perform size checks on various operations

Signed-off-by: Tavi <tavi@divested.dev>
Co-authored-by: =?UTF-8?q?Christian=20G=C3=B6ttsche?= <cgzones@googlemail.com>

memmove.c

@@ -0,0 +1,50 @@
+#include "musl.h"
+
+/*
+ * Copied from musl libc version 1.2.5 licensed under the MIT license
+ *
+ * Christian Göttsche: Added const qualifiers to retain const correctness.
+ */
+
+#include <string.h>
+#include <stdint.h>
+
+#ifdef __GNUC__
+typedef __attribute__((__may_alias__)) size_t WT;
+#define WS (sizeof(WT))
+#endif
+
+void *musl_memmove(void *dest, const void *src, size_t n)
+{
+	char *d = dest;
+	const char *s = src;
+
+	if (d==s) return d;
+	if ((uintptr_t)s-(uintptr_t)d-n <= -2*n) return musl_memcpy(d, s, n);
+
+	if (d<s) {
+#ifdef __GNUC__
+		if ((uintptr_t)s % WS == (uintptr_t)d % WS) {
+			while ((uintptr_t)d % WS) {
+				if (!n--) return dest;
+				*d++ = *s++;
+			}
+			for (; n>=WS; n-=WS, d+=WS, s+=WS) *(WT *)d = *(const WT *)s;
+		}
+#endif
+		for (; n; n--) *d++ = *s++;
+	} else {
+#ifdef __GNUC__
+		if ((uintptr_t)s % WS == (uintptr_t)d % WS) {
+			while ((uintptr_t)(d+n) % WS) {
+				if (!n--) return dest;
+				d[n] = s[n];
+			}
+			while (n>=WS) n-=WS, *(WT *)(d+n) = *(const WT *)(s+n);
+		}
+#endif
+		while (n) n--, d[n] = s[n];
+	}
+
+	return dest;
+}