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

Ilya Leoshkevich
2024-06-20T13:39:06Z

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!