Date:   Fri, 7 Jan 2022 20:08:10 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     David Laight <David.Laight@ACULAB.COM>
Cc:     Florian Weimer <fw@deneb.enyo.de>,
        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>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        carlos <carlos@redhat.com>
Message-ID: <333385690.14559.1641604090435.JavaMail.zimbra@efficios.com>
In-Reply-To: <701ff3888b3f42f4a6a1dded84b79078@AcuMS.aculab.com>
References: <20220107170302.8325-1-mathieu.desnoyers@efficios.com> <87a6g7ny0j.fsf@mid.deneb.enyo.de> <1968088162.13310.1641584935813.JavaMail.zimbra@efficios.com> <1300078200.13848.1641590867024.JavaMail.zimbra@efficios.com> <701ff3888b3f42f4a6a1dded84b79078@AcuMS.aculab.com>
Subject: Re: [RFC PATCH] rseq: x86: implement abort-at-ip extension
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----- On Jan 7, 2022, at 5:27 PM, David Laight David.Laight@ACULAB.COM wrote:

>> That being said, there might be an architecture agnostic alternative available.
>> On abort of a RSEQ_CS_FLAG_ABORT_AT_IP critical section, we could let the kernel
>> decrement/increment the stack pointer to make room for a pointer (depending if
>> the
>> stack grows down or up). It would then store the abort-at-ip value at the top of
>> stack.
> 
> Stack redzone in a leaf function?

Good point!

For x86-64 for instance, which has a redzone of 128 bytes, there are a few alternatives.
On abort:

Approach A) Move the stack pointer as little as possible

   1. On abort in kernel:
   1.1 Subtract 8 bytes from the stack pointer
   1.2 Store the abort-at-ip value at stack pointer - 128
   2. In abort handler (userspace)
   2.1 user-space knows that it can find the abort-at-ip value at stack pointer - 128
   2.2 after using that value, the abort handler needs to add 8 bytes to the stack pointer

Approach B) Move the stack pointer to skip the entire redzone

   1. On abort in kernel:
   1.1 Subtract 128 bytes from the stack pointer
   1.2 Store the abort-at-ip value at stack pointer - 8 (basically within a new red zone)
   2. In abort handler (userspace)
   2.1 user-space knows that it can find the abort-at-ip value at stack pointer - 8
   2.2 after using that value, the abort handler needs to add 128 bytes to the stack pointer

I suspect both approaches should work.

Any preference or other ideas ?

Thanks,

Mathieu

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