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ipv6: Fix preferred_lft not updating in some cases

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Consider the scenario where an IPv6 router is advertising a fixed
preferred_lft of 1800 seconds, while the valid_lft begins at 3600
seconds and counts down in realtime.

A client should reset its preferred_lft to 1800 every time the RA is
received, but a bug is causing Linux to ignore the update.

The core problem is here:
  if (prefered_lft != ifp->prefered_lft) {

Note that ifp->prefered_lft is an offset, so it doesn't decrease over
time.  Thus, the comparison is always (1800 != 1800), which fails to
trigger an update.

The most direct solution would be to compute a "stored_prefered_lft",
and use that value in the comparison.  But I think that trying to filter
out unnecessary updates here is a premature optimization.  In order for
the filter to apply, both of these would need to hold:

  - The advertised valid_lft and preferred_lft are both declining in
    real time.
  - No clock skew exists between the router & client.

So in this patch, I've set "update_lft = 1" unconditionally, which
allows the surrounding code to be greatly simplified.

Signed-off-by: Paul Marks <pmarks@google.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Paul Marks 2013-09-25 15:12:55 -07:00 committed by David S. Miller
parent d4a71b155c
commit c9d55d5bff
1 changed files with 15 additions and 37 deletions
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52
net/ipv6/addrconf.c
View File

@ -2220,43 +2220,21 @@ ok:
else
stored_lft = 0;
if (!update_lft && !create && stored_lft) {
if (valid_lft > MIN_VALID_LIFETIME ||
valid_lft > stored_lft)
update_lft = 1;
else if (stored_lft <= MIN_VALID_LIFETIME) {
/* valid_lft <= stored_lft is always true */
/*
* RFC 4862 Section 5.5.3e:
* "Note that the preferred lifetime of
* the corresponding address is always
* reset to the Preferred Lifetime in
* the received Prefix Information
* option, regardless of whether the
* valid lifetime is also reset or
* ignored."
*
* So if the preferred lifetime in
* this advertisement is different
* than what we have stored, but the
* valid lifetime is invalid, just
* reset prefered_lft.
*
* We must set the valid lifetime
* to the stored lifetime since we'll
* be updating the timestamp below,
* else we'll set it back to the
* minimum.
*/
if (prefered_lft != ifp->prefered_lft) {
valid_lft = stored_lft;
update_lft = 1;
}
} else {
valid_lft = MIN_VALID_LIFETIME;
if (valid_lft < prefered_lft)
prefered_lft = valid_lft;
update_lft = 1;
}
const u32 minimum_lft = min(
stored_lft, (u32)MIN_VALID_LIFETIME);
valid_lft = max(valid_lft, minimum_lft);
/* RFC4862 Section 5.5.3e:
* "Note that the preferred lifetime of the
* corresponding address is always reset to
* the Preferred Lifetime in the received
* Prefix Information option, regardless of
* whether the valid lifetime is also reset or
* ignored."
*
* So we should always update prefered_lft here.
*/
update_lft = 1;
}
if (update_lft) {