linux-kernel - Re: [PATCH 2/9] mm: implement new pkey

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Date:	Sat, 11 Jun 2016 11:47:44 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Dave Hansen <dave@...1.net>
cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-mm@...ck.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, dave.hansen@...ux.intel.com
Subject: Re: [PATCH 2/9] mm: implement new pkey_mprotect() system call

On Wed, 8 Jun 2016, Dave Hansen wrote:
> Proposed semantics:
> 1. protection key 0 is special and represents the default,
>    unassigned protection key.  It is always allocated.
> 2. mprotect() never affects a mapping's pkey_mprotect()-assigned
>    protection key. A protection key of 0 (even if set explicitly)
>    represents an unassigned protection key.
>    2a. mprotect(PROT_EXEC) on a mapping with an assigned protection
>        key may or may not result in a mapping with execute-only
>        properties.  pkey_mprotect() plus pkey_set() on all threads
>        should be used to _guarantee_ execute-only semantics.
> 3. mprotect(PROT_EXEC) may result in an "execute-only" mapping. The
>    kernel will internally attempt to allocate and dedicate a
>    protection key for the purpose of execute-only mappings.  This
>    may not be possible in cases where there are no free protection
>    keys available.

Shouldn't we just reserve a protection key for PROT_EXEC unconditionally?

Thanks,

	tglx