netlink: add attr parser utility functions

- nlattr_get_chara() to read a string into a char array, rather than to a char *
 - nlattr_get_bytes() to read an arbitrary (fixed length) byte sequence
 - nlattr_get_nested_ptr() to read a nested type to a struct foo *, rather than struct foo

Sponsored by:	Rubicon Communications, LLC ("Netgate")
Differential Revision:	https://reviews.freebsd.org/D42221

sys/netlink/netlink_message_parser.c

@@ -428,6 +428,23 @@ nlattr_get_ifpz(struct nlattr *nla, struct nl_pstate *npt, const void *arg, void
 	return (nlattr_get_ifp_internal(nla, npt, target, true));
 }
 
+int
+nlattr_get_chara(struct nlattr *nla, struct nl_pstate *npt, const void *arg, void *target)
+{
+	int maxlen = NLA_DATA_LEN(nla);
+	int target_size = (size_t)arg;
+	int len = strnlen((char *)NLA_DATA(nla), maxlen);
+
+	if (__predict_false(len >= maxlen) || __predict_false(len >= target_size)) {
+		NLMSG_REPORT_ERR_MSG(npt, "nla type %d size(%u) is not NULL-terminated or longer than %u",
+		    nla->nla_type, maxlen, target_size);
+		return (EINVAL);
+	}
+
+	strncpy((char *)target, (char *)NLA_DATA(nla), target_size);
+	return (0);
+}
+
 int
 nlattr_get_string(struct nlattr *nla, struct nl_pstate *npt, const void *arg, void *target)
 {
@@ -457,6 +474,20 @@ nlattr_get_stringn(struct nlattr *nla, struct nl_pstate *npt, const void *arg, v
 	*((char **)target) = buf;
 	return (0);
 }
+
+int
+nlattr_get_bytes(struct nlattr *nla, struct nl_pstate *npt, const void *arg, void *target)
+{
+	size_t size = (size_t)arg;
+
+	if (NLA_DATA_LEN(nla) != size)
+		return (EINVAL);
+
+	memcpy(target, NLA_DATA(nla), size);
+
+	return (0);
+}
+
 int
 nlattr_get_nla(struct nlattr *nla, struct nl_pstate *npt, const void *arg, void *target)
 {
@@ -476,6 +507,17 @@ nlattr_get_nested(struct nlattr *nla, struct nl_pstate *npt, const void *arg, vo
 	return (error);
 }
 
+int
+nlattr_get_nested_ptr(struct nlattr *nla, struct nl_pstate *npt, const void *arg, void *target)
+{
+	const struct nlhdr_parser *p = (const struct nlhdr_parser *)arg;
+	int error;
+
+	/* Assumes target points to the beginning of the structure */
+	error = nl_parse_header(NLA_DATA(nla), NLA_DATA_LEN(nla), p, npt, *(void **)target);
+	return (error);
+}
+
 int
 nlf_get_ifp(void *src, struct nl_pstate *npt, void *target)
 {

sys/netlink/netlink_message_parser.h

@@ -187,14 +187,20 @@ int nlattr_get_ifpz(struct nlattr *nla, struct nl_pstate *npt,
     const void *arg, void *target);
 int nlattr_get_ipvia(struct nlattr *nla, struct nl_pstate *npt,
     const void *arg, void *target);
+int nlattr_get_chara(struct nlattr *nla, struct nl_pstate *npt,
+    const void *arg, void *target);
 int nlattr_get_string(struct nlattr *nla, struct nl_pstate *npt,
     const void *arg, void *target);
 int nlattr_get_stringn(struct nlattr *nla, struct nl_pstate *npt,
     const void *arg, void *target);
+int nlattr_get_bytes(struct nlattr *nla, struct nl_pstate *npt,
+    const void *arg, void *target);
 int nlattr_get_nla(struct nlattr *nla, struct nl_pstate *npt,
     const void *arg, void *target);
 int nlattr_get_nested(struct nlattr *nla, struct nl_pstate *npt,
     const void *arg, void *target);
+int nlattr_get_nested_ptr(struct nlattr *nla, struct nl_pstate *npt,
+    const void *arg, void *target);
 
 bool nlmsg_report_err_msg(struct nl_pstate *npt, const char *fmt, ...);