Re: [PATCH v2 01/17] mm/gup: Fixup p*_access_permitted()

[Al Viro]

On Fri, Dec 15, 2017 at 04:20:31PM -0800, Linus Torvalds wrote:
> On Thu, Dec 14, 2017 at 11:51 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > So we actually need the pte_access_permitted() stuff if we want to
> > ensure we're not stepping on !PAGE_USER things.
> 
> We really don't. Not in that complex and broken format, and not for every level.
> 
> Also, while I think we *should* check the PAGE_USER bit when walking
> the page tables, like we used to, we should
> 
>  (a) do it much more simply, not with that broken interface that takes
> insane and pointless flags
> 
>  (b) not tie it together with this issue at all, since the PAGE_USER
> thing really is largely immaterial.
> 
> The fact is, if we have non-user mappings in the user part of the
> address space, we _need_ to teach access_ok() about them, because
> fundamentally any "get_user()/put_user()" will happily ignore the lack
> of PAGE_USER (since those happen from kernel space).

Details, please - how *can* access_ok() be taught of that?