kasan: test: silence intentional read overflow warnings

As done in commit d73dad4 ("kasan: test: bypass __alloc_size
checks") for __write_overflow warnings, also silence some more cases
that trip the __read_overflow warnings seen in 5.16-rc1[1]:

  In file included from include/linux/string.h:253,
                   from include/linux/bitmap.h:10,
                   from include/linux/cpumask.h:12,
                   from include/linux/mm_types_task.h:14,
                   from include/linux/mm_types.h:5,
                   from include/linux/page-flags.h:13,
                   from arch/arm64/include/asm/mte.h:14,
                   from arch/arm64/include/asm/pgtable.h:12,
                   from include/linux/pgtable.h:6,
                   from include/linux/kasan.h:29,
                   from lib/test_kasan.c:10:
  In function 'memcmp',
      inlined from 'kasan_memcmp' at lib/test_kasan.c:897:2:
  include/linux/fortify-string.h:263:25: error: call to '__read_overflow' declared with attribute error: detected read beyond size of object (1st parameter)
    263 |                         __read_overflow();
        |                         ^~~~~~~~~~~~~~~~~
  In function 'memchr',
      inlined from 'kasan_memchr' at lib/test_kasan.c:872:2:
  include/linux/fortify-string.h:277:17: error: call to '__read_overflow' declared with attribute error: detected read beyond size of object (1st parameter)
    277 |                 __read_overflow();
        |                 ^~~~~~~~~~~~~~~~~

[1] http://kisskb.ellerman.id.au/kisskb/buildresult/14660585/log/

Link: https://lkml.kernel.org/r/[email protected]
Fixes: d73dad4 ("kasan: test: bypass __alloc_size checks")
Signed-off-by: Kees Cook <[email protected]>
Reviewed-by: Andrey Konovalov <[email protected]>
Acked-by: Marco Elver <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Alexander Potapenko <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

lib/test_kasan.c

@@ -869,6 +869,7 @@ static void kasan_memchr(struct kunit *test)
 	ptr = kmalloc(size, GFP_KERNEL | __GFP_ZERO);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 
+	OPTIMIZER_HIDE_VAR(size);
 	KUNIT_EXPECT_KASAN_FAIL(test,
 		kasan_ptr_result = memchr(ptr, '1', size + 1));
 
@@ -894,6 +895,7 @@ static void kasan_memcmp(struct kunit *test)
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 	memset(arr, 0, sizeof(arr));
 
+	OPTIMIZER_HIDE_VAR(size);
 	KUNIT_EXPECT_KASAN_FAIL(test,
 		kasan_int_result = memcmp(ptr, arr, size+1));
 	kfree(ptr);