Re: [PATCH v4] mm, pkey: treat pkey-0 special

[Ram Pai <linuxram@us.ibm.com>]

On Fri, Mar 16, 2018 at 10:02:22PM +1100, Balbir Singh wrote:
> On Fri, Mar 16, 2018 at 9:33 PM, Ram Pai <linuxram@us.ibm.com> wrote:
> > Applications need the ability to associate an address-range with some
> > key and latter revert to its initial default key. Pkey-0 comes close to
> > providing this function but falls short, because the current
> > implementation disallows applications to explicitly associate pkey-0 to
> > the address range.
> >
> > Clarify the semantics of pkey-0 and provide the corresponding
> > implementation.
> >
> > Pkey-0 is special with the following semantics.
> > (a) it is implicitly allocated and can never be freed. It always exists.
> > (b) it is the default key assigned to any address-range.
> > (c) it can be explicitly associated with any address-range.
> >
> > Tested on powerpc only. Could not test on x86.
> 
> 
> Ram,
> 
> I was wondering if we should check the AMOR values on the ppc side to make sure
> that pkey0 is indeed available for use as default. I am still of the
> opinion that we

AMOR cannot be read/written by the OS in priviledge-non-hypervisor-mode.
We could try testing if key-0 is available to the OS by temproarily
changing the bits key-0 bits of AMR or IAMR register. But will be
dangeorous to do, for you might disable read,execute of all the pages,
since all pages are asscoiated with key-0 bydefault.

May be we can play with UAMOR register and check if its key-0 can be
modified. That is a good indication that key-0 is available.
If it is not available, disable the pkey-subsystem, and operate
the legacy way; no pkeys.


> should consider non-0 default pkey in the long run. I'm OK with the patches for
> now, but really 0 is not special except for it being the default bit
> values present
> in the PTE.

it will be a pain. Any new pte that gets instantiated will now have to
explicitly initialize its key to this default-non-zero-key.  I hope
we or any architecture goes there ever.

-- 
Ram Pai