rtnetlink: make the "split" NLM_DONE handling generic

Jaroslav reports Dell's OMSA Systems Management Data Engine
expects NLM_DONE in a separate recvmsg(), both for rtnl_dump_ifinfo()
and inet_dump_ifaddr(). We already added a similar fix previously in
commit 460b0d33 ("inet: bring NLM_DONE out to a separate recv() again")

Instead of modifying all the dump handlers, and making them look
different than modern for_each_netdev_dump()-based dump handlers -
put the workaround in rtnetlink code. This will also help us move
the custom rtnl-locking from af_netlink in the future (in net-next).

Note that this change is not touching rtnl_dump_all(). rtnl_dump_all()
is different kettle of fish and a potential problem. We now mix families
in a single recvmsg(), but NLM_DONE is not coalesced.

Tested:

  ./cli.py --dbg-small-recv 4096 --spec netlink/specs/rt_addr.yaml \
           --dump getaddr --json '{"ifa-family": 2}'

  ./cli.py --dbg-small-recv 4096 --spec netlink/specs/rt_route.yaml \
           --dump getroute --json '{"rtm-family": 2}'

  ./cli.py --dbg-small-recv 4096 --spec netlink/specs/rt_link.yaml \
           --dump getlink

Fixes: 3e41af90 ("rtnetlink: use xarray iterator to implement rtnl_dump_ifinfo()")
Fixes: cdb2f80f ("inet: use xa_array iterator to implement inet_dump_ifaddr()")
Reported-by: Jaroslav Pulchart <jaroslav.pulchart@gooddata.com>
Link: https://lore.kernel.org/all/CAK8fFZ7MKoFSEzMBDAOjoUt+vTZRRQgLDNXEOfdCCXSoXXKE0g@mail.gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/rtnetlink.h

@@ -13,6 +13,7 @@ enum rtnl_link_flags {
 	RTNL_FLAG_DOIT_UNLOCKED		= BIT(0),
 	RTNL_FLAG_BULK_DEL_SUPPORTED	= BIT(1),
 	RTNL_FLAG_DUMP_UNLOCKED		= BIT(2),
+	RTNL_FLAG_DUMP_SPLIT_NLM_DONE	= BIT(3),	/* legacy behavior */
 };
 
 enum rtnl_kinds {

net/core/rtnetlink.c

@@ -6484,6 +6484,46 @@ static int rtnl_mdb_del(struct sk_buff *skb, struct nlmsghdr *nlh,
 
 /* Process one rtnetlink message. */
 
+static int rtnl_dumpit(struct sk_buff *skb, struct netlink_callback *cb)
+{
+	rtnl_dumpit_func dumpit = cb->data;
+	int err;
+
+	/* Previous iteration have already finished, avoid calling->dumpit()
+	 * again, it may not expect to be called after it reached the end.
+	 */
+	if (!dumpit)
+		return 0;
+
+	err = dumpit(skb, cb);
+
+	/* Old dump handlers used to send NLM_DONE as in a separate recvmsg().
+	 * Some applications which parse netlink manually depend on this.
+	 */
+	if (cb->flags & RTNL_FLAG_DUMP_SPLIT_NLM_DONE) {
+		if (err < 0 && err != -EMSGSIZE)
+			return err;
+		if (!err)
+			cb->data = NULL;
+
+		return skb->len;
+	}
+	return err;
+}
+
+static int rtnetlink_dump_start(struct sock *ssk, struct sk_buff *skb,
+				const struct nlmsghdr *nlh,
+				struct netlink_dump_control *control)
+{
+	if (control->flags & RTNL_FLAG_DUMP_SPLIT_NLM_DONE) {
+		WARN_ON(control->data);
+		control->data = control->dump;
+		control->dump = rtnl_dumpit;
+	}
+
+	return netlink_dump_start(ssk, skb, nlh, control);
+}
+
 static int rtnetlink_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh,
 			     struct netlink_ext_ack *extack)
 {
@@ -6548,7 +6588,7 @@ static int rtnetlink_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh,
 				.module		= owner,
 				.flags		= flags,
 			};
-			err = netlink_dump_start(rtnl, skb, nlh, &c);
+			err = rtnetlink_dump_start(rtnl, skb, nlh, &c);
 			/* netlink_dump_start() will keep a reference on
 			 * module if dump is still in progress.
 			 */
@@ -6694,7 +6734,7 @@ void __init rtnetlink_init(void)
 	register_netdevice_notifier(&rtnetlink_dev_notifier);
 
 	rtnl_register(PF_UNSPEC, RTM_GETLINK, rtnl_getlink,
-		      rtnl_dump_ifinfo, 0);
+		      rtnl_dump_ifinfo, RTNL_FLAG_DUMP_SPLIT_NLM_DONE);
 	rtnl_register(PF_UNSPEC, RTM_SETLINK, rtnl_setlink, NULL, 0);
 	rtnl_register(PF_UNSPEC, RTM_NEWLINK, rtnl_newlink, NULL, 0);
 	rtnl_register(PF_UNSPEC, RTM_DELLINK, rtnl_dellink, NULL, 0);

net/ipv4/devinet.c

@@ -2805,7 +2805,7 @@ void __init devinet_init(void)
 	rtnl_register(PF_INET, RTM_NEWADDR, inet_rtm_newaddr, NULL, 0);
 	rtnl_register(PF_INET, RTM_DELADDR, inet_rtm_deladdr, NULL, 0);
 	rtnl_register(PF_INET, RTM_GETADDR, NULL, inet_dump_ifaddr,
-		      RTNL_FLAG_DUMP_UNLOCKED);
+		      RTNL_FLAG_DUMP_UNLOCKED | RTNL_FLAG_DUMP_SPLIT_NLM_DONE);
 	rtnl_register(PF_INET, RTM_GETNETCONF, inet_netconf_get_devconf,
 		      inet_netconf_dump_devconf,
 		      RTNL_FLAG_DOIT_UNLOCKED | RTNL_FLAG_DUMP_UNLOCKED);

net/ipv4/fib_frontend.c

@@ -1050,11 +1050,6 @@ static int inet_dump_fib(struct sk_buff *skb, struct netlink_callback *cb)
 			e++;
 		}
 	}
-
-	/* Don't let NLM_DONE coalesce into a message, even if it could.
-	 * Some user space expects NLM_DONE in a separate recv().
-	 */
-	err = skb->len;
 out:
 
 	cb->args[1] = e;
@@ -1665,5 +1660,5 @@ void __init ip_fib_init(void)
 	rtnl_register(PF_INET, RTM_NEWROUTE, inet_rtm_newroute, NULL, 0);
 	rtnl_register(PF_INET, RTM_DELROUTE, inet_rtm_delroute, NULL, 0);
 	rtnl_register(PF_INET, RTM_GETROUTE, NULL, inet_dump_fib,
-		      RTNL_FLAG_DUMP_UNLOCKED);
+		      RTNL_FLAG_DUMP_UNLOCKED | RTNL_FLAG_DUMP_SPLIT_NLM_DONE);
 }