On Thu, Jun 20, 2024 at 3:38 PM Ilya Leoshkevich [off-list ref] wrote:

On Thu, 2024-06-20 at 11:25 +0200, Alexander Gordeev wrote:

quoted

On Wed, Jun 19, 2024 at 05:44:11PM +0200, Ilya Leoshkevich wrote:

Hi Ilya,

quoted

+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.
+            */

quoted

+           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.

quoted

Is it a memory block that needs to be ignored instead?

quoted

+           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