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

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Message-ID: <878spudgro.fsf@x220.int.ebiederm.org>
Date:   Tue, 08 Oct 2019 17:16:59 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     "Michael Kerrisk \(man-pages\)" <mtk.manpages@...il.com>
Cc:     Christian Brauner <christian.brauner@...ntu.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

"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com> writes:

> On 10/8/19 9:40 PM, Eric W. Biederman wrote:
>> "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com> writes:
>> 
>>> Hello Eric,
>>>
>>>>>> Creating of a mount namespace in a user namespace automatically does
>>>>>> 'mount("", "/", MS_SLAVE | MS_REC, NULL);' if the starting mount
>>>>>> namespace was not created in that user namespace.  AKA creating
>>>>>> a mount namespace in a user namespace does the unshare for you.
>>>>>
>>>>> Oh -- I had forgotten that detail. But it is documented
>>>>> (by you, I think) in mount_namespaces(7):
>>>>>
>>>>>        *  A  mount  namespace  has  an  owner user namespace.  A
>>>>>           mount namespace whose owner user namespace is  differ‐
>>>>>           ent  from the owner user namespace of its parent mount
>>>>>           namespace is considered a less privileged mount names‐
>>>>>           pace.
>>>>>
>>>>>        *  When  creating  a  less  privileged  mount  namespace,
>>>>>           shared mounts are reduced to  slave  mounts.   (Shared
>>>>>           and  slave  mounts are discussed below.)  This ensures
>>>>>           that  mappings  performed  in  less  privileged  mount
>>>>>           namespaces will not propagate to more privileged mount
>>>>>           namespaces.
>>>>>
>>>>> There's one point that description that troubles me. There is a
>>>>> reference to "parent mount namespace", but as I understand things
>>>>> there is no parental relationship among mount namespaces instances
>>>>> (or am I wrong?). Should that wording not be rather something
>>>>> like "the mount namespace of the process that created this mount
>>>>> namespace"?
>>>>
>>>> How about "the mount namespace this mount namespace started as a copy of"
>>>>
>>>> You are absolutely correct there is no relationship between mount
>>>> namespaces.  There is just the propagation tree between mounts.  (Which
>>>> acts similarly to a parent/child relationship but is not at all the same
>>>> thing).
>>>
>>> Thanks. I made the text as follows:
>>>
>>>        *  Each  mount  namespace  has  an owner user namespace.  As noted
>>>           above, when a new mount namespace is  created,  it  inherits  a
>>>           copy  of  the  mount  points  from  the  mount namespace of the
>>>           process that created the new mount namespace.  If the two mount
>>>           namespaces are owned by different user namespaces, then the new
>>>           mount namespace is considered less privileged.
>> 
>> I hate to nitpick, 
>
> I love it when you nitpick. Thanks for your attention to the details 
> of my wording.
>
>> but I am going to say that when I read the text above
>> the phrase "mount namespace of the process that created the new mount
>> namespace" feels wrong.
>> 
>> Either you use unshare(2) and the mount namespace of the process that
>> created the mount namespace changes.
>> 
>> Or you use clone(2) and you could argue it is the new child that created
>> the mount namespace.
>> 
>> Having a different mount namespace at the end of the creation operation
>> feels like it makes your phrase confusing about what the starting
>> mount namespace is.  I hate to use references that are ambiguous when
>> things are changing.
>>
>> I agree that the term parent is also wrong.
>
> I see what you mean. My wording is imprecise.
>
> So, I tweaked text earlier in the page so that it now reads
> as follows:
>
>        A  new  mount  namespace  is  created  using  either  clone(2)  or
>        unshare(2) with the CLONE_NEWNS flag.  When a new mount  namespace
>        is created, its mount point list is initialized as follows:
>
>        *  If  the  namespace  is  created using clone(2), the mount point
>           list of the child's namespace is a copy of the mount point list
>           in the parent's namespace.
>
>        *  If  the  namespace is created using unshare(2), the mount point
>           list of the new namespace is a copy of the mount point list  in
>           the caller's previous mount namespace.
>
> And then I tweaked the text that we are currently discussing to read:
>
>        *  Each mount namespace has an owner user namespace.  As explained
>           above,  when  a new mount namespace is created, its mount point
>           list is initialized as a  copy  of  the  mount  point  list  of
>           another  mount namespace.  If the new namespaces and the names‐
>           pace from which the mount point list was copied  are  owned  by
>           different user namespaces, then the new mount namespace is con‐
>           sidered less privileged.
>
> How does this look to you now?

Much better thank you.

Eric