Re: pkeys: Support setting access rights for signal handlers

From: Dave Hansen <dave.hansen@intel.com>
To: Florian Weimer <fweimer@redhat.com>, Ram Pai <linuxram@us.ibm.com>
Cc: linux-mm <linux-mm@kvack.org>,
	x86@kernel.org, linux-arch <linux-arch@vger.kernel.org>,
	linux-x86_64@vger.kernel.org,
	Linux API <linux-api@vger.kernel.org>
Subject: Re: pkeys: Support setting access rights for signal handlers
Date: Wed, 13 Dec 2017 07:22:01 -0800	[thread overview]
Message-ID: <c220f36f-c04a-50ae-3fd7-2c6245e27057@intel.com> (raw)
In-Reply-To: <9f86d79e-165a-1b8e-32dd-7e4e8579da59@redhat.com>

On 12/13/2017 07:08 AM, Florian Weimer wrote:
> Okay, this model is really quite different from x86.  Is there a
> good reason for the difference?

Yes, both implementations are simple and take the "natural" behavior.
x86 changes XSAVE-controlled register values on entering a signal, so we
let them be changed (including PKRU).  POWER hardware does not do this
to its PKRU-equivalent, so we do not force it to.

x86 didn't have to do this for *signals*.  But, we kinda went on this
trajectory when we decided to clear/restore FPU state on
entering/exiting signals before XSAVE even existed.

FWIW, I do *not* think we have to do this for future XSAVE states.  But,
if we do that, we probably need an interface for apps to tell us which
states to save/restore and which state to set upon entering a signal
handler.  That's what I was trying to get you to consider instead of
just a one-off hack to fix this for pkeys.