linux-kernel - [PATCH] kmsan: fix memcpy tests

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Date:   Mon,  5 Dec 2022 14:25:58 +0100
From:   Alexander Potapenko <glider@...gle.com>
To:     glider@...gle.com
Cc:     linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        elver@...gle.com, dvyukov@...gle.com, linux-mm@...ck.org
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 <glider@...gle.com>
---
 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