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

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Message-ID: <ZHKG4xQ8vma7lfRQ@localhost>
Date:   Sun, 28 May 2023 00:40:35 +0200
From:   Pavel Machek <pavel@....cz>
To:     David Laight <David.Laight@...lab.com>
Cc:     "'Preble, Adam C'" <adam.c.preble@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: How do I force an IBT trap in a demo kernel module?

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.
								Pavel

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