[PATCH RFC] User namespaces: general cleanups

[Serge E. Hallyn]

Quoting David Howells (dhowells at redhat.com):
> Serge E. Hallyn <serue at us.ibm.com> wrote:
> 
> > +		new->uid = new->euid = new->suid = new->fsuid = 0;
> > +		new->gid = new->egid = new->sgid = new->fsgid = 0;
> 
> Should the supplementary groups be zapped too?  Do the GIDs therein still have
> meaning in the new user namespace?
> 
> Note also that eCryptFS is broken by your patch. 
> 
> I suggest adding the attached incremental patch.  It makes the following
> changes:
> 
>  (1) Provides a current_user_ns() macro to wrap accesses to current's user
>      namespace.
> 
>  (2) Fixes eCryptFS.
> 
>  (3) Renames create_new_userns() to create_user_ns() to be more consistent
>      with the other associated functions and because the 'new' in the name is
>      superfluous.
> 
>  (4) Moves the argument and permission checks made for CLONE_NEWUSER to the
>      beginning of do_fork() so that they're done prior to making any attempts
>      at allocation.
> 
>  (5) Calls create_user_ns() after prepare_creds(), and gives it the new creds
>      to fill in rather than have it return the new root user.  I don't imagine
>      the new root user being used for anything other than filling in a cred
>      struct.
> 
>      This also permits me to get rid of a get_uid() and a free_uid(), as the
>      reference the creds were holding on the old user_struct can just be
>      transferred to the new namespace's creator pointer.
> 
>  (6) Makes create_user_ns() reset the UIDs and GIDs of the creds under
>      preparation rather than doing it in copy_creds().

Hmm, with this patch, with CONFIG_KEYS=y users in child user_namespaces
never get freed.  Ones in the init_user_ns do, and with CONFIG_KEYS=n,
those in child user_namespaces do as well.

I don't see anything obvious in copy_creds() that would cause this...

(also, when CONFIG_KEYS=n, then the enomem label in copy_creds() is
unused - it might be kind of ugly to put just those two lines under
#ifdef, though)

-serge