linux-kernel - Re: [RFC v5 12/38] mm: ability to disable execute permission on a key at creation

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Message-ID: <9fbe72be-453f-57e2-861e-5d35fbe95c41@intel.com>
Date:   Tue, 11 Jul 2017 15:19:31 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Ram Pai <linuxram@...ibm.com>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        linux-arch@...r.kernel.org, corbet@....net, arnd@...db.de,
        linux-doc@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, mingo@...hat.com,
        paulus@...ba.org, aneesh.kumar@...ux.vnet.ibm.com,
        linux-kselftest@...r.kernel.org, akpm@...ux-foundation.org,
        linuxppc-dev@...ts.ozlabs.org, khandual@...ux.vnet.ibm.com
Subject: Re: [RFC v5 12/38] mm: ability to disable execute permission on a key
 at creation

On 07/11/2017 03:14 PM, Ram Pai wrote:
> Now how many does the kernel use to reserve for itself is something
> the kernel knows too and hence can expose it, though the information
> may change dynamically as the kernel reserves and releases the key
> based on its internal needs. 
> 
> So i think we can expose this informaton through procfs/sysfs and let
> the application decide how it wants to use the information.

Why bother?  On x86, you'll be told either 14 or 15 depending on whether
you tried to create a mapping in the process without execute permission.
 You can't use all 14 or 15 unless you actually call pkey_alloc() anyway
because the /proc check is inherently racy.

I'm just not sure I see the value in creating a new ABI for it.