lib: test_stackinit.c: XFAIL switch variable init tests

The tests for initializing a variable defined between a switch statement's
test and its first "case" statement are currently not initialized in
Clang[1] nor the proposed auto-initialization feature in GCC.

We should retain the test (so that we can evaluate compiler fixes), but
mark it as an "expected fail".  The rest of the kernel source will be
adjusted to avoid this corner case.

Also disable -Wswitch-unreachable for the test so that the intentionally
broken code won't trigger warnings for GCC (nor future Clang) when
initialization happens this unhandled place.

[1] https://bugs.llvm.org/show_bug.cgi?id=44916

Suggested-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Jann Horn <jannh@google.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Link: http://lkml.kernel.org/r/202002191358.2897A07C6@keescook
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/Makefile

@@ -87,6 +87,7 @@ obj-$(CONFIG_TEST_KMOD) += test_kmod.o
 obj-$(CONFIG_TEST_DEBUG_VIRTUAL) += test_debug_virtual.o
 obj-$(CONFIG_TEST_MEMCAT_P) += test_memcat_p.o
 obj-$(CONFIG_TEST_OBJAGG) += test_objagg.o
+CFLAGS_test_stackinit.o += $(call cc-disable-warning, switch-unreachable)
 obj-$(CONFIG_TEST_STACKINIT) += test_stackinit.o
 obj-$(CONFIG_TEST_BLACKHOLE_DEV) += test_blackhole_dev.o
 obj-$(CONFIG_TEST_MEMINIT) += test_meminit.o

lib/test_stackinit.c

@@ -92,8 +92,9 @@ static bool range_contains(char *haystack_start, size_t haystack_size,
  * @var_type: type to be tested for zeroing initialization
  * @which: is this a SCALAR, STRING, or STRUCT type?
  * @init_level: what kind of initialization is performed
+ * @xfail: is this test expected to fail?
  */
-#define DEFINE_TEST_DRIVER(name, var_type, which)		\
+#define DEFINE_TEST_DRIVER(name, var_type, which, xfail)	\
 /* Returns 0 on success, 1 on failure. */			\
 static noinline __init int test_ ## name (void)			\
 {								\
@@ -139,13 +140,14 @@ static noinline __init int test_ ## name (void)			\
 	for (sum = 0, i = 0; i < target_size; i++)		\
 		sum += (check_buf[i] == 0xFF);			\
 								\
-	if (sum == 0)						\
+	if (sum == 0) {						\
 		pr_info(#name " ok\n");				\
-	else							\
-		pr_warn(#name " FAIL (uninit bytes: %d)\n",	\
-			sum);					\
-								\
-	return (sum != 0);					\
+		return 0;					\
+	} else {						\
+		pr_warn(#name " %sFAIL (uninit bytes: %d)\n",	\
+			(xfail) ? "X" : "", sum);		\
+		return (xfail) ? 0 : 1;				\
+	}							\
 }
 #define DEFINE_TEST(name, var_type, which, init_level)		\
 /* no-op to force compiler into ignoring "uninitialized" vars */\
@@ -189,7 +191,7 @@ static noinline __init int leaf_ ## name(unsigned long sp,	\
 								\
 	return (int)buf[0] | (int)buf[sizeof(buf) - 1];		\
 }								\
-DEFINE_TEST_DRIVER(name, var_type, which)
+DEFINE_TEST_DRIVER(name, var_type, which, 0)
 
 /* Structure with no padding. */
 struct test_packed {
@@ -326,8 +328,14 @@ static noinline __init int leaf_switch_2_none(unsigned long sp, bool fill,
 	return __leaf_switch_none(2, fill);
 }
 
-DEFINE_TEST_DRIVER(switch_1_none, uint64_t, SCALAR);
-DEFINE_TEST_DRIVER(switch_2_none, uint64_t, SCALAR);
+/*
+ * These are expected to fail for most configurations because neither
+ * GCC nor Clang have a way to perform initialization of variables in
+ * non-code areas (i.e. in a switch statement before the first "case").
+ * https://bugs.llvm.org/show_bug.cgi?id=44916
+ */
+DEFINE_TEST_DRIVER(switch_1_none, uint64_t, SCALAR, 1);
+DEFINE_TEST_DRIVER(switch_2_none, uint64_t, SCALAR, 1);
 
 static int __init test_stackinit_init(void)
 {