linux-kernel - Re: [PATCH v10 23/40] arm64/signal: Set up and restore the GCS context for signal handlers

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Message-ID: <ZsYvXqYEMYvIar28@arm.com>
Date: Wed, 21 Aug 2024 19:18:06 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Mark Brown <broonie@...nel.org>
Cc: Will Deacon <will@...nel.org>, Jonathan Corbet <corbet@....net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Marc Zyngier <maz@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>,
	James Morse <james.morse@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Arnd Bergmann <arnd@...db.de>, Oleg Nesterov <oleg@...hat.com>,
	Eric Biederman <ebiederm@...ssion.com>,
	Shuah Khan <shuah@...nel.org>,
	"Rick P. Edgecombe" <rick.p.edgecombe@...el.com>,
	Deepak Gupta <debug@...osinc.com>, Ard Biesheuvel <ardb@...nel.org>,
	Szabolcs Nagy <Szabolcs.Nagy@....com>, Kees Cook <kees@...nel.org>,
	"H.J. Lu" <hjl.tools@...il.com>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
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	Christian Brauner <brauner@...nel.org>,
	Thiago Jung Bauermann <thiago.bauermann@...aro.org>,
	Ross Burton <ross.burton@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
	kvmarm@...ts.linux.dev, linux-fsdevel@...r.kernel.org,
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Subject: Re: [PATCH v10 23/40] arm64/signal: Set up and restore the GCS
 context for signal handlers

On Wed, Aug 21, 2024 at 07:03:13PM +0100, Mark Brown wrote:
> On Wed, Aug 21, 2024 at 06:28:49PM +0100, Catalin Marinas wrote:
> > On Thu, Aug 01, 2024 at 01:06:50PM +0100, Mark Brown wrote:
> > > +	/*
> > > +	 * Push a cap and the GCS entry for the trampoline onto the GCS.
> > > +	 */
> > > +	put_user_gcs((unsigned long)sigtramp, gcspr_el0 - 2, &ret);
> > > +	put_user_gcs(GCS_SIGNAL_CAP(gcspr_el0 - 1), gcspr_el0 - 1, &ret);
> > > +	if (ret != 0)
> > > +		return ret;
> 
> > Doesn't the second put_user_gcs() override the previous ret?
> 
> No, we only set ret on error - if the first one faults it'll set ret
> then the second one will either leave it unchanged or write the same
> error code depending on if it fails.  This idiom is used quite a lot in
> the signal code.

You are right, I missed that it's called 'err' in put_user_gcs(),
thought it's overridden.

-- 
Catalin