On Mon, 11 Sept 2023 at 16:57, Alexander Potapenko <glider@xxxxxxxxxx> wrote:
>
> Clang 18 learned to optimize away memcpy() calls of small uninitialized
> scalar values. To ensure that memcpy tests in kmsan_test.c still perform
> calls to memcpy() (which KMSAN replaces with __msan_memcpy()), declare a
> separate memcpy_noinline() function with volatile parameters, which
> won't be optimized.
>
> Also retire DO_NOT_OPTIMIZE(), as memcpy_noinline() is apparently
> enough.
>
> Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
Acked-by: Marco Elver <elver@xxxxxxxxxx>
> ---
> v2:
> - fix W=1 warnings reported by LKP test robot
> ---
> mm/kmsan/kmsan_test.c | 41 ++++++++++++++++-------------------------
> 1 file changed, 16 insertions(+), 25 deletions(-)
>
> diff --git a/mm/kmsan/kmsan_test.c b/mm/kmsan/kmsan_test.c
> index 312989aa2865c..a8d4ca4a1066d 100644
> --- a/mm/kmsan/kmsan_test.c
> +++ b/mm/kmsan/kmsan_test.c
> @@ -407,33 +407,25 @@ static void test_printk(struct kunit *test)
> KUNIT_EXPECT_TRUE(test, report_matches(&expect));
> }
>
> -/*
> - * Prevent the compiler from optimizing @var away. Without this, Clang may
> - * notice that @var is uninitialized and drop memcpy() calls that use it.
> - *
> - * There is OPTIMIZER_HIDE_VAR() in linux/compier.h that we cannot use here,
> - * because it is implemented as inline assembly receiving @var as a parameter
> - * and will enforce a KMSAN check. Same is true for e.g. barrier_data(var).
> - */
> -#define DO_NOT_OPTIMIZE(var) barrier()
> +/* Prevent the compiler from inlining a memcpy() call. */
> +static noinline void *memcpy_noinline(volatile void *dst,
> + const volatile void *src, size_t size)
> +{
> + return memcpy((void *)dst, (const void *)src, size);
> +}
>
> -/*
> - * Test case: ensure that memcpy() correctly copies initialized values.
> - * Also serves as a regression test to ensure DO_NOT_OPTIMIZE() does not cause
> - * extra checks.
> - */
> +/* Test case: ensure that memcpy() correctly copies initialized values. */
> static void test_init_memcpy(struct kunit *test)
> {
> EXPECTATION_NO_REPORT(expect);
> - volatile int src;
> - volatile int dst = 0;
> + volatile long long src;
> + volatile long long dst = 0;
>
> - DO_NOT_OPTIMIZE(src);
> src = 1;
> kunit_info(
> test,
> "memcpy()ing aligned initialized src to aligned dst (no reports)\n");
> - memcpy((void *)&dst, (void *)&src, sizeof(src));
> + memcpy_noinline((void *)&dst, (void *)&src, sizeof(src));
> kmsan_check_memory((void *)&dst, sizeof(dst));
> KUNIT_EXPECT_TRUE(test, report_matches(&expect));
> }
> @@ -451,8 +443,7 @@ static void test_memcpy_aligned_to_aligned(struct kunit *test)
> kunit_info(
> test,
> "memcpy()ing aligned uninit src to aligned dst (UMR report)\n");
> - DO_NOT_OPTIMIZE(uninit_src);
> - memcpy((void *)&dst, (void *)&uninit_src, sizeof(uninit_src));
> + memcpy_noinline((void *)&dst, (void *)&uninit_src, sizeof(uninit_src));
> kmsan_check_memory((void *)&dst, sizeof(dst));
> KUNIT_EXPECT_TRUE(test, report_matches(&expect));
> }
> @@ -474,8 +465,9 @@ static void test_memcpy_aligned_to_unaligned(struct kunit *test)
> kunit_info(
> test,
> "memcpy()ing aligned uninit src to unaligned dst (UMR report)\n");
> - DO_NOT_OPTIMIZE(uninit_src);
> - memcpy((void *)&dst[1], (void *)&uninit_src, sizeof(uninit_src));
> + kmsan_check_memory((void *)&uninit_src, sizeof(uninit_src));
> + memcpy_noinline((void *)&dst[1], (void *)&uninit_src,
> + sizeof(uninit_src));
> kmsan_check_memory((void *)dst, 4);
> KUNIT_EXPECT_TRUE(test, report_matches(&expect));
> }
> @@ -498,8 +490,8 @@ static void test_memcpy_aligned_to_unaligned2(struct kunit *test)
> kunit_info(
> test,
> "memcpy()ing aligned uninit src to unaligned dst - part 2 (UMR report)\n");
> - DO_NOT_OPTIMIZE(uninit_src);
> - memcpy((void *)&dst[1], (void *)&uninit_src, sizeof(uninit_src));
> + memcpy_noinline((void *)&dst[1], (void *)&uninit_src,
> + sizeof(uninit_src));
> kmsan_check_memory((void *)&dst[4], sizeof(uninit_src));
> KUNIT_EXPECT_TRUE(test, report_matches(&expect));
> }
> @@ -513,7 +505,6 @@ static void test_memcpy_aligned_to_unaligned2(struct kunit *test)
> \
> kunit_info(test, \
> "memset" #size "() should initialize memory\n"); \
> - DO_NOT_OPTIMIZE(uninit); \
> memset##size((uint##size##_t *)&uninit, 0, 1); \
> kmsan_check_memory((void *)&uninit, sizeof(uninit)); \
> KUNIT_EXPECT_TRUE(test, report_matches(&expect)); \
> --
> 2.42.0.283.g2d96d420d3-goog
>
