Messages in this thread | | | Date | Sun, 28 May 2023 00:40:35 +0200 | From | Pavel Machek <> | Subject | Re: How do I force an IBT trap in a demo kernel module? |
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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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