neighbor: fix proxy_delay usage when it is zero

When set to zero, the neighbor sysctl proxy_delay value
does not cause an immediate reply for ARP/ND requests
as expected, it instead causes a random delay between
[0, U32_MAX). Looking at this comment from
__get_random_u32_below() explains the reason:

/*
 * This function is technically undefined for ceil == 0, and in fact
 * for the non-underscored constant version in the header, we build bug
 * on that. But for the non-constant case, it's convenient to have that
 * evaluate to being a straight call to get_random_u32(), so that
 * get_random_u32_inclusive() can work over its whole range without
 * undefined behavior.
 */

Added helper function that does not call get_random_u32_below()
if proxy_delay is zero and just uses the current value of
jiffies instead, causing pneigh_enqueue() to respond
immediately.

Also added definition of proxy_delay to ip-sysctl.txt since
it was missing.

Signed-off-by: Brian Haley <haleyb.dev@gmail.com>
Link: https://lore.kernel.org/r/20230130171428.367111-1-haleyb.dev@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: brianphaley

Documentation/networking/ip-sysctl.rst

@@ -1592,6 +1592,14 @@ proxy_arp_pvlan - BOOLEAN
 	  Hewlett-Packard call it Source-Port filtering or port-isolation.
 	  Ericsson call it MAC-Forced Forwarding (RFC Draft).
 
+proxy_delay - INTEGER
+	Delay proxy response.
+
+	Delay response to a neighbor solicitation when proxy_arp
+	or proxy_ndp is enabled. A random value between [0, proxy_delay)
+	will be chosen, setting to zero means reply with no delay.
+	Value in jiffies. Defaults to 80.
+
 shared_media - BOOLEAN
 	Send(router) or accept(host) RFC1620 shared media redirects.
 	Overrides secure_redirects.

net/core/neighbour.c

@@ -1662,11 +1662,21 @@ static void neigh_proxy_process(struct timer_list *t)
 	spin_unlock(&tbl->proxy_queue.lock);
 }
 
+static unsigned long neigh_proxy_delay(struct neigh_parms *p)
+{
+	/* If proxy_delay is zero, do not call get_random_u32_below()
+	 * as it is undefined behavior.
+	 */
+	unsigned long proxy_delay = NEIGH_VAR(p, PROXY_DELAY);
+
+	return proxy_delay ?
+	       jiffies + get_random_u32_below(proxy_delay) : jiffies;
+}
+
 void pneigh_enqueue(struct neigh_table *tbl, struct neigh_parms *p,
 		    struct sk_buff *skb)
 {
-	unsigned long sched_next = jiffies +
-			get_random_u32_below(NEIGH_VAR(p, PROXY_DELAY));
+	unsigned long sched_next = neigh_proxy_delay(p);
 
 	if (p->qlen > NEIGH_VAR(p, PROXY_QLEN)) {
 		kfree_skb(skb);