linux-kernel - Re: pivot_root(".", ".") and the fchdir() dance

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Date:   Tue, 10 Sep 2019 13:21:16 +0200
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     mtk.manpages@...il.com,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-man <linux-man@...r.kernel.org>,
        Containers <containers@...ts.linux-foundation.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jordan Ogas <jogas@...l.gov>, werner@...esberger.net,
        Al Viro <viro@....linux.org.uk>
Subject: Re: pivot_root(".", ".") and the fchdir() dance

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/