From mboxrd@z Thu Jan 1 00:00:00 1970 From: Catalin Marinas Subject: Re: [RFC PATCH v2 6/6] prctl.2: Add tagged address ABI control prctls (arm64) Date: Wed, 10 Jun 2020 18:42:05 +0100 Message-ID: <20200610174205.GL26099@gaia> References: <1590614258-24728-1-git-send-email-Dave.Martin@arm.com> <1590614258-24728-7-git-send-email-Dave.Martin@arm.com> <20200609172232.GA63286@C02TF0J2HF1T.local> <20200610100641.GF25945@arm.com> <20200610152634.GJ26099@gaia> <20200610164209.GH25945@arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20200610164209.GH25945-5wv7dgnIgG8@public.gmane.org> Sender: linux-man-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Dave Martin Cc: Michael Kerrisk , linux-man-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-arch-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org, Will Deacon , Vincenzo Frascino List-Id: linux-arch.vger.kernel.org On Wed, Jun 10, 2020 at 05:42:09PM +0100, Dave P Martin wrote: > On Wed, Jun 10, 2020 at 04:26:34PM +0100, Catalin Marinas wrote: > > On Wed, Jun 10, 2020 at 11:06:42AM +0100, Dave P Martin wrote: > > > On Tue, Jun 09, 2020 at 06:22:32PM +0100, Catalin Marinas wrote: > > > > On Wed, May 27, 2020 at 10:17:38PM +0100, Dave P Martin wrote: > > > > > +.IP > > > > > +The level of support is selected by > > > > > +.IR "(unsigned int) arg2" , > > > > > > > > We use (unsigned long) for arg2. > > > > > > Hmmm, not quite sure how I came up with unsigned int here. I'll just > > > drop this: the type in the prctl() prototype is unsigned long anyway. > > > > > > The type is actually moot in this case, since the valid values all fit > > > in an unsigned int. > > > > Passing an int doesn't require that the top 32-bit of the long are > > zeroed (in case anyone writes the low-level SVC by hand). > > Fair point, I was forgetting that wrinkle. Anyway, the convention in > this page seems to be that if the type is unsigned long, we don't > mention it, because the prctl() prototype says that already. > > Question: the glibc prototype for prctl is variadic, so surely any > calls that don't explicitly cast the args to unsigned long are already > theoretically broken? The #defines (and 0) are all implicitly int. > This probably affects lots of prctls. > > We may get away with it because the compiler is almost certainly going > to favour a mov over a ldr for getting small integers into regs, and mov > fortunately zeroes the top bits for us anyway. So does LDR Wd. Anyway, I think glibc (or my reading of it) has something like like: register long _x1 asm ("x1") = _x1tmp; before invoking the SVC. I assume this would do the right conversion to long. I can't tell about other libraries but I'd say it's their responsibility to convert the args to long before calling the kernel's prctl(). -- Catalin From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from foss.arm.com ([217.140.110.172]:33814 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726254AbgFJRmO (ORCPT ); Wed, 10 Jun 2020 13:42:14 -0400 Date: Wed, 10 Jun 2020 18:42:05 +0100 From: Catalin Marinas Subject: Re: [RFC PATCH v2 6/6] prctl.2: Add tagged address ABI control prctls (arm64) Message-ID: <20200610174205.GL26099@gaia> References: <1590614258-24728-1-git-send-email-Dave.Martin@arm.com> <1590614258-24728-7-git-send-email-Dave.Martin@arm.com> <20200609172232.GA63286@C02TF0J2HF1T.local> <20200610100641.GF25945@arm.com> <20200610152634.GJ26099@gaia> <20200610164209.GH25945@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200610164209.GH25945@arm.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Dave Martin Cc: Michael Kerrisk , linux-man@vger.kernel.org, linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Will Deacon , Vincenzo Frascino Message-ID: <20200610174205.bFgIR8NMtA17oDjE85LNYx8Jx7_f6rsFGmFysMpLllQ@z> On Wed, Jun 10, 2020 at 05:42:09PM +0100, Dave P Martin wrote: > On Wed, Jun 10, 2020 at 04:26:34PM +0100, Catalin Marinas wrote: > > On Wed, Jun 10, 2020 at 11:06:42AM +0100, Dave P Martin wrote: > > > On Tue, Jun 09, 2020 at 06:22:32PM +0100, Catalin Marinas wrote: > > > > On Wed, May 27, 2020 at 10:17:38PM +0100, Dave P Martin wrote: > > > > > +.IP > > > > > +The level of support is selected by > > > > > +.IR "(unsigned int) arg2" , > > > > > > > > We use (unsigned long) for arg2. > > > > > > Hmmm, not quite sure how I came up with unsigned int here. I'll just > > > drop this: the type in the prctl() prototype is unsigned long anyway. > > > > > > The type is actually moot in this case, since the valid values all fit > > > in an unsigned int. > > > > Passing an int doesn't require that the top 32-bit of the long are > > zeroed (in case anyone writes the low-level SVC by hand). > > Fair point, I was forgetting that wrinkle. Anyway, the convention in > this page seems to be that if the type is unsigned long, we don't > mention it, because the prctl() prototype says that already. > > Question: the glibc prototype for prctl is variadic, so surely any > calls that don't explicitly cast the args to unsigned long are already > theoretically broken? The #defines (and 0) are all implicitly int. > This probably affects lots of prctls. > > We may get away with it because the compiler is almost certainly going > to favour a mov over a ldr for getting small integers into regs, and mov > fortunately zeroes the top bits for us anyway. So does LDR Wd. Anyway, I think glibc (or my reading of it) has something like like: register long _x1 asm ("x1") = _x1tmp; before invoking the SVC. I assume this would do the right conversion to long. I can't tell about other libraries but I'd say it's their responsibility to convert the args to long before calling the kernel's prctl(). -- Catalin