[5.7,254/376] kasan: stop tests being eliminated as dead code with FORTIFY_SOURCE

================================================

Of string and string-like functions, kasan_test tests:

 * strchr  ->  not affected, no fortified version
 * strrchr ->  likewise
 * strcmp  ->  likewise
 * strncmp ->  likewise

 * strnlen ->  not affected, the fortify source implementation calls the
               underlying strnlen implementation which is instrumented, not
               a builtin

 * strlen  ->  affected, the fortify souce implementation calls a __builtin
               version which the compiler can determine is dead.

 * memchr  ->  likewise
 * memcmp  ->  likewise

 * memset ->   not affected, the compiler knows that memset writes to its
	       first argument and therefore is not dead.

Why does this not affect the functions normally?
================================================

In string.h, these functions are not marked as __pure, so the compiler
cannot know that they do not have side effects.  If relevant functions are
marked as __pure in string.h, we see the following warnings and the
functions are elided:

lib/test_kasan.c: In function `kasan_memchr':
lib/test_kasan.c:606:2: warning: statement with no effect [-Wunused-value]
  memchr(ptr, '1', size + 1);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~
lib/test_kasan.c: In function `kasan_memcmp':
lib/test_kasan.c:622:2: warning: statement with no effect [-Wunused-value]
  memcmp(ptr, arr, size+1);
  ^~~~~~~~~~~~~~~~~~~~~~~~
lib/test_kasan.c: In function `kasan_strings':
lib/test_kasan.c:645:2: warning: statement with no effect [-Wunused-value]
  strchr(ptr, '1');
  ^~~~~~~~~~~~~~~~
...

This annotation would make sense to add and could be added at any point,
so the behaviour of test_kasan.c should change.

The fix
=======

Make all the functions that are pure write their results to a global,
which makes them live.  The strlen and memchr tests now pass.

The memcmp test still fails to trigger, which is addressed in the next
patch.

[dja@axtens.net: drop patch 3]
  Link: http://lkml.kernel.org/r/20200424145521.8203-2-dja@axtens.net
Fixes: 0c96350a2d2f ("lib/test_kasan.c: add tests for several string/memory API functions")
Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: David Gow <davidgow@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Daniel Micay <danielmicay@gmail.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Alexander Potapenko <glider@google.com>
Link: http://lkml.kernel.org/r/20200423154503.5103-1-dja@axtens.net
Link: http://lkml.kernel.org/r/20200423154503.5103-2-dja@axtens.net
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 lib/test_kasan.c | 29 +++++++++++++++++++----------
 1 file changed, 19 insertions(+), 10 deletions(-)

diff --git a/lib/test_kasan.c b/lib/test_kasan.c
index e3087d90e00d..dc2c6a51d11a 100644
--- a/lib/test_kasan.c
+++ b/lib/test_kasan.c
@@ -23,6 +23,14 @@ 
 
 #include <asm/page.h>
 
+/*
+ * We assign some test results to these globals to make sure the tests
+ * are not eliminated as dead code.
+ */
+
+int kasan_int_result;
+void *kasan_ptr_result;
+
 /*
  * Note: test functions are marked noinline so that their names appear in
  * reports.
@@ -622,7 +630,7 @@  static noinline void __init kasan_memchr(void)
 	if (!ptr)
 		return;
 
-	memchr(ptr, '1', size + 1);
+	kasan_ptr_result = memchr(ptr, '1', size + 1);
 	kfree(ptr);
 }
 
@@ -638,7 +646,7 @@  static noinline void __init kasan_memcmp(void)
 		return;
 
 	memset(arr, 0, sizeof(arr));
-	memcmp(ptr, arr, size+1);
+	kasan_int_result = memcmp(ptr, arr, size + 1);
 	kfree(ptr);
 }
 
@@ -661,22 +669,22 @@  static noinline void __init kasan_strings(void)
 	 * will likely point to zeroed byte.
 	 */
 	ptr += 16;
-	strchr(ptr, '1');
+	kasan_ptr_result = strchr(ptr, '1');
 
 	pr_info("use-after-free in strrchr\n");
-	strrchr(ptr, '1');
+	kasan_ptr_result = strrchr(ptr, '1');
 
 	pr_info("use-after-free in strcmp\n");
-	strcmp(ptr, "2");
+	kasan_int_result = strcmp(ptr, "2");
 
 	pr_info("use-after-free in strncmp\n");
-	strncmp(ptr, "2", 1);
+	kasan_int_result = strncmp(ptr, "2", 1);
 
 	pr_info("use-after-free in strlen\n");
-	strlen(ptr);
+	kasan_int_result = strlen(ptr);
 
 	pr_info("use-after-free in strnlen\n");
-	strnlen(ptr, 1);
+	kasan_int_result = strnlen(ptr, 1);
 }
 
 static noinline void __init kasan_bitops(void)
@@ -743,11 +751,12 @@  static noinline void __init kasan_bitops(void)
 	__test_and_change_bit(BITS_PER_LONG + BITS_PER_BYTE, bits);
 
 	pr_info("out-of-bounds in test_bit\n");
-	(void)test_bit(BITS_PER_LONG + BITS_PER_BYTE, bits);
+	kasan_int_result = test_bit(BITS_PER_LONG + BITS_PER_BYTE, bits);
 
 #if defined(clear_bit_unlock_is_negative_byte)
 	pr_info("out-of-bounds in clear_bit_unlock_is_negative_byte\n");
-	clear_bit_unlock_is_negative_byte(BITS_PER_LONG + BITS_PER_BYTE, bits);
+	kasan_int_result = clear_bit_unlock_is_negative_byte(BITS_PER_LONG +
+		BITS_PER_BYTE, bits);
 #endif
 	kfree(bits);
 }