From mboxrd@z Thu Jan  1 00:00:00 1970
From: ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org (Eric W. Biederman)
Subject: Re: [PATCH 3/4] devpts: Make the newinstance option historical
Date: Sun, 23 Sep 2012 00:00:19 -0700
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To: Al Viro <viro-3bDd1+5oDREiFSDQTTA3OLVCufUGDwFn@public.gmane.org>
Cc: Greg Kroah-Hartman <gregkh-hQyY1W1yCW8ekmWlsbkhG0B+6BGkLq7r@public.gmane.org>, containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, Kay Sievers <kay.sievers-tD+1rO4QERM@public.gmane.org>, Dave Hansen <haveblue-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Andy Whitcroft <apw-Z7WLFzj8eWMS+FvcfC7Uqw@public.gmane.org>, sukadev-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org, Linus Torvalds <torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Alan Cox <alan-qBU/x9rampVanCEyBjwyrvXRex20P6io@public.gmane.org>
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Al Viro <viro-3bDd1+5oDREiFSDQTTA3OLVCufUGDwFn@public.gmane.org> 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