From mboxrd@z Thu Jan  1 00:00:00 1970
From: Catalin Marinas <catalin.marinas@arm.com>
Subject: Re: [PATCH v17 03/15] arm64: Introduce prctl() options to control
 the tagged user addresses ABI
Date: Thu, 13 Jun 2019 16:57:55 +0100
Message-ID: <20190613155754.GX28951@C02TF0J2HF1T.local>
References: <cover.1560339705.git.andreyknvl@google.com>
 <a7a2933bea5fe57e504891b7eec7e9432e5e1c1a.1560339705.git.andreyknvl@google.com>
 <20190613111659.GX28398@e103592.cambridge.arm.com>
 <20190613153505.GU28951@C02TF0J2HF1T.local>
 <99cc257d-5e99-922a-fbe7-3bbaf3621e38@arm.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Return-path: <linux-kernel-owner@vger.kernel.org>
Content-Disposition: inline
In-Reply-To: <99cc257d-5e99-922a-fbe7-3bbaf3621e38@arm.com>
Sender: linux-kernel-owner@vger.kernel.org
To: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Dave Martin <Dave.Martin@arm.com>, Andrey Konovalov <andreyknvl@google.com>, linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, amd-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, linux-rdma@vger.kernel.org, linux-media@vger.kernel.org, kvm@vger.kernel.org, linux-kselftest@vger.kernel.org, Mark Rutland <mark.rutland@arm.com>, Szabolcs Nagy <Szabolcs.Nagy@arm.com>, Will Deacon <will.deacon@arm.com>, Kostya Serebryany <kcc@google.com>, Khalid Aziz <khalid.aziz@oracle.com>, Felix Kuehling <Felix.Kuehling@amd.com>, Jacob Bramley <Jacob.Bramley@arm.com>, Leon Romanovsky <leon@kernel.org>, Christoph Hellwig <hch@infradead.org>, Jason Gunthorpe <jgg@ziepe.ca>, Evgeniy Stepanov <eugenis@google.com>, Ke
List-Id: linux-rdma@vger.kernel.org

On Thu, Jun 13, 2019 at 04:45:54PM +0100, Vincenzo Frascino wrote:
> On 13/06/2019 16:35, Catalin Marinas wrote:
> > On Thu, Jun 13, 2019 at 12:16:59PM +0100, Dave P Martin wrote:
> >> On Wed, Jun 12, 2019 at 01:43:20PM +0200, Andrey Konovalov wrote:
> >>> +
> >>> +/*
> >>> + * Control the relaxed ABI allowing tagged user addresses into the kernel.
> >>> + */
> >>> +static unsigned int tagged_addr_prctl_allowed = 1;
> >>> +
> >>> +long set_tagged_addr_ctrl(unsigned long arg)
> >>> +{
> >>> +	if (!tagged_addr_prctl_allowed)
> >>> +		return -EINVAL;
> >>
> >> So, tagging can actually be locked on by having a process enable it and
> >> then some possibly unrelated process clearing tagged_addr_prctl_allowed.
> >> That feels a bit weird.
> > 
> > The problem is that if you disable the ABI globally, lots of
> > applications would crash. This sysctl is meant as a way to disable the
> > opt-in to the TBI ABI. Another option would be a kernel command line
> > option (I'm not keen on a Kconfig option).
> 
> Why you are not keen on a Kconfig option?

Because I don't want to rebuild the kernel/reboot just to be able to
test how user space handles the ABI opt-in. I'm ok with a Kconfig option
to disable this globally in addition to a run-time option (if actually
needed, I'm not sure).

-- 
Catalin