Re: [PATCH v5 36/37] s390/kmsan: Implement the architecture-specific functions

[Ilya Leoshkevich]

On Thu, 2024-06-20 at 11:25 +0200, Alexander Gordeev wrote:
> On Wed, Jun 19, 2024 at 05:44:11PM +0200, Ilya Leoshkevich wrote:
> 
> Hi Ilya,
> 
> > +static inline bool is_lowcore_addr(void *addr)
> > +{
> > +	return addr >= (void *)&S390_lowcore &&
> > +	       addr < (void *)(&S390_lowcore + 1);
> > +}
> > +
> > +static inline void *arch_kmsan_get_meta_or_null(void *addr, bool
> > is_origin)
> > +{
> > +	if (is_lowcore_addr(addr)) {
> > +		/*
> > +		 * Different lowcores accessed via S390_lowcore
> > are described
> > +		 * by the same struct page. Resolve the prefix
> > manually in
> > +		 * order to get a distinct struct page.
> > +		 */
> 
> > +		addr += (void
> > *)lowcore_ptr[raw_smp_processor_id()] -
> > +			(void *)&S390_lowcore;
> 
> If I am not mistaken neither raw_smp_processor_id() itself, nor
> lowcore_ptr[raw_smp_processor_id()] are atomic. Should the preemption
> be disabled while the addr is calculated?
> 
> But then the question arises - how meaningful the returned value is?
> AFAICT kmsan_get_metadata() is called from a preemptable context.
> So if the CPU is changed - how useful the previous CPU lowcore meta
> is?

This code path will only be triggered by instrumented code that
accesses lowcore. That code is supposed to disable preemption;
if it didn't, it's a bug in that code and it should be fixed there.

> 
> Is it a memory block that needs to be ignored instead?
> 
> > +		if (WARN_ON_ONCE(is_lowcore_addr(addr)))
> > +			return NULL;
> 
> lowcore_ptr[] pointing into S390_lowcore is rather a bug.

Right, but AFAIK BUG() calls are discouraged. I guess in a debug tool
the rules are more relaxed, but we can recover from this condition here
easily, that's why I still went for WARN_ON_ONCE().

> > +		return kmsan_get_metadata(addr, is_origin);
> > +	}
> > +	return NULL;
> > +}
> 
> Thanks!