refcount: Annotated intentional signed integer wrap-around

Mark the various refcount_t functions with __signed_wrap, as we depend
on the wrapping behavior to detect the overflow and perform saturation.
Silences warnings seen with the LKDTM REFCOUNT_* tests:

  UBSAN: signed-integer-overflow in ../include/linux/refcount.h:189:11
  2147483647 + 1 cannot be represented in type 'int'
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/r/20240221051634.work.287-kees@kernel.orgSigned-off-by: Kees Cook <keescook@chromium.org>

include/linux/refcount.h

@@ -136,7 +136,8 @@ static inline unsigned int refcount_read(const refcount_t *r)
 	return atomic_read(&r->refs);
 }
 
-static inline __must_check bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp)
+static inline __must_check __signed_wrap
+bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp)
 {
 	int old = refcount_read(r);
 
@@ -177,7 +178,8 @@ static inline __must_check bool refcount_add_not_zero(int i, refcount_t *r)
 	return __refcount_add_not_zero(i, r, NULL);
 }
 
-static inline void __refcount_add(int i, refcount_t *r, int *oldp)
+static inline __signed_wrap
+void __refcount_add(int i, refcount_t *r, int *oldp)
 {
 	int old = atomic_fetch_add_relaxed(i, &r->refs);
 
@@ -256,7 +258,8 @@ static inline void refcount_inc(refcount_t *r)
 	__refcount_inc(r, NULL);
 }
 
-static inline __must_check bool __refcount_sub_and_test(int i, refcount_t *r, int *oldp)
+static inline __must_check __signed_wrap
+bool __refcount_sub_and_test(int i, refcount_t *r, int *oldp)
 {
 	int old = atomic_fetch_sub_release(i, &r->refs);