Re: [PATCH] [RFCv3] add manpages for Memory Protection Keys

["Michael Kerrisk (man-pages)" <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On 06/03/2016 09:53 AM, Dave Hansen wrote:
> On 06/02/2016 05:25 PM, Michael Kerrisk (man-pages) wrote:
>> The convention for man-pages is that new sentences always start 
>> of new source lines. (This makes subsequent patches less "noisy", 
>> since the common unit of change in a text is a sentence.)
>> Could you fix this throughout please?
> 
> Yep, I can do that, and I'll also integrate all of your comments,
> although I won't respond to all of them individually, I will integrate them.
> 
>>> +no longer be used in any protection-key-related operations.
>>> +.PP
>>> +.RB ( pkey_alloc ())
>>> +.I flags
>>> +may contain zero or more disable operations:
>>
>> Why "zero or more" rather than "zero or one"? I mean:
>> what sense could it make to OR together PKEY_DISABLE_ACCESS and
>> PKEY_DISABLE_WRITE?
> 
> This is one of the attributes of the hardware that I carried up in to
> the interfaces.  The hardware contains two bits: one to write-disable
> and one to access-disable.  You're allowed to set both at the same time,
> even though the "access" bit overrules the "write" bit when set.
> 
> So, it doesn't make a ton of logical sense with these two flags, but it
> might if we ever got an "execute disable" feature or some other feature
> that could be combined more arbitrarily.

So, I have a suggestion. How about tightening the constraint here, so
that only one of these flags is allowed for now. (EINVAL if both
are specified.) That constraint could always be relaxed later , if
desired, and adding it now may allow some wriggle room later in terms
of modifying the API or allowing for different architectural choices.

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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