[PATCH] netlink: decode NLMSG_DONE message

[Dmitry V. Levin]

On Sun, Jun 04, 2017 at 03:57:08PM +0800, JingPiao Chen wrote:
> On Fri, Jun 02, 2017 at 06:25:28PM +0300, Dmitry V. Levin wrote:
> > On Fri, May 05, 2017 at 06:21:17PM +0800, JingPiao Chen wrote:
> > > * netlink.c (decode_payload): Decode NLMSG_DONE message.
> > > * tests/netlink_protocol.c: Add check for decoding
> > > of NLMSG_DONE message.
> >
> > Unlike NLMSG_ERROR, there are no universal NLMSG_DONE messages.  While
> > many NLMSG_DONE messages indeed have payload containing just one integer,
> > there are exceptions.  For example,
> >
> > net/netfilter/nfnetlink_log.c:__nfulnl_send() sends struct nfgenmsg,
> > drivers/connector/connector.c:cn_netlink_send_mult() sends struct cn_msg,
> > drivers/scsi/scsi_transport_iscsi.c:iscsi_if_send_reply() sends struct
> iscsi_uevent,
> > kernel/auditfilter.c:audit_list_rules() sends struct audit_rule_data,
> > kernel/audit.c:audit_get_feature() sends struct audit_features,
> > kernel/audit.c:audit_receive_msg() sends struct audit_status,
> audit_sig_info,
> > audit_tty_status, and so on.
> 
> I am decoding audit, I understand kernel/auditfilter.c: audit_list_rules()
> sends struct audit_rule_data, but I do not understand
> kernel/audit.c: audit_get_feature() sends struct audit_features
> 
> static int audit_get_feature(struct sk_buff *skb)
> {
> ...
> audit_send_reply(skb, seq, AUDIT_GET_FEATURE, 0, 0, &af, sizeof(af));
> ...
> }
> 
> done = 0, why is it send NLMSG_DONE messages?
> Can you help me understand this? Thank you.

I must have missed this case, apparently, audit_get_feature does not send
NLMSG_DONE messages.

> > I suggest implementing a default decoder of NLMSG_DONE messages that
> > would print the integer in case of len == sizeof(int) and fall back
> > to  printstrn for other lengths.
> 
> I updated the patch. default decoder[1]:

Thanks.

> diff --git a/netlink.c b/netlink.c
> index 678343c..104a65f 100644
> --- a/netlink.c
> +++ b/netlink.c
> @@ -120,6 +120,12 @@ decode_payload(struct tcb *const tcp,
>   if (nlmsghdr->nlmsg_type == NLMSG_ERROR) {
>   decode_nlmsgerr(tcp, addr, len);
>   return;
> + } else if (nlmsghdr->nlmsg_type == NLMSG_DONE && len == sizeof(int)) {
> + int total_len;

btw, why do you call it total_len?  This integer can contain anything.


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