net: ipv6/addrconf: introduce a regen_min_advance sysctl

In RFC 8981, REGEN_ADVANCE cannot be less than 2 seconds, and the RFC
does not permit the creation of temporary addresses with lifetimes
shorter than that:

> When processing a Router Advertisement with a
> Prefix Information option carrying a prefix for the purposes of
> address autoconfiguration (i.e., the A bit is set), the host MUST
> perform the following steps:

> 5.  A temporary address is created only if this calculated preferred
>     lifetime is greater than REGEN_ADVANCE time units.

However, some users want to change their IPv6 address as frequently as
possible regardless of the RFC's arbitrary minimum lifetime. For the
benefit of those users, add a regen_min_advance sysctl parameter that
can be set to below or above 2 seconds.

Link: https://datatracker.ietf.org/doc/html/rfc8981
Signed-off-by: Alex Henrie <alexhenrie24@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Documentation/networking/ip-sysctl.rst

@@ -2535,6 +2535,16 @@ max_desync_factor - INTEGER
 
 	Default: 600
 
+regen_min_advance - INTEGER
+	How far in advance (in seconds), at minimum, to create a new temporary
+	address before the current one is deprecated. This value is added to
+	the amount of time that may be required for duplicate address detection
+	to determine when to create a new address. Linux permits setting this
+	value to less than the default of 2 seconds, but a value less than 2
+	does not conform to RFC 8981.
+
+	Default: 2
+
 regen_max_retry - INTEGER
 	Number of attempts before give up attempting to generate
 	valid temporary addresses.

include/linux/ipv6.h

@@ -27,6 +27,7 @@ struct ipv6_devconf {
 	__s32		use_tempaddr;
 	__s32		temp_valid_lft;
 	__s32		temp_prefered_lft;
+	__s32		regen_min_advance;
 	__s32		regen_max_retry;
 	__s32		max_desync_factor;
 	__s32		max_addresses;

include/net/addrconf.h

@@ -8,8 +8,9 @@
 
 #define MIN_VALID_LIFETIME		(2*3600)	/* 2 hours */
 
-#define TEMP_VALID_LIFETIME		(7*86400)
+#define TEMP_VALID_LIFETIME		(7*86400)       /* 1 week */
-#define TEMP_PREFERRED_LIFETIME		(86400)
+#define TEMP_PREFERRED_LIFETIME		(86400)         /* 24 hours */
+#define REGEN_MIN_ADVANCE		(2)             /* 2 seconds */
 #define REGEN_MAX_RETRY			(3)
 #define MAX_DESYNC_FACTOR		(600)
 

net/ipv6/addrconf.c

@@ -195,6 +195,7 @@ static struct ipv6_devconf ipv6_devconf __read_mostly = {
 	.use_tempaddr		= 0,
 	.temp_valid_lft		= TEMP_VALID_LIFETIME,
 	.temp_prefered_lft	= TEMP_PREFERRED_LIFETIME,
+	.regen_min_advance	= REGEN_MIN_ADVANCE,
 	.regen_max_retry	= REGEN_MAX_RETRY,
 	.max_desync_factor	= MAX_DESYNC_FACTOR,
 	.max_addresses		= IPV6_MAX_ADDRESSES,
@@ -257,6 +258,7 @@ static struct ipv6_devconf ipv6_devconf_dflt __read_mostly = {
 	.use_tempaddr		= 0,
 	.temp_valid_lft		= TEMP_VALID_LIFETIME,
 	.temp_prefered_lft	= TEMP_PREFERRED_LIFETIME,
+	.regen_min_advance	= REGEN_MIN_ADVANCE,
 	.regen_max_retry	= REGEN_MAX_RETRY,
 	.max_desync_factor	= MAX_DESYNC_FACTOR,
 	.max_addresses		= IPV6_MAX_ADDRESSES,
@@ -1341,7 +1343,7 @@ out:
 
 static unsigned long ipv6_get_regen_advance(struct inet6_dev *idev)
 {
-	return 2 + idev->cnf.regen_max_retry *
+	return idev->cnf.regen_min_advance + idev->cnf.regen_max_retry *
 			idev->cnf.dad_transmits *
 			max(NEIGH_VAR(idev->nd_parms, RETRANS_TIME), HZ/100) / HZ;
 }
@@ -6819,6 +6821,13 @@ static const struct ctl_table addrconf_sysctl[] = {
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec,
 	},
+	{
+		.procname       = "regen_min_advance",
+		.data           = &ipv6_devconf.regen_min_advance,
+		.maxlen         = sizeof(int),
+		.mode           = 0644,
+		.proc_handler   = proc_dointvec,
+	},
 	{
 		.procname	= "regen_max_retry",
 		.data		= &ipv6_devconf.regen_max_retry,