kasan: test: bypass __alloc_size checks

Intentional overflows, as performed by the KASAN tests, are detected at
compile time[1] (instead of only at run-time) with the addition of
__alloc_size. Fix this by forcing the compiler into not being able to
trust the size used following the kmalloc()s.

[1] https://lore.kernel.org/lkml/[e-mail]

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Kees Cook <[e-mail]>
Cc: Andrey Ryabinin <[e-mail]>
Cc: Alexander Potapenko <[e-mail]>
Cc: Andrey Konovalov <[e-mail]>
Cc: Dmitry Vyukov <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

lib/test_kasan.c

···
 	ptr = kmalloc(size, GFP_KERNEL);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 
+	OPTIMIZER_HIDE_VAR(size);
 	KUNIT_EXPECT_KASAN_FAIL(test, memset(ptr + size - 1, 0, 2));
 	kfree(ptr);
 }
···
 	ptr = kmalloc(size, GFP_KERNEL);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 
+	OPTIMIZER_HIDE_VAR(size);
 	KUNIT_EXPECT_KASAN_FAIL(test, memset(ptr + size - 3, 0, 4));
 	kfree(ptr);
 }
···
 	ptr = kmalloc(size, GFP_KERNEL);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 
+	OPTIMIZER_HIDE_VAR(size);
 	KUNIT_EXPECT_KASAN_FAIL(test, memset(ptr + size - 7, 0, 8));
 	kfree(ptr);
 }
···
 	ptr = kmalloc(size, GFP_KERNEL);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 
+	OPTIMIZER_HIDE_VAR(size);
 	KUNIT_EXPECT_KASAN_FAIL(test, memset(ptr + size - 15, 0, 16));
 	kfree(ptr);
 }
···
 	ptr = kmalloc(size, GFP_KERNEL);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 
+	OPTIMIZER_HIDE_VAR(size);
 	KUNIT_EXPECT_KASAN_FAIL(test,
 				memset(ptr, 0, size + KASAN_GRANULE_SIZE));
 	kfree(ptr);
···
 {
 	char *ptr;
 	size_t size = 64;
-	volatile size_t invalid_size = -2;
+	size_t invalid_size = -2;
 
 	/*
 	 * Hardware tag-based mode doesn't check memmove for negative size.
···
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
 
 	memset((char *)ptr, 0, 64);
+	OPTIMIZER_HIDE_VAR(invalid_size);
 	KUNIT_EXPECT_KASAN_FAIL(test,
 		memmove((char *)ptr, (char *)ptr + 4, invalid_size));
 	kfree(ptr);

lib/test_kasan_module.c

···
 		return;
 	}
 
+	OPTIMIZER_HIDE_VAR(size);
+
 	pr_info("out-of-bounds in copy_from_user()\n");
 	unused = copy_from_user(kmem, usermem, size + 1);