On Tue, Aug 05, 2025 at 01:51:40PM -0400, Zi Yan 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.
Oops, sorry about that! I had incorrectly assumed typeof() decayed the type when I wrote the guard-regions test code, and hadn't considered that we were casting to (t **) and dereffing that.
And as discussed off-list, if you deref a char array like that, and are at the end of an array, that's err... not brilliant :)
Fix it by implementing a simplified READ_ONCE() instead.
I sort of intended to make this easier for pointers, but the semantics of this are actually potentially a bit nicer - it's more like READ_ONCE() and you're passing in the value you're actually reading so this is probably better.
Fixes: 3f6bfd4789a0 ("selftests/mm: reuse FORCE_READ to replace "asm volatile("" : "+r" (XXX));"") Signed-off-by: Zi Yan ziy@nvidia.com
LGTM, so:
Reviewed-by: Lorenzo Stoakes lorenzo.stoakes@oracle.com
But see nits below.
FORCE_READ() comes from commit 876320d71f51 ("selftests/mm: add self tests for guard page feature"). I will a separate patch to stable tree.
tools/testing/selftests/mm/cow.c | 4 ++-- tools/testing/selftests/mm/guard-regions.c | 2 +- tools/testing/selftests/mm/hugetlb-madvise.c | 4 +++- tools/testing/selftests/mm/migration.c | 2 +- tools/testing/selftests/mm/pagemap_ioctl.c | 2 +- tools/testing/selftests/mm/split_huge_page_test.c | 7 +++++-- tools/testing/selftests/mm/vm_util.h | 2 +- 7 files changed, 14 insertions(+), 9 deletions(-)
diff --git a/tools/testing/selftests/mm/cow.c b/tools/testing/selftests/mm/cow.c index d30625c18259..c744c603d688 100644 --- a/tools/testing/selftests/mm/cow.c +++ b/tools/testing/selftests/mm/cow.c @@ -1554,8 +1554,8 @@ static void run_with_zeropage(non_anon_test_fn fn, const char *desc) }
/* Read from the page to populate the shared zeropage. */
- FORCE_READ(mem);
- FORCE_READ(smem);
FORCE_READ(*mem);
FORCE_READ(*smem);
fn(mem, smem, pagesize);
munmap: diff --git a/tools/testing/selftests/mm/guard-regions.c b/tools/testing/selftests/mm/guard-regions.c index b0d42eb04e3a..8dd81c0a4a5a 100644 --- a/tools/testing/selftests/mm/guard-regions.c +++ b/tools/testing/selftests/mm/guard-regions.c @@ -145,7 +145,7 @@ static bool try_access_buf(char *ptr, bool write) if (write) *ptr = 'x'; else
FORCE_READ(ptr);
FORCE_READ(*ptr);}
signal_jump_set = false;
diff --git a/tools/testing/selftests/mm/hugetlb-madvise.c b/tools/testing/selftests/mm/hugetlb-madvise.c index 1afe14b9dc0c..c5940c0595be 100644 --- a/tools/testing/selftests/mm/hugetlb-madvise.c +++ b/tools/testing/selftests/mm/hugetlb-madvise.c @@ -50,8 +50,10 @@ void read_fault_pages(void *addr, unsigned long nr_pages) unsigned long i;
for (i = 0; i < nr_pages; i++) {
unsigned long *addr2 =((unsigned long *)(addr + (i * huge_page_size)));
FORCE_READ(((unsigned long *)(addr + (i * huge_page_size))));
FORCE_READ(*addr2);}
diff --git a/tools/testing/selftests/mm/migration.c b/tools/testing/selftests/mm/migration.c index c5a73617796a..ea945eebec2f 100644 --- a/tools/testing/selftests/mm/migration.c +++ b/tools/testing/selftests/mm/migration.c @@ -110,7 +110,7 @@ void *access_mem(void *ptr) * the memory access actually happens and prevents the compiler * from optimizing away this entire loop. */
FORCE_READ((uint64_t *)ptr);
FORCE_READ(*(uint64_t *)ptr);}
return NULL;
diff --git a/tools/testing/selftests/mm/pagemap_ioctl.c b/tools/testing/selftests/mm/pagemap_ioctl.c index 0d4209eef0c3..e6face7c0166 100644 --- a/tools/testing/selftests/mm/pagemap_ioctl.c +++ b/tools/testing/selftests/mm/pagemap_ioctl.c @@ -1525,7 +1525,7 @@ void zeropfn_tests(void)
ret = madvise(mem, hpage_size, MADV_HUGEPAGE); if (!ret) {
FORCE_READ(mem);
FORCE_READ(*mem);ret = pagemap_ioctl(mem, hpage_size, &vec, 1, 0, 0, PAGE_IS_PFNZERO, 0, 0, PAGE_IS_PFNZERO);
diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c index 718daceb5282..3c761228e451 100644 --- a/tools/testing/selftests/mm/split_huge_page_test.c +++ b/tools/testing/selftests/mm/split_huge_page_test.c @@ -440,8 +440,11 @@ int create_pagecache_thp_and_fd(const char *testfile, size_t fd_size, int *fd, } madvise(*addr, fd_size, MADV_HUGEPAGE);
- for (size_t i = 0; i < fd_size; i++)
FORCE_READ((*addr + i));
for (size_t i = 0; i < fd_size; i++) {
char *addr2 = *addr + i;FORCE_READ(*addr2);}
if (!check_huge_file(*addr, fd_size / pmd_pagesize, pmd_pagesize)) { ksft_print_msg("No large pagecache folio generated, please provide a filesystem supporting large folio\n");
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))
NIT: but wonder if const is necessary, and also (as discussed off-list again :) will this work with a (void) prefixed, just to a. make it clear we're reading but discarding and b. to avoid any possible compiler warning on this?
I know for some reason this form doesn't generate one currently (not sure why), but we may hit that in future.
extern unsigned int __page_size; extern unsigned int __page_shift; -- 2.47.2
Cheers, Lorenzo