Re: pivot_root(".", ".") and the fchdir() dance - Michael Kerrisk (man-pages)

From: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
To: Christian Brauner <christian.brauner@ubuntu.com>
Cc: mtk.manpages@gmail.com,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	linux-man <linux-man@vger.kernel.org>,
	Containers <containers@lists.linux-foundation.org>,
	lkml <linux-kernel@vger.kernel.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Jordan Ogas <jogas@lanl.gov>,
	werner@almesberger.net, Al Viro <viro@ftp.linux.org.uk>
Subject: Re: pivot_root(".", ".") and the fchdir() dance
Date: Tue, 10 Sep 2019 13:21:16 +0200	[thread overview]
Message-ID: <30545c5c-ff4c-8b87-e591-40cc0a631304@gmail.com> (raw)
In-Reply-To: <20190910111551.scam5payogqqvlri@wittgenstein>

Hello Christian,

>> All: I plan to add the following text to the manual page:
>>
>>        new_root and put_old may be the same  directory.   In  particular,
>>        the following sequence allows a pivot-root operation without need‐
>>        ing to create and remove a temporary directory:
>>
>>            chdir(new_root);
>>            pivot_root(".", ".");
>>            umount2(".", MNT_DETACH);
> 
> Hm, should we mention that MS_PRIVATE or MS_SLAVE is usually needed
> before the umount2()? Especially for the container case... I think we
> discussed this briefly yesterday in person.
Thanks for noticing. That detail (more precisely: not MS_SHARED) is
already covered in the numerous other changes that I have pending
for this page:

       The following restrictions apply:
       ...
       -  The propagation type of new_root and its parent mount must  not
          be MS_SHARED; similarly, if put_old is an existing mount point,
          its propagation type must not be MS_SHARED.

Thanks,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/