fortify: Add KUnit test for FORTIFY_SOURCE internals

Add lib/fortify_kunit.c KUnit test for checking the expected behavioral
characteristics of FORTIFY_SOURCE internals.

Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Tom Rix <trix@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: "Steven Rostedt (Google)" <rostedt@goodmis.org>
Cc: Yury Norov <yury.norov@gmail.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Sander Vanheule <sander@svanheule.net>
Cc: linux-hardening@vger.kernel.org
Cc: llvm@lists.linux.dev
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

MAINTAINERS

@@ -8002,6 +8002,7 @@ L:	linux-hardening@vger.kernel.org
 S:	Supported
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git for-next/hardening
 F:	include/linux/fortify-string.h
+F:	lib/fortify_kunit.c
 F:	lib/test_fortify/*
 F:	scripts/test_fortify.sh
 K:	\b__NO_FORTIFY\b

lib/Kconfig.debug

@@ -2542,6 +2542,15 @@ config STACKINIT_KUNIT_TEST
 	  CONFIG_GCC_PLUGIN_STRUCTLEAK, CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF,
 	  or CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL.
 
+config FORTIFY_KUNIT_TEST
+	tristate "Test fortified str*() and mem*() function internals at runtime" if !KUNIT_ALL_TESTS
+	depends on KUNIT && FORTIFY_SOURCE
+	default KUNIT_ALL_TESTS
+	help
+	  Builds unit tests for checking internals of FORTIFY_SOURCE as used
+	  by the str*() and mem*() family of functions. For testing runtime
+	  traps of FORTIFY_SOURCE, see LKDTM's "FORTIFY_*" tests.
+
 config TEST_UDELAY
 	tristate "udelay test driver"
 	help

lib/Makefile

@@ -381,6 +381,7 @@ obj-$(CONFIG_IS_SIGNED_TYPE_KUNIT_TEST) += is_signed_type_kunit.o
 obj-$(CONFIG_OVERFLOW_KUNIT_TEST) += overflow_kunit.o
 CFLAGS_stackinit_kunit.o += $(call cc-disable-warning, switch-unreachable)
 obj-$(CONFIG_STACKINIT_KUNIT_TEST) += stackinit_kunit.o
+obj-$(CONFIG_FORTIFY_KUNIT_TEST) += fortify_kunit.o
 
 obj-$(CONFIG_GENERIC_LIB_DEVMEM_IS_ALLOWED) += devmem_is_allowed.o
 

lib/fortify_kunit.c

@@ -0,0 +1,77 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Runtime test cases for CONFIG_FORTIFY_SOURCE that aren't expected to
+ * Oops the kernel on success. (For those, see drivers/misc/lkdtm/fortify.c)
+ *
+ * For corner cases with UBSAN, try testing with:
+ *
+ * ./tools/testing/kunit/kunit.py run --arch=x86_64 \
+ *	--kconfig_add CONFIG_FORTIFY_SOURCE=y \
+ *	--kconfig_add CONFIG_UBSAN=y \
+ *	--kconfig_add CONFIG_UBSAN_TRAP=y \
+ *	--kconfig_add CONFIG_UBSAN_BOUNDS=y \
+ *	--kconfig_add CONFIG_UBSAN_LOCAL_BOUNDS=y \
+ *	--make_options LLVM=1 fortify
+ */
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <kunit/test.h>
+#include <linux/string.h>
+#include <linux/init.h>
+
+static const char array_of_10[] = "this is 10";
+static const char *ptr_of_11 = "this is 11!";
+static char array_unknown[] = "compiler thinks I might change";
+
+static void known_sizes_test(struct kunit *test)
+{
+	KUNIT_EXPECT_EQ(test, __compiletime_strlen("88888888"), 8);
+	KUNIT_EXPECT_EQ(test, __compiletime_strlen(array_of_10), 10);
+	KUNIT_EXPECT_EQ(test, __compiletime_strlen(ptr_of_11), 11);
+
+	KUNIT_EXPECT_EQ(test, __compiletime_strlen(array_unknown), SIZE_MAX);
+	/* Externally defined and dynamically sized string pointer: */
+	KUNIT_EXPECT_EQ(test, __compiletime_strlen(saved_command_line), SIZE_MAX);
+}
+
+/* This is volatile so the optimizer can't perform DCE below. */
+static volatile int pick;
+
+/* Not inline to keep optimizer from figuring out which string we want. */
+static noinline size_t want_minus_one(int pick)
+{
+	const char *str;
+
+	switch (pick) {
+	case 1:
+		str = "4444";
+		break;
+	case 2:
+		str = "333";
+		break;
+	default:
+		str = "1";
+		break;
+	}
+	return __compiletime_strlen(str);
+}
+
+static void control_flow_split_test(struct kunit *test)
+{
+	KUNIT_EXPECT_EQ(test, want_minus_one(pick), SIZE_MAX);
+}
+
+static struct kunit_case fortify_test_cases[] = {
+	KUNIT_CASE(known_sizes_test),
+	KUNIT_CASE(control_flow_split_test),
+	{}
+};
+
+static struct kunit_suite fortify_test_suite = {
+	.name = "fortify",
+	.test_cases = fortify_test_cases,
+};
+
+kunit_test_suite(fortify_test_suite);
+
+MODULE_LICENSE("GPL");