Re: [PATCH v3] powerpc, pkey: make protection key 0 less special

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

On Fri, May 04, 2018 at 12:59:27PM -0700, Dave Hansen wrote:
> On 05/04/2018 12:22 PM, Ram Pai wrote:
> > @@ -407,9 +414,6 @@ static bool pkey_access_permitted(int pkey, bool write, bool execute)
> >  	int pkey_shift;
> >  	u64 amr;
> >  
> > -	if (!pkey)
> > -		return true;
> > -
> >  	if (!is_pkey_enabled(pkey))
> >  		return true;
> 
> Looks fine to me.  Obviously doesn't have any impact on x86 or the
> generic code.
> 
> One question, though.  Which other check makes up for this removed !pkey
> check?

is_pkey_enabled() does take care of it.  we do not enable userspace to
change permissions on pkey-0. This information is tracked in
UAMOR register.  is_pkey_enabled() refers to UAMOR to determine
if the given key is modifiable by userspace. since UAMOR has the bit
corresponding to key-0 set to 0, is_pkey_enabled(key-0) will return
false. 

The deleted code above, would have done the same job without
referring UAMOR. However having special checks on pkey-0 makes
pkey-0 special. It defeats the purpose of this patch; which is to make
pkey-0 less special :).


-- 
Ram Pai