platform/netlink: cleanup nla_memcpy()

- use size_t arguments for the memory sizes. While sizes from netlink
  API currently are int typed and inherrently limited, use the more
  appropriate data type.

- rename the arguments. The "count" is really the size of the
  destination buffer.

- return how many bytes we wanted to write (like g_strlcpy()).
  That makes more sense than how many bytes we actually wrote
  because previously, we could not detect truncation.
  Anyway, none of the callers cared about the return-value either
  way.

src/platform/nm-netlink.c

@@ -420,18 +420,33 @@ nla_strlcpy (char *dst,
 	return 0;
 }
 
-int
-nla_memcpy (void *dest, const struct nlattr *src, int count)
+size_t
+nla_memcpy (void *dst, const struct nlattr *nla, size_t dstsize)
 {
-	int minlen;
+	size_t len;
+	int srclen;
 
-	if (!src)
+	if (!nla)
 		return 0;
 
-	minlen = NM_MIN (count, (int) nla_len (src));
-	memcpy (dest, nla_data (src), minlen);
+	srclen = nla_len (nla);
 
-	return minlen;
+	if (srclen <= 0) {
+		nm_assert (srclen == 0);
+		return 0;
+	}
+
+	len = NM_MIN ((size_t) srclen, dstsize);
+	if (len > 0) {
+		/* there is a crucial difference between nla_strlcpy() and nla_memcpy().
+		 * The former always write @dstsize bytes (akin to strncpy()), here, we only
+		 * write the bytes that we actually have (leaving the remainder undefined). */
+		memcpy (dst,
+		        nla_data (nla),
+		        len);
+	}
+
+	return srclen;
 }
 
 int

src/platform/nm-netlink.h

@@ -195,7 +195,24 @@ nla_get_string (const struct nlattr *nla)
 
 size_t nla_strlcpy (char *dst, const struct nlattr *nla, size_t dstsize);
 
-int nla_memcpy (void *dest, const struct nlattr *src, int count);
+size_t nla_memcpy (void *dst, const struct nlattr *nla, size_t dstsize);
+
+#define nla_memcpy_checked_size(dst, nla, dstsize) \
+	G_STMT_START { \
+		void *const _dst = (dst); \
+		const struct nlattr *const _nla = (nla); \
+		const size_t _dstsize = (dstsize); \
+		size_t _srcsize; \
+		\
+		/* assert that, if @nla is given, that it has the exact expected
+		 * size. This implies that the caller previously verified the length
+		 * of the attribute (via minlen/maxlen at nla_parse()). */ \
+		\
+		if (_nla) { \
+			_srcsize = nla_memcpy (_dst, _nla, _dstsize); \
+			nm_assert (_srcsize == _dstsize); \
+		} \
+	} G_STMT_END
 
 int nla_put (struct nl_msg *msg, int attrtype, int datalen, const void *data);