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Date: Sun, 23 Sep 2012 07:34:45 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Kay Sievers <kay.sievers@vrfy.org>,
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        Linus Torvalds <torvalds@linux-foundation.org>,
        Alan Cox <alan@lxorguk.ukuu.org.uk>,
        Serge Hallyn <serge.hallyn@canonical.com>
Subject: Re: [PATCH 3/4] devpts: Make the newinstance option historical
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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.