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 <elver@google.com>
Signed-off-by: Alexander Potapenko <glider@google.com>
---
 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

On Mon, 5 Dec 2022 at 15:57, Alexander Potapenko <glider@google.com> 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 <elver@google.com>
> Signed-off-by: Alexander Potapenko <glider@google.com>

Reviewed-by: Marco Elver <elver@google.com>

> ---
>  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
>