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Subject: Re: pivot_root(".", ".") and the fchdir() dance
To:     "Eric W. Biederman" <ebiederm@xmission.com>
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From:   "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Message-ID: <448138b8-0d0c-5eb3-d5e5-04a26912d3a8@gmail.com>
Date:   Sun, 15 Sep 2019 10:12:09 +0200
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Hello Eric,

On 9/11/19 1:06 AM, Eric W. Biederman wrote:
> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
> 
>> 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.
> 
> Ugh.  That is close but not quite correct.
> 
> A better explanation:
> 
>     The pivot_root system call will never propagate any changes it makes.
>     The pivot_root system call ensures this is safe by verifying that
>     none of put_old, the parent of new_root, and parent of the root directory
>     have a propagation type of MS_SHARED.

Thanks for that. However, another question. You text has two changes.
First, I understand why you reword the discussion to indicate the
_purpose_ of the rules. However, you also, AFAICS, list a different set of
of directories that can't be MS_SHARED:

I said: new_root, the parent of new_root, and put_old
You said: the parent of new_root, and put_old, and parent of the
root directory.

Was I wrong on this detail also?

> The concern from our conversation at the container mini-summit was that
> there is a pathology if in your initial mount namespace all of the
> mounts are marked MS_SHARED like systemd does (and is almost necessary
> if you are going to use mount propagation), that if new_root itself
> is MS_SHARED then unmounting the old_root could propagate.
> 
> So I believe the desired sequence is:
> 
>>>>            chdir(new_root);
> +++            mount("", ".", MS_SLAVE | MS_REC, NULL);
>>>>            pivot_root(".", ".");
>>>>            umount2(".", MNT_DETACH);
> 
> The change to new new_root could be either MS_SLAVE or MS_PRIVATE.  So
> long as it is not MS_SHARED the mount won't propagate back to the
> parent mount namespace.

Thanks. I made that change.

Cheers,

Michael
 

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