I am in the process of catching up with the last two years or so.
Right now, I am at the changes involving user namespaces.

I have two possible implementations, both working equally well in a
shared environment. Since I am not familiar with namespaces in general
and user namespaces in particular, I would like you to look over the
patches and tell me, what you think.

Are the patches good so far? Are there are any things I missed and must
consider? Maybe, I am completely off track? Anything else?

I included both patches inline below. The patches are also available as
separate branches at github 

https://github.com/olafdietsche/linux-accessfs/tree/tmp-user-ns-1
https://github.com/olafdietsche/linux-accessfs/tree/tmp-user-ns-2

I am leaning toward the second patch. Although it is a little bit longer
than the first one, it involves no user id conversions.