linux-kernel - Re: Official documentation from Intel stating that poking INT3 (single-byte) concurrently is OK ?

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Message-ID: <5774aace-23f3-c53d-8e65-b90b588dbbe3@efficios.com>
Date:   Tue, 21 Feb 2023 13:42:58 -0500
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Olivier Dion <odion@...icios.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Jiri Kosina <jkosina@...e.cz>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: Official documentation from Intel stating that poking INT3
 (single-byte) concurrently is OK ?

On 2023-02-21 12:50, Steven Rostedt wrote:
> On Tue, 21 Feb 2023 11:44:42 -0500
> Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> 
>> Hi Peter,
>>
>> I have emails from you dating from a few years back unofficially stating
>> that it's OK to update the first byte of an instruction with a single-byte
>> int3 concurrently:
>>
>> https://lkml.indiana.edu/hypermail/linux/kernel/1001.1/01530.html
>>
>> It is referred in the original implementation of text_poke_bp():
>> commit fd4363fff3d9 ("x86: Introduce int3 (breakpoint)-based instruction patching")
>>
>> Olivier Dion is working on the libpatch [1,2] project aiming to use this
>> property for low-latency/low-overhead live code patching in user-space as
>> well, but we cannot find an official statement from Intel that guarantees
>> this breakpoint-bypass technique is indeed OK without stopping the world
>> while patching.
>>
>> Do you know where I could find an official statement of this guarantee ?
>>
> 
> The fact that we have been using it for over 10 years without issue should
> be a good guarantee ;-)
> 
> I know you probably prefer an official statement, and I thought they
> actually gave one, but can't seem to find it.

I recall an in-person discussion with Peter Anvin shortly after he got 
the official confirmation, but I cannot find any public trace of it. I 
suspect Intel may have documented this internally only.

  Anyway. how does the dynamic
> linker do this? Doesn't it update code on the fly as well?

The dynamic linker is similar to the module loader in the kernel: the 
code modification is done before the loaded code is ever executed, and 
is therefore inherently safe with respect to cross-modification of 
concurrently executing code.

Thanks,

Mathieu

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