Date:   Wed, 12 Jan 2022 16:34:08 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     David Laight <David.Laight@aculab.com>
Cc:     'Mathieu Desnoyers' <mathieu.desnoyers@efficios.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Christian Brauner <brauner@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        paulmck <paulmck@kernel.org>, Boqun Feng <boqun.feng@gmail.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Paul Turner <pjt@google.com>,
        linux-api <linux-api@vger.kernel.org>,
        Florian Weimer <fw@deneb.enyo.de>, carlos <carlos@redhat.com>
Subject: Re: [RFC PATCH v2 1/2] rseq: x86: implement abort-at-ip extension
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On Wed, Jan 12, 2022 at 03:15:27PM +0000, David Laight wrote:
> From: Mathieu Desnoyers
> > Sent: 12 January 2022 15:06
> > 
> > ----- On Jan 12, 2022, at 9:58 AM, David Laight David.Laight@ACULAB.COM wrote:
> > 
> > >>  * [*] The openrisc, powerpc64 and x86-64 architectures define a "redzone" as a
> > >>  *     stack area beyond the stack pointer which can be used by the compiler
> > >>  *     to store local variables in leaf functions.
> > >
> > > I wonder if that is really worth the trouble it causes!
> > > By the time a function is spilling values to stack the cost
> > > of a %sp update is almost certainly noise.
> > >
> > > Someone clearly thought it was a 'good idea (tm)'.
> > 
> > I must admit that I've been surprised to learn about these redzones. Thanks for
> > pointing them out to me, it was clearly a blind spot. I suspect it would be useful
> > to introduce per-architecture KERNEL_REDZONE, USER_REDZONE and COMPAT_USER_REDZONE
> > with a asm-generic version defining them to 0, with proper documentation. It would
> > make it clearer to kernel developers working on stuff similar to signal handler
> > delivery that they need to consider these carefully.
> 
> They can never be used in kernel - any ISR would overwrite them.

That depends on how the architecture does exceptions; also consider:

  https://www.intel.com/content/www/us/en/develop/download/flexible-return-and-event-delivery-specification.html