Revert "net: ipv6/addrconf: clamp preferred_lft to the minimum required"

The commit had a bug and might not have been the right approach anyway.

Fixes: 629df670 ("net: ipv6/addrconf: clamp preferred_lft to the minimum required")
Fixes: ec575f88 ("Documentation: networking: explain what happens if temp_prefered_lft is too small or too large")
Reported-by: Dan Moulding <dan@danm.net>
Closes: https://lore.kernel.org/netdev/20231221231115.12402-1-dan@danm.net/
Link: https://lore.kernel.org/netdev/CAMMLpeTdYhd=7hhPi2Y7pwdPCgnnW5JYh-bu3hSc7im39uxnEA@mail.gmail.com/Signed-off-by: Alex Henrie <alexhenrie24@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20231230043252.10530-1-alexhenrie24@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/networking/ip-sysctl.rst

@@ -2511,7 +2511,7 @@ temp_valid_lft - INTEGER
 temp_prefered_lft - INTEGER
 	Preferred lifetime (in seconds) for temporary addresses. If
 	temp_prefered_lft is less than the minimum required lifetime (typically
-	5 seconds), the preferred lifetime is the minimum required. If
+	5 seconds), temporary addresses will not be created. If
 	temp_prefered_lft is greater than temp_valid_lft, the preferred lifetime
 	is temp_valid_lft.
 

net/ipv6/addrconf.c

@@ -1407,23 +1407,15 @@ static int ipv6_create_tempaddr(struct inet6_ifaddr *ifp, bool block)
 
 	write_unlock_bh(&idev->lock);
 
-	/* From RFC 4941:
-	 *
-	 *     A temporary address is created only if this calculated Preferred
-	 *     Lifetime is greater than REGEN_ADVANCE time units.  In
-	 *     particular, an implementation must not create a temporary address
-	 *     with a zero Preferred Lifetime.
-	 *
-	 * Clamp the preferred lifetime to a minimum of regen_advance, unless
-	 * that would exceed valid_lft.
-	 *
+	/* A temporary address is created only if this calculated Preferred
+	 * Lifetime is greater than REGEN_ADVANCE time units.  In particular,
+	 * an implementation must not create a temporary address with a zero
+	 * Preferred Lifetime.
 	 * Use age calculation as in addrconf_verify to avoid unnecessary
 	 * temporary addresses being generated.
 	 */
 	age = (now - tmp_tstamp + ADDRCONF_TIMER_FUZZ_MINUS) / HZ;
-	if (cfg.preferred_lft <= regen_advance + age)
-		cfg.preferred_lft = regen_advance + age + 1;
-	if (cfg.preferred_lft > cfg.valid_lft) {
+	if (cfg.preferred_lft <= regen_advance + age) {
 		in6_ifa_put(ifp);
 		in6_dev_put(idev);
 		ret = -1;