[PATCH 1/2] fs: Extend mount_ns with support for a fast namespace to vfsmount function

[Al Viro]

On Sat, Mar 24, 2018 at 11:12:02AM -0500, Eric W. Biederman wrote:

> > This is completely wrong.  Look:
> > 	* SB_KERNMOUNT and !SB_KERNMOUNT cases are almost entirely isolated;
> > completely so once that ns_to_mnt becomes unconditionally non-NULL.  
> > 	* in !SB_KERNMOUNT passing ns_to_mnt() is pointless - you might as
> > well pass existing vfsmount (or ERR_PTR()) and use _that_.  fill_super()
> > is not used at all in that case.
> > 	* is SB_KERNMOUNT ns_to_mnt serves only as a flag, eventually
> > constant true.
> >
> > So let's split it in two helpers and give them sane arguments.
> 
> Everything I look at with multiple helpers feels even worse to me.
> The above has the advantage it is the minimal change to fix the
> regression.  So I am not worried about code correctness.

> I keep wondering is the intention long term to fix sget so it has an
> efficient data structure for finding super blocks (like an rbtree) or if
> the intention is to deprecate sget entirely and just have everything
> call alloc_super, and be responsible for their own data structures for
> finding existing superblocks.
>
> At this point since we are not in agreement on a proper fix I am going
> to plan on just queueing up a revert.   So that we don't ship 4.16 with
> a regression in a permission check.

Permission check is trivial to put back in; I'll do that.

FWIW, I don't believe that sget_userns() is a good place for any kind of
universal permission checks.  It's a library helper, not a place everything
must come through when mounting something.  So's mount_ns(), etc.

BTW, will you be at LSF?  I would suggest discussing the architectural
issues there - they are directly related to fsmount() proposals...