On Tue, Aug 05, 2025 at 02:48:25PM -0400, Zi Yan wrote: > On 5 Aug 2025, at 14:38, Jann Horn wrote: > > > On Tue, Aug 5, 2025 at 7:51 PM Zi Yan <[email protected]> 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. :)
We were having this discussion on IRC, generally I'm fine with it as-is, though my version sort of intended to make life easier by passing a ptr, but this version makes it closer to READ_ONCE() so probably semantically a little better. :) > > Best Regards, > Yan, Zi
