[PATCH 2/2] kmsan: prevent optimizations in memcpy tests

[Alexander Potapenko]

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>
---
 mm/kmsan/kmsan_test.c | 37 ++++++++++++++-----------------------
 1 file changed, 14 insertions(+), 23 deletions(-)

diff --git a/mm/kmsan/kmsan_test.c b/mm/kmsan/kmsan_test.c
index 312989aa2865c..0c32c917b489a 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(dst, 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;
 
-	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(&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