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SubjectRe: pivot_root(".", ".") and the fchdir() dance
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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/

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