Date:   Wed, 12 Jan 2022 10:24:13 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     David Laight <David.Laight@ACULAB.COM>
Cc:     Christian Brauner <christian.brauner@ubuntu.com>,
        Christian Brauner <brauner@kernel.org>,
        Peter Zijlstra <peterz@infradead.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>
Message-ID: <2040942183.24657.1642001053391.JavaMail.zimbra@efficios.com>
In-Reply-To: <0088806280f54211b3f90b2c1a82a140@AcuMS.aculab.com>
References: <20220110171611.8351-1-mathieu.desnoyers@efficios.com> <20220111110556.inteixgtl5vpmka7@wittgenstein> <1626924888.21447.1641922985771.JavaMail.zimbra@efficios.com> <20220112084617.32bjjo774n7vvyct@wittgenstein> <1475639366.24565.1641998849957.JavaMail.zimbra@efficios.com> <71e7d09733df4a899d12b7ef25198bbc@AcuMS.aculab.com> <1953851780.24610.1641999934047.JavaMail.zimbra@efficios.com> <0088806280f54211b3f90b2c1a82a140@AcuMS.aculab.com>
Subject: Re: [RFC PATCH v2 1/2] rseq: x86: implement abort-at-ip extension
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Thread-Topic: rseq: x86: implement abort-at-ip extension
Thread-Index: AdgHxNJPmHALYHCBSGGqQq0pyoAGXDSYsRN7NJggl0DQZ+TsQA==
Precedence: bulk

----- On Jan 12, 2022, at 10:15 AM, David Laight David.Laight@ACULAB.COM 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.

arch/powerpc/include/asm/ptrace.h define those for ppc64:

113:#define USER_REDZONE_SIZE	512
114:#define KERNEL_REDZONE_SIZE	288

and then uses the kernel redzone size for:

#define STACK_INT_FRAME_SIZE    (sizeof(struct pt_regs) + \
                                 STACK_FRAME_OVERHEAD + KERNEL_REDZONE_SIZE)

which AFAIU should ensure that ISR don't overwrite the redzone within the kernel.

Thanks,

Mathieu

> 
>	David
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT,
> UK
> Registration No: 1397386 (Wales)

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com