LinuxLists.cc - [PATCH] kmsan: fix memcpy tests

2022-12-05 14:49:11

Subject: [PATCH] kmsan: fix memcpy tests

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 <[email protected]>
---
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

2022-12-05 14:52:56

by Marco Elver

[permalink] [raw]

Subject: Re: [PATCH] kmsan: fix memcpy tests

On Mon, 5 Dec 2022 at 14:26, Alexander Potapenko <[email protected]> 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 <[email protected]>

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
>

2022-12-05 16:28:13

by Alexander Potapenko

[permalink] [raw]

Subject: [PATCH v2] kmsan: fix memcpy tests

2022-12-05 16:44:27

by Marco Elver

[permalink] [raw]

Subject: Re: [PATCH v2] kmsan: fix memcpy tests

On Mon, 5 Dec 2022 at 15:57, Alexander Potapenko <[email protected]> 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.
>
> Use OPTIMIZER_HIDE_VAR() to hide the uninitialized var from the
> too-smart compiler.
>
> Suggested-by: Marco Elver <[email protected]>
> Signed-off-by: Alexander Potapenko <[email protected]>

Reviewed-by: Marco Elver <[email protected]>

> ---
> mm/kmsan/kmsan_test.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/mm/kmsan/kmsan_test.c b/mm/kmsan/kmsan_test.c
> index 9a29ea2dbfb9b..eb44ef3c5f290 100644
> --- a/mm/kmsan/kmsan_test.c
> +++ b/mm/kmsan/kmsan_test.c
> @@ -419,6 +419,7 @@ static void test_memcpy_aligned_to_aligned(struct kunit *test)
> kunit_info(
> test,
> "memcpy()ing aligned uninit src to aligned dst (UMR report)\n");
> + OPTIMIZER_HIDE_VAR(uninit_src);
> memcpy((void *)&dst, (void *)&uninit_src, sizeof(uninit_src));
> kmsan_check_memory((void *)&dst, sizeof(dst));
> KUNIT_EXPECT_TRUE(test, report_matches(&expect));
> @@ -441,6 +442,7 @@ static void test_memcpy_aligned_to_unaligned(struct kunit *test)
> kunit_info(
> test,
> "memcpy()ing aligned uninit src to unaligned dst (UMR report)\n");
> + OPTIMIZER_HIDE_VAR(uninit_src);
> memcpy((void *)&dst[1], (void *)&uninit_src, sizeof(uninit_src));
> kmsan_check_memory((void *)dst, 4);
> KUNIT_EXPECT_TRUE(test, report_matches(&expect));
> @@ -464,6 +466,7 @@ 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");
> + OPTIMIZER_HIDE_VAR(uninit_src);
> 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
>