linux-kernel - RE: [RFC PATCH v2 1/2] rseq: x86: implement abort-at-ip extension

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Date:   Wed, 12 Jan 2022 15:15:27 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Mathieu Desnoyers' <mathieu.desnoyers@...icios.com>
CC:     Christian Brauner <christian.brauner@...ntu.com>,
        Christian Brauner <brauner@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        paulmck <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
        "H. Peter Anvin" <hpa@...or.com>, Paul Turner <pjt@...gle.com>,
        linux-api <linux-api@...r.kernel.org>,
        Florian Weimer <fw@...eb.enyo.de>, carlos <carlos@...hat.com>
Subject: RE: [RFC PATCH v2 1/2] rseq: x86: implement abort-at-ip extension

From: Mathieu Desnoyers
> Sent: 12 January 2022 15:06
> 
> ----- On Jan 12, 2022, at 9:58 AM, David Laight David.Laight@...LAB.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.

	David

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