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 22:34:06 +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 09/04/2015 01:13 PM, Florian Weimer wrote:
> ...
>>>>> >>>  #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. :)
>
> Any suggestions?

It's difficult.  I don't know what kind of programming model you
expect.  Could glibc use these bits for its own implementation?  Or
OpenSSL?  Or is this intended for tightly integrated language
run-times which have a very precise idea what kind of stuff runs
within the same address space?

> Are you thinking that we want a completely separate syscall and
> completely avoid using the PROT_* bits?

Yes, that would seem more future-proof.
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