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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: <71cad40b-0f9f-24de-b650-8bc4fce78fa8@gmail.com>
Date:   Sat, 28 Sep 2019 17:05:29 +0200
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Hello Eric,

A ping on my question below. Could you take a look please?

Thanks,

Michael

>>>> 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.
>>
>> For what it is worth.  The sequence above without the change in mount
>> attributes will fail if it is necessary to change the mount attributes
>> as "." is both put_old as well as new_root.
>>
>> When I initially suggested the change I saw "." was new_root and forgot
>> "." was also put_old.  So I thought there was a silent danger without
>> that sequence.
> 
> So, now I am a little confused by the comments you added here. Do you
> now mean that the 
> 
> mount("", ".", MS_SLAVE | MS_REC, NULL);
> 
> call is not actually necessary?
> 
> Thanks,
> 
> Michael
> 


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