linux-kernel - Re: [PATCH] x86, powerpc : pkey-mprotect must allow pkey-0

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Message-Id: <20180310055544.GU1060@ram.oc3035372033.ibm.com>
Date:   Fri, 9 Mar 2018 21:55:44 -0800
From:   Ram Pai <linuxram@...ibm.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     mpe@...erman.id.au, mingo@...hat.com, akpm@...ux-foundation.org,
        linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org, x86@...nel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        benh@...nel.crashing.org, paulus@...ba.org,
        khandual@...ux.vnet.ibm.com, aneesh.kumar@...ux.vnet.ibm.com,
        bsingharora@...il.com, hbabu@...ibm.com, mhocko@...nel.org,
        bauerman@...ux.vnet.ibm.com, ebiederm@...ssion.com, corbet@....net,
        arnd@...db.de, fweimer@...hat.com, msuchanek@...e.com,
        Ulrich.Weigand@...ibm.com
Subject: Re: [PATCH] x86, powerpc : pkey-mprotect must allow pkey-0

On Fri, Mar 09, 2018 at 02:40:32PM -0800, Dave Hansen wrote:
> On 03/09/2018 12:12 AM, Ram Pai wrote:
> > Once an address range is associated with an allocated pkey, it cannot be
> > reverted back to key-0. There is no valid reason for the above behavior.  On
> > the contrary applications need the ability to do so.
> 
> Why don't we just set pkey 0 to be allocated in the allocation bitmap by
> default?

ok. that will make it allocatable. But it will not be associatable,
given the bug in the current code. And what will be the
default key associated with a pte? zero? or something else?

> 
> We *could* also just not let it be special and let it be freed.  An app
> could theoretically be careful and make sure nothing is using it.

unable to see how this solves the problem. Need some more explaination.


RP