platform/netlink: add nlmsg_parse_error() helper

src/libnm-glib-aux/nm-errno.c

@@ -28,6 +28,7 @@ static NM_UTILS_LOOKUP_STR_DEFINE(
     NM_UTILS_LOOKUP_STR_ITEM(NME_NL_ATTRSIZE, "NME_NL_ATTRSIZE"),
     NM_UTILS_LOOKUP_STR_ITEM(NME_NL_BAD_SOCK, "NME_NL_BAD_SOCK"),
     NM_UTILS_LOOKUP_STR_ITEM(NME_NL_DUMP_INTR, "NME_NL_DUMP_INTR"),
+    NM_UTILS_LOOKUP_STR_ITEM(NME_NL_MSG_INVAL, "NME_NL_MSG_INVAL"),
     NM_UTILS_LOOKUP_STR_ITEM(NME_NL_MSG_OVERFLOW, "NME_NL_MSG_OVERFLOW"),
     NM_UTILS_LOOKUP_STR_ITEM(NME_NL_MSG_TOOSHORT, "NME_NL_MSG_TOOSHORT"),
     NM_UTILS_LOOKUP_STR_ITEM(NME_NL_MSG_TRUNC, "NME_NL_MSG_TRUNC"),

src/libnm-glib-aux/nm-errno.h

@@ -39,6 +39,7 @@ enum _NMErrno {
 
     /* netlink errors. */
     NME_NL_SEQ_MISMATCH,
+    NME_NL_MSG_INVAL,
     NME_NL_MSG_TRUNC,
     NME_NL_MSG_TOOSHORT,
     NME_NL_DUMP_INTR,

src/libnm-platform/nm-netlink.c

@@ -291,6 +291,52 @@ nlmsg_reserve(struct nl_msg *n, uint32_t len, uint32_t pad)
 
 /*****************************************************************************/
 
+int
+nlmsg_parse_error(const struct nlmsghdr *nlh, const char **out_extack_msg)
+{
+    const struct nlmsgerr *e;
+
+    nm_assert(nlh);
+
+    NM_SET_OUT(out_extack_msg, NULL);
+
+    if (nlh->nlmsg_type != NLMSG_ERROR)
+        return -NME_NL_MSG_INVAL;
+
+    if (nlh->nlmsg_len < nlmsg_size(sizeof(struct nlmsgerr))) {
+        /* Truncated error message, the default action
+         * is to stop parsing. The user may overrule
+         * this action by returning NL_SKIP or
+         * NL_PROCEED (dangerous) */
+        return -NME_NL_MSG_TRUNC;
+    }
+
+    e = nlmsg_data(nlh);
+
+    if (!e->error)
+        return 0;
+
+    if (NM_FLAGS_HAS(nlh->nlmsg_flags, NLM_F_ACK_TLVS) && out_extack_msg
+        && nlh->nlmsg_len >= sizeof(*e) + e->msg.nlmsg_len) {
+        static const struct nla_policy policy[] = {
+            [NLMSGERR_ATTR_MSG]  = {.type = NLA_STRING},
+            [NLMSGERR_ATTR_OFFS] = {.type = NLA_U32},
+        };
+        struct nlattr *tb[G_N_ELEMENTS(policy)];
+        struct nlattr *tlvs;
+
+        tlvs = (struct nlattr *) ((char *) e + sizeof(*e) + e->msg.nlmsg_len - NLMSG_HDRLEN);
+        if (nla_parse_arr(tb, tlvs, nlh->nlmsg_len - sizeof(*e) - e->msg.nlmsg_len, policy) >= 0) {
+            if (tb[NLMSGERR_ATTR_MSG])
+                *out_extack_msg = nla_get_string(tb[NLMSGERR_ATTR_MSG]);
+        }
+    }
+
+    return -nm_errno_from_native(e->error);
+}
+
+/*****************************************************************************/
+
 struct nlattr *
 nla_reserve(struct nl_msg *msg, int attrtype, int attrlen)
 {

src/libnm-platform/nm-netlink.h

@@ -497,6 +497,8 @@ nlmsg_find_attr(struct nlmsghdr *nlh, int hdrlen, int attrtype)
     return nla_find(nlmsg_attrdata(nlh, hdrlen), nlmsg_attrlen(nlh, hdrlen), attrtype);
 }
 
+int nlmsg_parse_error(const struct nlmsghdr *nlh, const char **out_extack_msg);
+
 int nlmsg_parse(const struct nlmsghdr   *nlh,
                 int                      hdrlen,
                 struct nlattr           *tb[],