+ kmsan-prevent-optimizations-in-memcpy-tests.patch added to mm-unstable branch

+ kmsan-prevent-optimizations-in-memcpy-tests.patch added to mm-unstable branch

[Andrew Morton]

The patch titled
     Subject: kmsan: prevent optimizations in memcpy tests
has been added to the -mm mm-unstable branch.  Its filename is
     kmsan-prevent-optimizations-in-memcpy-tests.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/kmsan-prevent-optimizations-in-memcpy-tests.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Alexander Potapenko <glider@google.com>
Subject: kmsan: prevent optimizations in memcpy tests
Date: Mon, 11 Sep 2023 16:57:00 +0200

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.

Link: https://lkml.kernel.org/r/20230911145702.2663753-2-glider@google.com
Signed-off-by: Alexander Potapenko <glider@google.com>
Acked-by: Marco Elver <elver@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/kmsan/kmsan_test.c |   41 +++++++++++++++-------------------------
 1 file changed, 16 insertions(+), 25 deletions(-)

--- a/mm/kmsan/kmsan_test.c~kmsan-prevent-optimizations-in-memcpy-tests
+++ a/mm/kmsan/kmsan_test.c
@@ -407,33 +407,25 @@ static void test_printk(struct kunit *te
 	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((void *)dst, (const void *)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;
+	volatile long long src;
+	volatile long long 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_align
 	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_unali
 	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((void *)&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_unali
 	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_unali
                                                                             \
 		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));           \
_

Patches currently in -mm which might be from glider@google.com are

kmsan-simplify-kmsan_internal_memmove_metadata.patch
kmsan-prevent-optimizations-in-memcpy-tests.patch
kmsan-merge-test_memcpy_aligned_to_unaligned2-together.patch
kmsan-introduce-test_memcpy_initialized_gap.patch