Re: pivot_root(".", ".") and the fchdir() dance
From: Michael Kerrisk (man-pages)
Date: Tue Oct 08 2019 - 17:40:34 EST
On 10/8/19 9:40 PM, Eric W. Biederman wrote:
> "Michael Kerrisk (man-pages)" <mtk.manpages@xxxxxxxxx> 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 differâ
>>>> ent from the owner user namespace of its parent mount
>>>> namespace is considered a less privileged mount namesâ
>>>> * 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
>>>> 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
>>> 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
>> 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
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 namesâ
pace from which the mount point list was copied are owned by
different user namespaces, then the new mount namespace is conâ
sidered less privileged.
How does this look to you now?
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