netlink: advertise incomplete dumps

Consider the following situation:
 * a dump that would show 8 entries, four in the first
   round, and four in the second
 * between the first and second rounds, 6 entries are
   removed
 * now the second round will not show any entry, and
   even if there is a sequence/generation counter the
   application will not know

To solve this problem, add a new flag NLM_F_DUMP_INTR
to the netlink header that indicates the dump wasn't
consistent, this flag can also be set on the MSG_DONE
message that terminates the dump, and as such above
situation can be detected.

To achieve this, add a sequence counter to the netlink
callback struct. Of course, netlink code still needs
to use this new functionality. The correct way to do
that is to always set cb->seq when a dumpit callback
is invoked and call nl_dump_check_consistent() for
each new message. The core code will also call this
function for the final MSG_DONE message.

To make it usable with generic netlink, a new function
genlmsg_nlhdr() is needed to obtain the netlink header
from the genetlink user header.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/netlink.h b/include/linux/netlink.h
index 4c4ac3f..8d1bcec 100644
--- a/include/linux/netlink.h
+++ b/include/linux/netlink.h
@@ -48,6 +48,7 @@ struct nlmsghdr {
 #define NLM_F_MULTI		2	/* Multipart message, terminated by NLMSG_DONE */
 #define NLM_F_ACK		4	/* Reply with ack, with zero or error code */
 #define NLM_F_ECHO		8	/* Echo this request 		*/
+#define NLM_F_DUMP_INTR		16	/* Dump was inconsistent due to sequence change */
 
 /* Modifiers to GET request */
 #define NLM_F_ROOT	0x100	/* specify tree	root	*/
@@ -221,6 +222,7 @@ struct netlink_callback {
 					struct netlink_callback *cb);
 	int			(*done)(struct netlink_callback *cb);
 	int			family;
+	unsigned int		prev_seq, seq;
 	long			args[6];
 };