kmsan: introduce test_unpoison_memory()

Add a regression test to ensure that kmsan_unpoison_memory() works the
same as an unpoisoning operation added by the instrumentation.

The test has two subtests: one that checks the instrumentation, and one
that checks kmsan_unpoison_memory().  Each subtest initializes the first
byte of a 4-byte buffer, then checks that the other 3 bytes are
uninitialized.

[glider@google.com: change description, remove comment about failing test case]
Link: https://lkml.kernel.org/r/20240528104807.738758-2-glider@google.comSigned-off-by: Brian Johannesmeyer <bjohannesmeyer@gmail.com>
Link: https://lore.kernel.org/lkml/20240524232804.1984355-1-bjohannesmeyer@gmail.com/T/Signed-off-by: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Marco Elver <elver@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/kmsan/kmsan_test.c

@@ -614,6 +614,32 @@ static void test_stackdepot_roundtrip(struct kunit *test)
 	KUNIT_EXPECT_TRUE(test, report_matches(&expect));
 }
 
+/*
+ * Test case: ensure that kmsan_unpoison_memory() and the instrumentation work
+ * the same.
+ */
+static void test_unpoison_memory(struct kunit *test)
+{
+	EXPECTATION_UNINIT_VALUE_FN(expect, "test_unpoison_memory");
+	volatile char a[4], b[4];
+
+	kunit_info(
+		test,
+		"unpoisoning via the instrumentation vs. kmsan_unpoison_memory() (2 UMR reports)\n");
+
+	/* Initialize a[0] and check a[1]--a[3]. */
+	a[0] = 0;
+	kmsan_check_memory((char *)&a[1], 3);
+	KUNIT_EXPECT_TRUE(test, report_matches(&expect));
+
+	report_reset();
+
+	/* Initialize b[0] and check b[1]--b[3]. */
+	kmsan_unpoison_memory((char *)&b[0], 1);
+	kmsan_check_memory((char *)&b[1], 3);
+	KUNIT_EXPECT_TRUE(test, report_matches(&expect));
+}
+
 static struct kunit_case kmsan_test_cases[] = {
 	KUNIT_CASE(test_uninit_kmalloc),
 	KUNIT_CASE(test_init_kmalloc),
@@ -637,6 +663,7 @@ static struct kunit_case kmsan_test_cases[] = {
 	KUNIT_CASE(test_memset64),
 	KUNIT_CASE(test_long_origin_chain),
 	KUNIT_CASE(test_stackdepot_roundtrip),
+	KUNIT_CASE(test_unpoison_memory),
 	{},
 };