src: initial extended netlink error reporting

This patch correlates the in-kernel extended netlink error offset and
the location information.

Assuming 'foo' table does not exist, then error reporting shows:

 # nft delete table foo
 Error: Could not process rule: No such file or directory
 delete table foo
              ^^^

Similarly, if table uniquely identified by handle '1234' does not exist,
then error reporting shows:

 # nft delete table handle 1234
 Error: Could not process rule: No such file or directory
 delete table handle 1234
                     ^^^^

 Assuming 'bar' chain does not exists in the kernel, while 'foo' does:

 # nft delete chain foo bar
 Error: Could not process rule: No such file or directory
 delete chain foo bar
                  ^^^

 This also gives us a hint when adding rules:

 # nft add rule ip foo bar counter
 Error: Could not process rule: No such file or directory
 add rule ip foo bar counter
             ^^^

This is based on ("src: basic support for extended netlink errors") from
Florian Westphal, posted in 2018, with no netlink offset correlation
support.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/src/monitor.c b/src/monitor.c
index 142cc929..bb269c02 100644
--- a/src/monitor.c
+++ b/src/monitor.c
@@ -906,7 +906,8 @@ static int netlink_events_cb(const struct nlmsghdr *nlh, void *data)
 
 int netlink_echo_callback(const struct nlmsghdr *nlh, void *data)
 {
-	struct netlink_ctx *ctx = data;
+	struct netlink_cb_data *nl_cb_data = data;
+	struct netlink_ctx *ctx = nl_cb_data->nl_ctx;
 	struct netlink_mon_handler echo_monh = {
 		.format = NFTNL_OUTPUT_DEFAULT,
 		.ctx = ctx,