[PATCH 2/4] kmsan: another take at fixing memcpy tests

[Alexander Potapenko]

commit 5478afc55a21 ("kmsan: fix memcpy tests") uses OPTIMIZER_HIDE_VAR()
to hide the uninitialized var from the compiler optimizations.

However OPTIMIZER_HIDE_VAR(uninit) enforces an immediate check of
@uninit, so memcpy tests did not actually check the behavior of memcpy(),
because they always contained a KMSAN report.

Replace OPTIMIZER_HIDE_VAR() with a file-local macro that just clobbers
the memory with a barrier(), and add a test case for memcpy() that does not
expect an error report.

Also reflow kmsan_test.c with clang-format.

Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
---
v2:
 - replace inline assembly with a barrier(), update comments
---
 mm/kmsan/kmsan_test.c | 44 +++++++++++++++++++++++++++++++++++++------
 1 file changed, 38 insertions(+), 6 deletions(-)

diff --git a/mm/kmsan/kmsan_test.c b/mm/kmsan/kmsan_test.c
index 088e21a48dc4b..aeddfdd4f679f 100644
--- a/mm/kmsan/kmsan_test.c
+++ b/mm/kmsan/kmsan_test.c
@@ -407,6 +407,37 @@ 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()
+
+/*
+ * 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.
+ */
+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));
+	kmsan_check_memory((void *)&dst, sizeof(dst));
+	KUNIT_EXPECT_TRUE(test, report_matches(&expect));
+}
+
 /*
  * Test case: ensure that memcpy() correctly copies uninitialized values between
  * aligned `src` and `dst`.
@@ -420,7 +451,7 @@ 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);
+	DO_NOT_OPTIMIZE(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));
@@ -443,7 +474,7 @@ 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);
+	DO_NOT_OPTIMIZE(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));
@@ -467,13 +498,14 @@ 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);
+	DO_NOT_OPTIMIZE(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));
 }
 
-static noinline void fibonacci(int *array, int size, int start) {
+static noinline void fibonacci(int *array, int size, int start)
+{
 	if (start < 2 || (start == size))
 		return;
 	array[start] = array[start - 1] + array[start - 2];
@@ -482,8 +514,7 @@ static noinline void fibonacci(int *array, int size, int start) {
 
 static void test_long_origin_chain(struct kunit *test)
 {
-	EXPECTATION_UNINIT_VALUE_FN(expect,
-				    "test_long_origin_chain");
+	EXPECTATION_UNINIT_VALUE_FN(expect, "test_long_origin_chain");
 	/* (KMSAN_MAX_ORIGIN_DEPTH * 2) recursive calls to fibonacci(). */
 	volatile int accum[KMSAN_MAX_ORIGIN_DEPTH * 2 + 2];
 	int last = ARRAY_SIZE(accum) - 1;
@@ -515,6 +546,7 @@ static struct kunit_case kmsan_test_cases[] = {
 	KUNIT_CASE(test_uaf),
 	KUNIT_CASE(test_percpu_propagate),
 	KUNIT_CASE(test_printk),
+	KUNIT_CASE(test_init_memcpy),
 	KUNIT_CASE(test_memcpy_aligned_to_aligned),
 	KUNIT_CASE(test_memcpy_aligned_to_unaligned),
 	KUNIT_CASE(test_memcpy_aligned_to_unaligned2),
-- 
2.40.0.rc0.216.gc4246ad0f0-goog