sys: Add sys/ckdint.h

We have a C23 stdckdint.h header for userspace, which provides checked
addition, subtraction and multiplication.  We lack similar helpers in
the kernel, where they are regularly needed.

Let's just adopt the C23 macros.  For bonus points, I added a wrapper to
ensure that ignored an return value is raised as an error by the
compiler.

Reviewed by:	kib, emaste
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D58773

include/stdckdint.h

@@ -7,33 +7,11 @@
 #ifndef __STDC_VERSION_STDCKDINT_H__
 #define __STDC_VERSION_STDCKDINT_H__ 202311L
 
-#include <sys/cdefs.h>
+#include <sys/_visible.h>
 
 #if __BSD_VISIBLE || __ISO_C_VISIBLE >= 2023
 
-#if __GNUC_PREREQ__(5, 1) || __has_builtin(__builtin_add_overflow)
-#define ckd_add(result, a, b)						\
-	__builtin_add_overflow((a), (b), (result))
-#else
-#define ckd_add(result, a, b)						\
-	_Static_assert(0, "checked addition not supported")
-#endif
-
-#if __GNUC_PREREQ__(5, 1) || __has_builtin(__builtin_sub_overflow)
-#define ckd_sub(result, a, b)						\
-	__builtin_sub_overflow((a), (b), (result))
-#else
-#define ckd_sub(result, a, b)						\
-	_Static_assert(0, "checked subtraction not supported")
-#endif
-
-#if __GNUC_PREREQ__(5, 1) || __has_builtin(__builtin_mul_overflow)
-#define ckd_mul(result, a, b)						\
-	__builtin_mul_overflow((a), (b), (result))
-#else
-#define ckd_mul(result, a, b)						\
-	_Static_assert(0, "checked multiplication not supported")
-#endif
+#include <sys/ckdint.h>
 
 #endif
 

sys/sys/ckdint.h

@@ -0,0 +1,47 @@
+/*-
+ * Copyright (c) 2023 Dag-Erling Smørgrav
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#ifndef _SYS_CKDINT_H_
+#define	_SYS_CKDINT_H_
+
+#include <sys/cdefs.h>
+
+#ifdef _KERNEL
+/* If you're not checking the return value, what's the point? */
+__nodiscard static inline bool
+__ckd_result(bool result)
+{
+	return (result);
+}
+#else
+#define	__ckd_result(result)	(result)
+#endif
+
+#if __GNUC_PREREQ__(5, 1) || __has_builtin(__builtin_add_overflow)
+#define ckd_add(result, a, b)						\
+	__ckd_result(__builtin_add_overflow((a), (b), (result)))
+#else
+#define ckd_add(result, a, b)						\
+	_Static_assert(0, "checked addition not supported")
+#endif
+
+#if __GNUC_PREREQ__(5, 1) || __has_builtin(__builtin_sub_overflow)
+#define ckd_sub(result, a, b)						\
+	__ckd_result(__builtin_sub_overflow((a), (b), (result)))
+#else
+#define ckd_sub(result, a, b)						\
+	_Static_assert(0, "checked subtraction not supported")
+#endif
+
+#if __GNUC_PREREQ__(5, 1) || __has_builtin(__builtin_mul_overflow)
+#define ckd_mul(result, a, b)						\
+	__ckd_result(__builtin_mul_overflow((a), (b), (result)))
+#else
+#define ckd_mul(result, a, b)						\
+	_Static_assert(0, "checked multiplication not supported")
+#endif
+
+#endif /* _SYS_CKDINT_H_ */