On 5 Aug 2025, at 14:38, Jann Horn wrote:
On Tue, Aug 5, 2025 at 7:51 PM Zi Yan ziy@nvidia.com wrote:
FORCE_READ() converts input value x to its pointer type then reads from address x. This is wrong. If x is a non-pointer, it would be caught it easily. But all FORCE_READ() callers are trying to read from a pointer and FORCE_READ() basically reads a pointer to a pointer instead of the original typed pointer. Almost no access violation was found, except the one from split_huge_page_test.
[...]
diff --git a/tools/testing/selftests/mm/vm_util.h b/tools/testing/selftests/mm/vm_util.h index c20298ae98ea..b55d1809debc 100644 --- a/tools/testing/selftests/mm/vm_util.h +++ b/tools/testing/selftests/mm/vm_util.h @@ -23,7 +23,7 @@
- anything with it in order to trigger a read page fault. We therefore must use
- volatile to stop the compiler from optimising this away.
*/ -#define FORCE_READ(x) (*(volatile typeof(x) *)x) +#define FORCE_READ(x) (*(const volatile typeof(x) *)&(x))
So is the problem with the old code basically that it should have been something like
#define FORCE_READ(x) (*(volatile typeof(*(x)) *)(x))
to actually cast the normal pointer to a volatile pointer?
Yeah. That works too. I would rename it to FORCE_READ_PTR to avoid misuse. :)
Best Regards, Yan, Zi