diff --git a/include/ssp/ssp.h b/include/ssp/ssp.h
--- a/include/ssp/ssp.h
+++ b/include/ssp/ssp.h
@@ -87,12 +87,19 @@
 #define __ssp_redirect0(rtype, fun, args, call) \
     __ssp_redirect_raw(rtype, fun, fun, args, call, 1, __ssp_bos0, __len)
 
-/*
- * Take caution when using __ssp_overlap!  Don't use it in contexts where we
- * can end up with double-evaluation of a statement with some side-effects.
- */
-#define __ssp_overlap(a, b, l) \
-    (((a) <= (b) && (b) < (a) + (l)) || ((b) <= (a) && (a) < (b) + (l)))
+#include <machine/_stdint.h>
+
+static inline int
+__ssp_overlap(const void *leftp, const void *rightp, size_t sz)
+{
+	__uintptr_t left = (__uintptr_t)leftp;
+	__uintptr_t right = (__uintptr_t)rightp;
+
+	if (left <= right)
+		return (SIZE_MAX - sz < left || right < left + sz);
+
+	return (SIZE_MAX - sz < right || left < right + sz);
+}
 
 #include <sys/_iovec.h>
 
diff --git a/include/ssp/string.h b/include/ssp/string.h
--- a/include/ssp/string.h
+++ b/include/ssp/string.h
@@ -125,7 +125,7 @@
 	if (len > slen)
 		__chk_fail();
 
-	if (__ssp_overlap((const char *)src, (const char *)buf, len))
+	if (__ssp_overlap(src, buf, len))
 		__chk_fail();
 
 	return (__ssp_real(mempcpy)(buf, src, len));
diff --git a/include/ssp/wchar.h b/include/ssp/wchar.h
--- a/include/ssp/wchar.h
+++ b/include/ssp/wchar.h
@@ -31,6 +31,15 @@
 
 #if __SSP_FORTIFY_LEVEL > 0
 
+static inline int
+__ssp_wchar_overlap(const void *leftp, const void *rightp, size_t len)
+{
+
+	if (len > SIZE_MAX / sizeof(wchar_t))
+		return (1);
+	return (__ssp_overlap(leftp, rightp, len * sizeof(wchar_t)));
+}
+
 /*
  * __ssp_wbos for w*() calls where the size parameters are in sizeof(wchar_t)
  * units, so the result needs to be scaled appropriately.
@@ -54,7 +63,7 @@
 
 	if (len > slen)
 		__chk_fail();
-	if (__ssp_overlap(src, buf, len))
+	if (__ssp_wchar_overlap(src, buf, len))
 		__chk_fail();
 
 	return (__ssp_real(wmemcpy)(buf, src, len));
@@ -67,7 +76,7 @@
 
 	if (len > slen)
 		__chk_fail();
-	if (__ssp_overlap(src, buf, len))
+	if (__ssp_wchar_overlap(src, buf, len))
 		__chk_fail();
 
 	return (__ssp_real(wmempcpy)(buf, src, len));
@@ -102,7 +111,7 @@
 
 	if (len >= slen)
 		__chk_fail();
-	if (__ssp_overlap(buf, src, len))
+	if (__ssp_wchar_overlap(buf, src, len))
 		__chk_fail();
 
 	(void)__ssp_real(wmemcpy)(buf, src, len + 1);
@@ -116,7 +125,7 @@
 
 	if (len > slen)
 		__chk_fail();
-	if (__ssp_overlap(buf, src, len))
+	if (__ssp_wchar_overlap(buf, src, len))
 		__chk_fail();
 
 	return (__ssp_real(wcpncpy)(buf, src, len));
@@ -155,7 +164,7 @@
 
 	if (len > slen)
 		__chk_fail();
-	if (__ssp_overlap(buf, src, len))
+	if (__ssp_wchar_overlap(buf, src, len))
 		__chk_fail();
 
 	return (__ssp_real(wmemcpy)(buf, src, len));
@@ -170,7 +179,7 @@
 		return (buf);
 	if (len > slen)
 		__chk_fail();
-	if (__ssp_overlap(buf, src, len))
+	if (__ssp_wchar_overlap(buf, src, len))
 		__chk_fail();
 
 	return (__ssp_real(wcsncat)(buf, src, len));
@@ -183,7 +192,7 @@
 
 	if (len > slen)
 		__chk_fail();
-	if (__ssp_overlap(buf, src, len))
+	if (__ssp_wchar_overlap(buf, src, len))
 		__chk_fail();
 
 	return (__ssp_real(wcslcat)(buf, src, len));
@@ -196,7 +205,7 @@
 
 	if (len > slen)
 		__chk_fail();
-	if (__ssp_overlap(buf, src, len))
+	if (__ssp_wchar_overlap(buf, src, len))
 		__chk_fail();
 
 	return (__ssp_real(wcsncpy)(buf, src, len));
@@ -209,7 +218,7 @@
 
 	if (len > slen)
 		__chk_fail();
-	if (__ssp_overlap(buf, src, len))
+	if (__ssp_wchar_overlap(buf, src, len))
 		__chk_fail();
 
 	return (__ssp_real(wcslcpy)(buf, src, len));
diff --git a/lib/libc/secure/memcpy_chk.c b/lib/libc/secure/memcpy_chk.c
--- a/lib/libc/secure/memcpy_chk.c
+++ b/lib/libc/secure/memcpy_chk.c
@@ -44,7 +44,7 @@
 	if (len > slen)
 		__chk_fail();
 
-	if (__ssp_overlap((const char *)src, (const char *)dst, len))
+	if (__ssp_overlap(src, dst, len))
 		__chk_fail();
 
 	return (memcpy(dst, src, len));