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From: Kees Cook <keescook@chromium.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org,
	Alexander Potapenko <glider@google.com>,
	Jann Horn <jannh@google.com>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	clang-built-linux@googlegroups.com
Subject: [PATCH] lib: test_stackinit.c: XFAIL switch variable init tests
Date: Wed, 19 Feb 2020 14:02:32 -0800
Message-ID: <202002191358.2897A07C6@keescook> (raw)

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>
---
 lib/Makefile         |  1 +
 lib/test_stackinit.c | 28 ++++++++++++++++++----------
 2 files changed, 19 insertions(+), 10 deletions(-)

diff --git a/lib/Makefile b/lib/Makefile
index 611872c06926..08c2b6d32900 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -86,6 +86,7 @@ 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
diff --git a/lib/test_stackinit.c b/lib/test_stackinit.c
index 2d7d257a430e..f93b1e145ada 100644
--- a/lib/test_stackinit.c
+++ b/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)
 {
-- 
2.20.1


-- 
Kees Cook

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