linux-kernel - Re: [PATCH 07/12] mm: Pass the 4-bit protection key in via PROT

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Date:	Fri, 04 Sep 2015 22:13:46 +0200
From:	Florian Weimer <fw@...eb.enyo.de>
To:	Dave Hansen <dave@...1.net>
Cc:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH 07/12] mm: Pass the 4-bit protection key in via PROT_ bits to syscalls

* Dave Hansen:

> On 05/07/2015 12:11 PM, One Thousand Gnomes wrote:
>>> diff -puN
>>> include/uapi/asm-generic/mman-common.h~pkeys-11-user-abi-bits
>>> include/uapi/asm-generic/mman-common.h
>>> --- a/include/uapi/asm-generic/mman-common.h~pkeys-11-user-abi-bits
>>> 2015-05-07 10:31:43.367276719 -0700
>>> +++ b/include/uapi/asm-generic/mman-common.h 2015-05-07
>>> 10:31:43.370276855 -0700
>>> @@ -10,6 +10,10 @@
>>>  #define PROT_WRITE	0x2		/* page can be written */
>>>  #define PROT_EXEC	0x4		/* page can be executed */
>>>  #define PROT_SEM	0x8		/* page may be used for atomic ops */
>>> +#define PROT_PKEY0	0x10		/* protection key value (bit 0) */
>>> +#define PROT_PKEY1	0x20		/* protection key value (bit 1) */
>>> +#define PROT_PKEY2	0x40		/* protection key value (bit 2) */
>>> +#define PROT_PKEY3	0x80		/* protection key value (bit 3) */
>> 
>> Thats leaking deep Intelisms into asm-generic which makes me very
>> uncomfortable. Whether we need to reserve some bits for "arch specific"
>> is one question, what we do with them ought not to be leaking out.
>> 
>> To start with trying to port code people will want to do
>> 
>> #define PROT_PKEY0	0
>> #define PROT_PKEY1	0
>
> Yeah, I feel pretty uncomfortable with it as well.  I really don't
> expect these to live like this in asm-generic when I submit this.
>
> Powerpc and ia64 have _something_ resembling protection keys, so the
> concept isn't entirely x86 or Intel-specific.  My hope would be that we
> do this in a way that other architectures can use.

It will also be very painful to add additional bits.  We went through
this with the CPU affinity mask, and it still hurts it.  Please use a
more sensible interface from the start. :)
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