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

[Steven Rostedt]

On Tue, 21 Feb 2023 11:44:42 -0500
Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> 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