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

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Message-ID: <20230221125032.0b02d309@gandalf.local.home>
Date:   Tue, 21 Feb 2023 12:50:32 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
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 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. Anyway. how does the dynamic
linker do this? Doesn't it update code on the fly as well?

-- Steve