kasan: test: Silence GCC 12 warnings

GCC 12 continues to get smarter about array accesses. The KASAN tests
are expecting to explicitly test out-of-bounds conditions at run-time,
so hide the variable from GCC, to avoid warnings like:

../lib/test_kasan.c: In function 'ksize_uaf':
../lib/test_kasan.c:790:61: warning: array subscript 120 is outside array bounds of 'void[120]' [-Warray-bounds]
  790 |         KUNIT_EXPECT_KASAN_FAIL(test, ((volatile char *)ptr)[size]);
      |                                       ~~~~~~~~~~~~~~~~~~~~~~^~~~~~
../lib/test_kasan.c:97:9: note: in definition of macro 'KUNIT_EXPECT_KASAN_FAIL'
   97 |         expression; \
      |         ^~~~~~~~~~

Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: kasan-dev@googlegroups.com
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220608214024.1068451-1-keescook@chromium.org

lib/test_kasan.c

@@ -131,6 +131,7 @@ static void kmalloc_oob_right(struct kunit *test)
 	ptr = kmalloc(size, GFP_KERNEL);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 
+	OPTIMIZER_HIDE_VAR(ptr);
 	/*
 	 * An unaligned access past the requested kmalloc size.
 	 * Only generic KASAN can precisely detect these.
@@ -159,6 +160,7 @@ static void kmalloc_oob_left(struct kunit *test)
 	ptr = kmalloc(size, GFP_KERNEL);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 
+	OPTIMIZER_HIDE_VAR(ptr);
 	KUNIT_EXPECT_KASAN_FAIL(test, *ptr = *(ptr - 1));
 	kfree(ptr);
 }
@@ -171,6 +173,7 @@ static void kmalloc_node_oob_right(struct kunit *test)
 	ptr = kmalloc_node(size, GFP_KERNEL, 0);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 
+	OPTIMIZER_HIDE_VAR(ptr);
 	KUNIT_EXPECT_KASAN_FAIL(test, ptr[0] = ptr[size]);
 	kfree(ptr);
 }
@@ -191,6 +194,7 @@ static void kmalloc_pagealloc_oob_right(struct kunit *test)
 	ptr = kmalloc(size, GFP_KERNEL);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 
+	OPTIMIZER_HIDE_VAR(ptr);
 	KUNIT_EXPECT_KASAN_FAIL(test, ptr[size + OOB_TAG_OFF] = 0);
 
 	kfree(ptr);
@@ -271,6 +275,7 @@ static void kmalloc_large_oob_right(struct kunit *test)
 	ptr = kmalloc(size, GFP_KERNEL);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 
+	OPTIMIZER_HIDE_VAR(ptr);
 	KUNIT_EXPECT_KASAN_FAIL(test, ptr[size] = 0);
 	kfree(ptr);
 }
@@ -410,6 +415,8 @@ static void kmalloc_oob_16(struct kunit *test)
 	ptr2 = kmalloc(sizeof(*ptr2), GFP_KERNEL);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr2);
 
+	OPTIMIZER_HIDE_VAR(ptr1);
+	OPTIMIZER_HIDE_VAR(ptr2);
 	KUNIT_EXPECT_KASAN_FAIL(test, *ptr1 = *ptr2);
 	kfree(ptr1);
 	kfree(ptr2);
@@ -756,6 +763,8 @@ static void ksize_unpoisons_memory(struct kunit *test)
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 	real_size = ksize(ptr);
 
+	OPTIMIZER_HIDE_VAR(ptr);
+
 	/* This access shouldn't trigger a KASAN report. */
 	ptr[size] = 'x';
 
@@ -778,6 +787,7 @@ static void ksize_uaf(struct kunit *test)
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 	kfree(ptr);
 
+	OPTIMIZER_HIDE_VAR(ptr);
 	KUNIT_EXPECT_KASAN_FAIL(test, ksize(ptr));
 	KUNIT_EXPECT_KASAN_FAIL(test, ((volatile char *)ptr)[0]);
 	KUNIT_EXPECT_KASAN_FAIL(test, ((volatile char *)ptr)[size]);