fix undefined pointer comparison in memmove

the comparison must take place in the address space model as an
integer type, since comparing pointers that are not pointing into the
same array is undefined.

the subsequent d<s comparison however is valid, because it's only
reached in the case where the source and dest overlap, in which case
they are necessarily pointing to parts of the same array.

to make the comparison, use an unsigned range check for dist(s,d)>=n,
algebraically !(-n<s-d<n). subtracting n yields !(-2*n<s-d-n<0), which
mapped into unsigned modular arithmetic is !(-2*n<s-d-n) or rather
-2*n>=s-d-n.

diff --git a/src/string/memmove.c b/src/string/memmove.c
index 27f670e1a31fe860ff6ce17a453559ef42706aeb..f225bb30b84be684ef7798f55680bc9b596124b1 100644 (file)
--- a/src/string/memmove.c
+++ b/src/string/memmove.c
@@ -10,7 +10,7 @@ void *memmove(void *dest, const void *src, size_t n)
        const char *s = src;
 
        if (d==s) return d;
-       if (s+n <= d || d+n <= s) return memcpy(d, s, n);
+       if ((uintptr_t)s-(uintptr_t)d-n <= -2*n) return memcpy(d, s, n);
 
        if (d<s) {
                if ((uintptr_t)s % WS == (uintptr_t)d % WS) {