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 <[email protected]>
LGTM, so:
Reviewed-by: Lorenzo Stoakes <[email protected]>
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)));
> /* Prevent the compiler from optimizing out the entire loop: */
> - 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
