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

[Alexander Potapenko <glider@google.com>]

On Thu, Jun 20, 2024 at 3:38 PM Ilya Leoshkevich <iii@linux.ibm.com> wrote:
>
> 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().

We have KMSAN_WARN_ON() for that, sorry for not pointing it out
earlier: https://elixir.bootlin.com/linux/latest/source/mm/kmsan/kmsan.h#L46