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Subject: Re: pivot_root(".", ".") and the fchdir() dance
To:     Christian Brauner <christian.brauner@ubuntu.com>
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From:   "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Message-ID: <30545c5c-ff4c-8b87-e591-40cc0a631304@gmail.com>
Date:   Tue, 10 Sep 2019 13:21:16 +0200
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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/