rtnetlink: add RTNL_FLAG_DUMP_UNLOCKED flag

Similarly to RTNL_FLAG_DOIT_UNLOCKED, this new flag
allows dump operations registered via rtnl_register()
or rtnl_register_module() to opt-out from RTNL protection.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netlink.h

@@ -291,6 +291,7 @@ struct netlink_callback {
 	u16			answer_flags;
 	u32			min_dump_alloc;
 	unsigned int		prev_seq, seq;
+	int			flags;
 	bool			strict_check;
 	union {
 		u8		ctx[48];
@@ -323,6 +324,7 @@ struct netlink_dump_control {
 	void *data;
 	struct module *module;
 	u32 min_dump_alloc;
+	int flags;
 };
 
 int __netlink_dump_start(struct sock *ssk, struct sk_buff *skb,

include/net/rtnetlink.h

@@ -12,6 +12,7 @@ typedef int (*rtnl_dumpit_func)(struct sk_buff *, struct netlink_callback *);
 enum rtnl_link_flags {
 	RTNL_FLAG_DOIT_UNLOCKED		= BIT(0),
 	RTNL_FLAG_BULK_DEL_SUPPORTED	= BIT(1),
+	RTNL_FLAG_DUMP_UNLOCKED		= BIT(2),
 };
 
 enum rtnl_kinds {

net/core/rtnetlink.c

@@ -6532,6 +6532,7 @@ static int rtnetlink_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh,
 		}
 		owner = link->owner;
 		dumpit = link->dumpit;
+		flags = link->flags;
 
 		if (type == RTM_GETLINK - RTM_BASE)
 			min_dump_alloc = rtnl_calcit(skb, nlh);
@@ -6549,6 +6550,7 @@ static int rtnetlink_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh,
 				.dump		= dumpit,
 				.min_dump_alloc	= min_dump_alloc,
 				.module		= owner,
+				.flags		= flags,
 			};
 			err = netlink_dump_start(rtnl, skb, nlh, &c);
 			/* netlink_dump_start() will keep a reference on

net/netlink/af_netlink.c

@@ -2261,6 +2261,8 @@ static int netlink_dump(struct sock *sk, bool lock_taken)
 
 		cb->extack = &extack;
 
+		if (cb->flags & RTNL_FLAG_DUMP_UNLOCKED)
+			extra_mutex = NULL;
 		if (extra_mutex)
 			mutex_lock(extra_mutex);
 		nlk->dump_done_errno = cb->dump(skb, cb);
@@ -2355,6 +2357,7 @@ int __netlink_dump_start(struct sock *ssk, struct sk_buff *skb,
 	cb->data = control->data;
 	cb->module = control->module;
 	cb->min_dump_alloc = control->min_dump_alloc;
+	cb->flags = control->flags;
 	cb->skb = skb;
 
 	cb->strict_check = nlk_test_bit(STRICT_CHK, NETLINK_CB(skb).sk);