Re: design of user namespaces

[ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org (Eric W. Biederman)]

"Serge E. Hallyn" <serue-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org> writes:

> Quoting Eric W. Biederman (ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org):
>> 
>> The very important points are that it is a remount of an existing mount
>> so that we don't have to worry about corrupted filesystem attacks, and
>> that authentication is performed at mount time.
>
> Conceptually that (making corrupted fs attacks a non-issue) is
> wonderful.  Practically, I may be missing something:  When you say
> remount, it seems you must either mean a bind mount or a remount.  If
> remount, then that will want to change superblock flags.  If the
> child userns(+child mntns) does a real remount, then that will change
> the flags for the parent ns as well, right?
>
> If instead we do a bind mount we don't have that problem, but then the
> fs can't be the one doing the user namespace work.
>
> I'm probably missing something.

Essentially I am creating a new mount operation that is a
cousin of a remount.

Unlike a real remount you can't change the super flags.
Unlike a bind mount you get the fs involved, and you pass in a string of flags
that the fs can interpret in a standard way.

I expect the flags you pass in would be a subset of what is allowed
in a normal remount. 

Which is why I was calling it nativemount.  Although usernsmount
may be better.

Eric