Re: [PATCH] selftests/mm: fix FORCE_READ to read input value correctly.

[Jann Horn <jannh@google.com>]

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?