Subject: [PATCH v2 net-next 01/11] netlink: Add answer_flags to netlink_callback

From: David Ahern <[email-masked]>
Date: Mon, 15 Oct 2018 18:56:41 -0700

From: David Ahern <[email-masked]>

With dump filtering we need a way to ensure the NLM_F_DUMP_FILTERED
flag is set on a message back to the user if the data returned is
influenced by some input attributes. Normally this can be done as
messages are added to the skb, but if the filter results in no data
being returned, the user could be confused as to why.

This patch adds answer_flags to the netlink_callback allowing dump
handlers to set the NLM_F_DUMP_FILTERED at a minimum in the
NLMSG_DONE message ensuring the flag gets back to the user.

The netlink_callback space is initialized to 0 via a memset in
__netlink_dump_start, so init of the new answer_flags is covered.

Signed-off-by: David Ahern <[email-masked]>
---
 include/linux/netlink.h  | 1 +
 net/netlink/af_netlink.c | 3 ++-
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/linux/netlink.h b/include/linux/netlink.h
index 72580f1a72a2..4da90a6ab536 100644
--- a/include/linux/netlink.h
+++ b/include/linux/netlink.h
@@ -180,6 +180,7 @@ struct netlink_callback {
 	u16			family;
 	u16			min_dump_alloc;
 	bool			strict_check;
+	u16			answer_flags;
 	unsigned int		prev_seq, seq;
 	long			args[6];
 };
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index e613a9f89600..6bb9f3cde0b0 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -2257,7 +2257,8 @@ static int netlink_dump(struct sock *sk)
 	}
 
 	nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE,
-			       sizeof(nlk->dump_done_errno), NLM_F_MULTI);
+			       sizeof(nlk->dump_done_errno),
+			       NLM_F_MULTI | cb->answer_flags);
 	if (WARN_ON(!nlh))
 		goto errout_skb;
 
-- 
2.11.0