fix aliasing-based undefined behavior in string functions

use the GNU C may_alias attribute if available, and fallback to naive
byte-by-byte loops if __GNUC__ is not defined.

this patch has been written to minimize changes so that history
remains reviewable; it does not attempt to bring the affected code
into a more consistent or elegant form.

1 files changed, 5 insertions, 3 deletions

diff --git a/src/string/stpncpy.c b/src/string/stpncpy.c
index d6d92ffc..f57fa6b7 100644
--- a/src/string/stpncpy.c
+++ b/src/string/stpncpy.c
@@ -9,9 +9,10 @@
 
 char *__stpncpy(char *restrict d, const char *restrict s, size_t n)
 {
-	size_t *wd;
-	const size_t *ws;
-
+#ifdef __GNUC__
+	typedef size_t __attribute__((__may_alias__)) word;
+	word *wd;
+	const word *ws;
 	if (((uintptr_t)s & ALIGN) == ((uintptr_t)d & ALIGN)) {
 		for (; ((uintptr_t)s & ALIGN) && n && (*d=*s); n--, s++, d++);
 		if (!n || !*s) goto tail;
@@ -20,6 +21,7 @@ char *__stpncpy(char *restrict d, const char *restrict s, size_t n)
 		       n-=sizeof(size_t), ws++, wd++) *wd = *ws;
 		d=(void *)wd; s=(const void *)ws;
 	}
+#endif
 	for (; n && (*d=*s); n--, s++, d++);
 tail:
 	memset(d, 0, n);