Re: [PATCH v4 2/9] stop_machine: Add NMI-based execution path

[Chang S. Bae]

On 11/9/2026 6:46 PM, Borislav Petkov wrote:
> On Wed, Nov 04, 2026 at 06:23:15PM +0000, Chang S. Bae wrote:
> > +/*
> > + * Instrumentation may trigger nested exceptions such as #INT3, #DB,
> > + * or #PF. IRET from those would re-enable NMIs, which opposes the goal
> > + * of this NMI stop-machine facility.
> > + */
> > +bool noinstr stop_machine_nmi_handler(void)
>
> stop_machine_nmi()
>
> > +{
> > +	struct multi_stop_data *msdata = raw_cpu_read(nmi_stop.data);
> > +	unsigned int cpu = raw_smp_processor_id();
> > +	int ret;
> > +
> > +	if (!msdata || !arch_test_and_clear_bit(cpu, cpumask_bits(msdata->nmi_cpus)))
> > +		return false;
> > +
> > +	/*
> > +	 * The indirect call to @nmisafe_fn() is indistinguishable at
> > +	 * post-compilation. Temporarily enabling instrumentation avoids
> > +	 * objtool false positives.
> > +	 */
> > +	instrumentation_begin();
> > +	ret = msdata->nmisafe_fn(msdata->data);
> > +	instrumentation_end();
>
> This is just silly. We should fix objtool here.

Here is your previous thought:

  This one would be hard to fix because that's the

	err = msdata->fn(msdata->data);

  function pointer which objtool can't see through.

https://lore.kernel.org/lkml/20260127144947.GKaXjQi8R9cE17WYKM@fat_crate.local/