Re: [PATCH] fix NULL pointer + success return in route lookup path

On 21-06-2009 19:11, Neil Horman wrote:

On Sat, Jun 20, 2009 at 04:47:48PM -0700, David Miller wrote:

quoted

From: Jarek Poplawski <redacted>
Date: Sat, 20 Jun 2009 18:39:25 +0200

quoted

Jarek Poplawski wrote, On 06/20/2009 02:37 PM:

quoted

Neil Horman wrote, On 06/19/2009 07:18 PM:

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Don't drop route if we're not caching	

...

quoted

quoted

 route.c |   14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index cd76b3c..65b3a8b 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c

@@ -1085,8 +1085,16 @@ restart:
 	now = jiffies;
 
 	if (!rt_caching(dev_net(rt->u.dst.dev))) {
-		rt_drop(rt);

One more question: if this rt is assigned to an skb, there is only
skb_dst_drop() in kfree_skb(), but it seems we skip rt_free() part,
or I miss something?

This whole code path was buggy, if it returns success it should
do as the normal success code path does which is assign for
non-SKB case to *rp, or skb_dst_set().

So I'm a bit confused.  I see how my patch corrects the path we take through
rt_intern_hash, doing the same thing that we normally do in the success case.
What I don't see is how we clean up those dst entries when we're done with them.
Since their not placed in the route cache (assuming rt_caching returns zero),
then don't we have a leak, since the garbage collector will never see it in the
cache to reap.

Assuming thats the case, I was thinking about closing that leak by setting
DST_NOHASH in rt_intern_hash for any dst_entry that was submitted when
rt_caching returns zero.  Then in skb_dst_drop, we can check for dst->_refcnt
== 0, and if flags & DST_NOHASH is true, then we can call dst_free on it.  Or
does that remove the dst_entry before a caller might be done with it in some
cases?

Maybe it can work, but it needs a thorough checking now and adds a new
code path to track later while looking for bugs. So, I wonder if it's
not better to link such dsts in rt_intern_hash anyway, probably as a
separate list, scanned only for expired entries.

Thanks,
Jarek P.