kasan: test: improve failure message in KUNIT_EXPECT_KASAN_FAIL()

The KUNIT_EXPECT_KASAN_FAIL() macro currently uses KUNIT_EXPECT_EQ() to
compare fail_data.report_expected and fail_data.report_found.  This always
gave a somewhat useless error message on failure, but the addition of
extra compile-time checking with READ_ONCE() has caused it to get much
longer, and be truncated before anything useful is displayed.

Instead, just check fail_data.report_found by hand (we've just set
report_expected to 'true'), and print a better failure message with
KUNIT_FAIL().  Because of this, report_expected is no longer used
anywhere, and can be removed.

Beforehand, a failure in:
KUNIT_EXPECT_KASAN_FAIL(test, ((volatile char *)area)[3100]);
would have looked like:
[22:00:34] [FAILED] vmalloc_oob
[22:00:34]     # vmalloc_oob: EXPECTATION FAILED at lib/test_kasan.c:991
[22:00:34]     Expected ({ do { extern void __compiletime_assert_705(void) __attribute__((__error__("Unsupported access size for {READ,WRITE}_ONCE()."))); if (!((sizeof(fail_data.report_expected) == sizeof(char) || sizeof(fail_data.repp
[22:00:34]     not ok 45 - vmalloc_oob

With this change, it instead looks like:
[22:04:04] [FAILED] vmalloc_oob
[22:04:04]     # vmalloc_oob: EXPECTATION FAILED at lib/test_kasan.c:993
[22:04:04]     KASAN failure expected in "((volatile char *)area)[3100]", but none occurred
[22:04:04]     not ok 45 - vmalloc_oob

Also update the example failure in the documentation to reflect this.

Link: https://lkml.kernel.org/r/20210606005531.165954-1-davidgow@google.comSigned-off-by: David Gow <davidgow@google.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Reviewed-by: Marco Elver <elver@google.com>
Acked-by: Brendan Higgins <brendanhiggins@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Daniel Axtens <dja@axtens.net>
Cc: David Gow <davidgow@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/dev-tools/kasan.rst

@@ -447,11 +447,10 @@ When a test fails due to a failed ``kmalloc``::
 
 When a test fails due to a missing KASAN report::
 
-        # kmalloc_double_kzfree: EXPECTATION FAILED at lib/test_kasan.c:629
-        Expected kasan_data->report_expected == kasan_data->report_found, but
-        kasan_data->report_expected == 1
-        kasan_data->report_found == 0
-        not ok 28 - kmalloc_double_kzfree
+        # kmalloc_double_kzfree: EXPECTATION FAILED at lib/test_kasan.c:974
+        KASAN failure expected in "kfree_sensitive(ptr)", but none occurred
+        not ok 44 - kmalloc_double_kzfree
+
 
 At the end the cumulative status of all KASAN tests is printed. On success::
 

include/linux/kasan.h

@@ -17,7 +17,6 @@ struct task_struct;
 
 /* kasan_data struct is used in KUnit tests for KASAN expected failures */
 struct kunit_kasan_expectation {
-	bool report_expected;
 	bool report_found;
 };
 

lib/test_kasan.c

@@ -55,7 +55,6 @@ static int kasan_test_init(struct kunit *test)
 	multishot = kasan_save_enable_multi_shot();
 	kasan_set_tagging_report_once(false);
 	fail_data.report_found = false;
-	fail_data.report_expected = false;
 	kunit_add_named_resource(test, NULL, NULL, &resource,
 					"kasan_data", &fail_data);
 	return 0;
@@ -94,20 +93,20 @@ static void kasan_test_exit(struct kunit *test)
 	    !kasan_async_mode_enabled())				\
 		migrate_disable();					\
 	KUNIT_EXPECT_FALSE(test, READ_ONCE(fail_data.report_found));	\
-	WRITE_ONCE(fail_data.report_expected, true);			\
 	barrier();							\
 	expression;							\
 	barrier();							\
-	KUNIT_EXPECT_EQ(test,						\
-			READ_ONCE(fail_data.report_expected),		\
-			READ_ONCE(fail_data.report_found));		\
+	if (!READ_ONCE(fail_data.report_found)) {			\
+		KUNIT_FAIL(test, KUNIT_SUBTEST_INDENT "KASAN failure "	\
+				"expected in \"" #expression		\
+				 "\", but none occurred");		\
+	}								\
 	if (IS_ENABLED(CONFIG_KASAN_HW_TAGS)) {				\
 		if (READ_ONCE(fail_data.report_found))			\
 			kasan_enable_tagging_sync();			\
 		migrate_enable();					\
 	}								\
 	WRITE_ONCE(fail_data.report_found, false);			\
-	WRITE_ONCE(fail_data.report_expected, false);			\
 } while (0)
 
 #define KASAN_TEST_NEEDS_CONFIG_ON(test, config) do {			\