage

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Date:	Fri, 07 Dec 2007 14:11:05 +0300
From:	"Denis V. Lunev" <den@...ru>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	David Woodhouse <dwmw2@...radead.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Pavel Machek <pavel@....cz>,
	kernel list <linux-kernel@...r.kernel.org>,
	netdev <netdev@...r.kernel.org>,
	Pavel Emelyanov <xemul@...nvz.org>,
	"Denis V. Lunev" <den@...nvz.org>
Subject: Re: [PATCH 2.6.24-rc3] Fix /proc/net breakage

Andrew Morton wrote:
> On Fri, 07 Dec 2007 04:51:37 +0000 David Woodhouse <dwmw2@...radead.org> wrote:
> 
>> On Mon, 2007-11-26 at 15:17 -0700, Eric W. Biederman wrote:
>>> Well I clearly goofed when I added the initial network namespace support
>>> for /proc/net.  Currently things work but there are odd details visible
>>> to user space, even when we have a single network namespace.
>>>
>>> Since we do not cache proc_dir_entry dentries at the moment we can
>>> just modify ->lookup to return a different directory inode depending
>>> on the network namespace of the process looking at /proc/net, replacing
>>> the current technique of using a magic and fragile follow_link method.
>>>
>>> To accomplish that this patch:
>>> - introduces a shadow_proc method to allow different dentries to
>>>   be returned from proc_lookup.
>>> - Removes the old /proc/net follow_link magic
>>> - Fixes a weakness in our not caching of proc generic dentries.
>>>
>>> As shadow_proc uses a task struct to decided which dentry to return we
>>> can go back later and fix the proc generic caching without modifying any code that
>>> uses the shadow_proc method.
>>>
>>> Signed-off-by: Eric W. Biederman <ebiederm@...ssion.com>
>>> ---
>>>  fs/proc/generic.c       |   12 ++++++-
>>>  fs/proc/proc_net.c      |   86 +++--------------------------------------------
>>>  include/linux/proc_fs.h |    3 ++
>>>  3 files changed, 19 insertions(+), 82 deletions(-)
>> (commit 2b1e300a9dfc3196ccddf6f1d74b91b7af55e416)
>>
>> This seems to have broken the use of /proc/bus/usb as a mountpoint. It
>> always appears empty now, whatever's supposed to be mounted there.
>>
> 
> Yes.  Denis and Eric are tossing around competing patches but afaik nobody
> is happy with any of them.  Guys, could we get this sorted soonish please?
> 

Andrew, I become too relaxed after receiving
"Tested-by: Giacomo Catenazzi <cate@...ian.org>"

Eric, I believe that reverting an original behavior is better than your
new one as
- you introduce search into the depth by calling have_submounts(dentry)
during revalidation for all(!) /proc dentries
- your shadowing behavior will be broken if you'll mount something in
the depth of shadowed tree (this can be done as a DoS attempt)

As a last minute call, may be it will be better to pin network namespace
like a pid namespace during mount to avoid this crap at all?

Regards,
	Den
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