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

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Message-ID: <PH7PR11MB65237A81D8258BB1E10969A9A9409@PH7PR11MB6523.namprd11.prod.outlook.com>
Date:   Tue, 23 May 2023 19:28:52 +0000
From:   "Preble, Adam C" <adam.c.preble@...el.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: How do I force an IBT trap in a demo kernel module?

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:

__attribute__((nocf_check))
static void indirect_jump_to_me(void)
{
    pr_info("Reached indirect jump\n");
}

I think that took care of my code generation, but objtool is offended by what I am doing:

/root/module/ibt_test.o: warning: objtool: init_ibt_test_module+0x11: relocation to !ENDBR: indirect_jump_to_me+0x0
  MODPOST /root/module/Module.symvers
ERROR: modpost: "__x86_indirect_thunk_nt_rax" [/root/module/ibt_test.ko] undefined!
make[4]: *** [/usr/src/my-header-6.3.0/scripts/Makefile.modpost:136: /root/module/Module.symvers] Error 1

So what do I have to do to tell objtool to allow to me deliberately shoot myself in the foot here?