Re: MPK: removing a pkey

[Dave Hansen <dave.hansen-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>]

On 11/22/2017 08:21 AM, Florian Weimer wrote:
> On 11/22/2017 05:10 PM, Dave Hansen wrote:
>> On 11/22/2017 04:15 AM, Florian Weimer wrote:
>>> On 11/22/2017 09:18 AM, Vlastimil Babka wrote:
>>>> And, was the pkey == -1 internal wiring supposed to be exposed to the
>>>> pkey_mprotect() signal, or should there have been a pre-check returning
>>>> EINVAL in SYSCALL_DEFINE4(pkey_mprotect), before calling
>>>> do_mprotect_pkey())? I assume it's too late to change it now anyway (or
>>>> not?), so should we also document it?
>>>
>>> I think the -1 case to the set the default key is useful because it
>>> allows you to use a key value of -1 to mean “MPK is not supported”, and
>>> still call pkey_mprotect.
>>
>> The behavior to not allow 0 to be set was unintentional and is a bug.
>> We should fix that.
> 
> On the other hand, x86-64 has no single default protection key due to
> the PROT_EXEC emulation.

No, the default is clearly 0 and documented to be so.  The PROT_EXEC
emulation one should be inaccessible in all the APIs so does not even
show up as *being* a key in the API.  The fact that it's implemented
with pkeys should be pretty immaterial other than the fact that you
can't touch the high bits in PKRU.