Re: How do I force an IBT trap in a demo kernel module?

From: Pavel Machek
Date: Sat May 27 2023 - 18:40:44 EST

On Wed 2023-05-24 08:00:36, David Laight wrote:
> From: Preble, Adam C
> > Sent: 23 May 2023 20:29
> >
> > I am debugging why a kernel module of ours triggers the IBT (Indirect Branch Tracking) trap, and while
> > doing that, I was trying to write a standalone demo that would forcefully trigger it on purpose. This
> > has turned out to be much more difficult than I thought! What can I do to get a module to generate an
> > indirect branch without an endbr64? Creating the indirect branch itself doesn't appear to be hard:
> >
> > 1. Set up a function call
> > 2. Assign it to a function pointer
> > 3. Call the function pointer
> > 4. ...maybe add a compiler flag so it doesn't optimize the call to a direct branch.
> >
> > I am primarily building in a Debian environment with gcc (Debian 10.2.1-6) 10.2.1 20210110. By
> > default, the branch does get optimized away. I had to set the -mforce-indirect-call flag. The endbr64
> > instruction would still emit so I added a function attribute to suppress it:
> ...
> > So what do I have to do to tell objtool to allow to me deliberately shoot myself in the foot here?
> You could try adjusting the function pointer by the size of the endbr64 instrauction.
> Oh, put the function pointer variable into static data.
> That should stop it all being optimised away.

volatile will help, too.

Or simply write the code in assembly.