linux-kernel - Re: [PATCH 1/3] x86, pkeys: do not special case protection key 0

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Date:   Sun, 18 Mar 2018 10:30:48 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ram Pai <linuxram@...ibm.com>
cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        dave.hansen@...el.com, mpe@...erman.id.au, mingo@...nel.org,
        akpm@...ux-foundation.org, shuah@...nel.org
Subject: Re: [PATCH 1/3] x86, pkeys: do not special case protection key 0

On Sat, 17 Mar 2018, Ram Pai wrote:
> On Fri, Mar 16, 2018 at 02:46:56PM -0700, Dave Hansen wrote:
> > 
> > From: Dave Hansen <dave.hansen@...ux.intel.com>
> > 
> > mm_pkey_is_allocated() treats pkey 0 as unallocated.  That is
> > inconsistent with the manpages, and also inconsistent with
> > mm->context.pkey_allocation_map.  Stop special casing it and only
> > disallow values that are actually bad (< 0).
> > 
> > The end-user visible effect of this is that you can now use
> > mprotect_pkey() to set pkey=0.
> > 
> > This is a bit nicer than what Ram proposed because it is simpler
> > and removes special-casing for pkey 0.  On the other hand, it does
> > allow applciations to pkey_free() pkey-0, but that's just a silly
> > thing to do, so we are not going to protect against it.
> 
> So your proposal 
> (a) allocates pkey 0 implicitly, 
> (b) does not stop anyone from freeing pkey-0
> (c) and allows pkey-0 to be explicitly associated with any address range.
> correct?
> 
> My proposal
> (a) allocates pkey 0 implicitly, 
> (b) stops anyone from freeing pkey-0
> (c) and allows pkey-0 to be explicitly associated with any address range.
> 
> So the difference between the two proposals is just the freeing part i.e (b).
> Did I get this right?

Yes, and that's consistent with the other pkeys.

> Its a philosophical debate; allow the user to shoot-in-the-feet or stop
> from not doing so. There is no clear answer either way. I am fine either
> way.

The user can shoot himself already with the other pkeys, so adding another
one does not matter and is again consistent.

Thanks,

	tglx