cdefs: Introduce __result_use_or_ignore_check

Try to paper over inconsistent semantics for __warn_unused_result__
between clang and gcc.  See
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66425 for a spirited
discussion of these semantics.

Introduce __result_use_or_ignore_check, which allows callers to
explicitly ignore the return value with a cast to void.  Use that to
restore some checking for copyout() and friends, previously removed in
commit d07acc58d898 ("systm: Relax __result_use_check annotations").

Reviewed by:	olce, rpokala, kib, emaste
MFC after:	1 month
Differential Revision:	https://reviews.freebsd.org/D43426

2 files changed, 20 insertions, 8 deletions

diff --git a/sys/sys/cdefs.h b/sys/sys/cdefs.h
index 206cc569c55a..e47a7072e1f1 100644
--- a/sys/sys/cdefs.h
+++ b/sys/sys/cdefs.h
@@ -326,6 +326,17 @@
 #if __GNUC_PREREQ__(3, 4)
 #define	__fastcall	__attribute__((__fastcall__))
 #define	__result_use_check	__attribute__((__warn_unused_result__))
+#ifdef __clang__
+/*
+ * clang and gcc have different semantics for __warn_unused_result__: the latter
+ * does not permit the use of a void cast to suppress the warning.  Use
+ * __result_use_or_ignore_check in places where a void cast is acceptable.
+ * This can be implemented by [[nodiscard]] from C23.
+ */
+#define	__result_use_or_ignore_check	__result_use_check
+#else
+#define	__result_use_or_ignore_check
+#endif /* !__clang__ */
 #else
 #define	__fastcall
 #define	__result_use_check
diff --git a/sys/sys/systm.h b/sys/sys/systm.h
index 29c8bfc3c768..376a35e7e8e2 100644
--- a/sys/sys/systm.h
+++ b/sys/sys/systm.h
@@ -299,10 +299,11 @@ int __result_use_check copyin(const void * __restrict udaddr,
     void * _Nonnull __restrict kaddr, size_t len);
 int __result_use_check copyin_nofault(const void * __restrict udaddr,
     void * _Nonnull __restrict kaddr, size_t len);
-int copyout(const void * _Nonnull __restrict kaddr,
-    void * __restrict udaddr, size_t len);
-int copyout_nofault(const void * _Nonnull __restrict kaddr,
+int __result_use_or_ignore_check copyout(const void * _Nonnull __restrict kaddr,
     void * __restrict udaddr, size_t len);
+int __result_use_or_ignore_check copyout_nofault(
+    const void * _Nonnull __restrict kaddr, void * __restrict udaddr,
+    size_t len);
 
 #ifdef SAN_NEEDS_INTERCEPTORS
 int	SAN_INTERCEPTOR(copyin)(const void *, void *, size_t);
@@ -323,11 +324,11 @@ int64_t	fuword64(volatile const void *base);
 int __result_use_check fueword(volatile const void *base, long *val);
 int __result_use_check fueword32(volatile const void *base, int32_t *val);
 int __result_use_check fueword64(volatile const void *base, int64_t *val);
-int subyte(volatile void *base, int byte);
-int suword(volatile void *base, long word);
-int suword16(volatile void *base, int word);
-int suword32(volatile void *base, int32_t word);
-int suword64(volatile void *base, int64_t word);
+int __result_use_or_ignore_check subyte(volatile void *base, int byte);
+int __result_use_or_ignore_check suword(volatile void *base, long word);
+int __result_use_or_ignore_check suword16(volatile void *base, int word);
+int __result_use_or_ignore_check suword32(volatile void *base, int32_t word);
+int __result_use_or_ignore_check suword64(volatile void *base, int64_t word);
 uint32_t casuword32(volatile uint32_t *base, uint32_t oldval, uint32_t newval);
 u_long	casuword(volatile u_long *p, u_long oldval, u_long newval);
 int	casueword32(volatile uint32_t *base, uint32_t oldval, uint32_t *oldvalp,