[RFC v2 09/13] mm: Restrict memory encryption to anonymous VMA's

[Peter Zijlstra]

On Tue, Dec 04, 2018 at 09:30:20PM -0800, Alison Schofield wrote:
> On Tue, Dec 04, 2018 at 10:10:44AM +0100, Peter Zijlstra wrote:
> > > + * Encrypted mprotect is only supported on anonymous mappings.
> > > + * All VMA's in the requested range must be anonymous. If this
> > > + * test fails on any single VMA, the entire mprotect request fails.
> > > + */
> > > +bool mem_supports_encryption(struct vm_area_struct *vma, unsigned long end)
> > 
> > That's a 'weird' interface and cannot do what the comment says it should
> > do.
> 
> More please? With MKTME, only anonymous memory supports encryption.
> Is it the naming that's weird, or you don't see it doing what it says?

It's weird because you don't fully speficy the range -- ie. it cannot
verify the vma argument. It is also weird because the start and end are
not of the same type -- or rather, there is no start at all.

So while the comment talks about a range, there is not in fact a range
(only the implied @start is somewhere inside @vma). The comment also
states all vmas in the range, but again, because of a lack of range
specification it cannot verify this statement.

Now, I don't necessarily object to the function and its implementation,
but that comment is just plain misleading.