Re: [PATCH] kmsan: fix memcpy tests
From: Marco Elver
Date: Mon Dec 05 2022 - 09:12:11 EST
On Mon, 5 Dec 2022 at 14:26, Alexander Potapenko <glider@xxxxxxxxxx> wrote:
>
> Recent Clang changes may cause it to delete calls of memcpy(), if the
> source is an uninitialized volatile local.
> This happens because passing a pointer to a volatile local into memcpy()
> discards the volatile qualifier, giving the compiler a free hand to
> optimize the memcpy() call away.
>
> To outsmart the compiler, we call __msan_memcpy() instead of memcpy()
> in test_memcpy_aligned_to_aligned(), test_memcpy_aligned_to_unaligned()
> and test_memcpy_aligned_to_unaligned2(), because it's the behavior of
> __msan_memcpy() we are testing here anyway.
>
> Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
It might be nice to retain memcpy() calls somehow, as that tests
end-to-end that the compiler does the right thing here i.e. replacing
the memcpy() calls with instrumented versions.
Does OPTIMIZER_HIDE_VAR() help? This should prevent the compiler from
seeing it's uninitialized.
> ---
> mm/kmsan/kmsan_test.c | 16 +++++++++++++---
> 1 file changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/mm/kmsan/kmsan_test.c b/mm/kmsan/kmsan_test.c
> index 9a29ea2dbfb9b..8e4f206a900ae 100644
> --- a/mm/kmsan/kmsan_test.c
> +++ b/mm/kmsan/kmsan_test.c
> @@ -406,6 +406,16 @@ static void test_printk(struct kunit *test)
> KUNIT_EXPECT_TRUE(test, report_matches(&expect));
> }
>
> +/*
> + * The test_memcpy_xxx tests below should be calling memcpy() to copy an
> + * uninitialized value from a volatile int. But such calls discard the volatile
> + * qualifier, so Clang may optimize them away, breaking the tests.
> + * Because KMSAN instrumentation pass would just replace memcpy() with
> + * __msan_memcpy(), do that explicitly to trick the optimizer into preserving
> + * the calls.
> + */
> +void *__msan_memcpy(void *, const void *, size_t);
> +
> /*
> * Test case: ensure that memcpy() correctly copies uninitialized values between
> * aligned `src` and `dst`.
> @@ -419,7 +429,7 @@ static void test_memcpy_aligned_to_aligned(struct kunit *test)
> kunit_info(
> test,
> "memcpy()ing aligned uninit src to aligned dst (UMR report)\n");
> - memcpy((void *)&dst, (void *)&uninit_src, sizeof(uninit_src));
> + __msan_memcpy((void *)&dst, (void *)&uninit_src, sizeof(uninit_src));
> kmsan_check_memory((void *)&dst, sizeof(dst));
> KUNIT_EXPECT_TRUE(test, report_matches(&expect));
> }
> @@ -441,7 +451,7 @@ static void test_memcpy_aligned_to_unaligned(struct kunit *test)
> kunit_info(
> test,
> "memcpy()ing aligned uninit src to unaligned dst (UMR report)\n");
> - memcpy((void *)&dst[1], (void *)&uninit_src, sizeof(uninit_src));
> + __msan_memcpy((void *)&dst[1], (void *)&uninit_src, sizeof(uninit_src));
> kmsan_check_memory((void *)dst, 4);
> KUNIT_EXPECT_TRUE(test, report_matches(&expect));
> }
> @@ -464,7 +474,7 @@ 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");
> - memcpy((void *)&dst[1], (void *)&uninit_src, sizeof(uninit_src));
> + __msan_memcpy((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));
> }
> --
> 2.39.0.rc0.267.gcb52ba06e7-goog
>