linux-kernel - Re: [PATCH 3/4] devpts: Make the newinstance option historical

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Date:	Sun, 23 Sep 2012 00:00:19 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Kay Sievers <kay.sievers@...y.org>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	containers@...ts.linux-foundation.org,
	Dave Hansen <haveblue@...ibm.com>,
	linux-kernel@...r.kernel.org, Andy Whitcroft <apw@...onical.com>,
	sukadev@...ux.vnet.ibm.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Serge Hallyn <serge.hallyn@...onical.com>
Subject: Re: [PATCH 3/4] devpts: Make the newinstance option historical

Al Viro <viro@...IV.linux.org.uk> writes:

> On Sun, Sep 23, 2012 at 07:30:38AM +0100, Al Viro wrote:
>> On Sat, Sep 22, 2012 at 10:59:04PM -0700, Eric W. Biederman wrote:
>> 
>> > The test:
>> > >> +	if (filp->f_vfsmnt->mnt_root == filp->f_dentry)
>> > kicks in and no redirection is performed.
>> 
>> Umm...  OK, after the first round of recursion.  Unless you bind /something/pts
>> on /something.
>
> Other way round, of course...  E.g.
> mount --bind /dev /dev/pts
> mount --bind /dev /dev/pts/pts
> etc. enough times to overflow the kernel stack on recursion.

Except of course I gave the wrong explanation.

I did the open with O_PATH.  Since O_PATH never calls open recursion
is impossible.

Eric
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