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

From: Neil Horman <[email-masked]>
Date: Sat, 20 Jun 2009 12:44:46 -0400

On Sat, Jun 20, 2009 at 02:37:00PM +0200, Jarek Poplawski wrote:
> Neil Horman wrote, On 06/19/2009 07:18 PM:
> 
> > Don't drop route if we're not caching	
> > 
> > 	I recently got a report of an oops on a route lookup.  Maxime was
> > testing what would happen if route caching was turned off (doing so by setting
> > making rt_caching always return 0), and found that it triggered an oops.  I
> > looked at it and found that the problem stemmed from the fact that the route
> > lookup routines were returning success from their lookup paths (which is good),
> > but never set the **rp pointer to anything (which is bad).  This happens because
> > in rt_intern_hash, if rt_caching returns false, we call rt_drop and return 0.
> > This almost emulates slient success.  What we should be doing is assigning *rp =
> > rt and _not_ dropping the route.  This way, during slow path lookups, when we
> > create a new route cache entry, we don't immediately discard it, rather we just
> > don't add it into the cache hash table, but we let this one lookup use it for
> > the purpose of this route request.  Maxime has tested and reports it prevents
> > the oops.
> 
> Hmm... So, IOW, do you mean the same Maxime, by whom it was "Reported-by" and
> "Tested-by", and probably anonymous on the Cc list, or I miss something?
> 
Yes, they are all one in the same person, I honestly had not thought of the
Reported-by tag in my email, apologies.  I had asked Maxime to follow up on the
list to add the tag, but that never seems to have happened.
Neil

> Regards,
> Jarek P.
> 
> > There is still a subsequent routing issue that I'm looking into
> > further, but I'm confident that, even if its related to this same path, this
> > patch makes sense to take.
> > 
> > Regards
> > Neil
> >     
> > Signed-off-by: Neil Horman <[email-masked]>
> > 
> > 
> >  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);
> > -		return 0;
> > +		/*
> > +		 * If we're not caching, just tell the caller we
> > +		 * were successful and don't touch the route.  The
> > +		 * caller hold the sole reference to the cache entry, and
> > +		 * it will be released when the caller is done with it.
> > +		 * If we drop it here, the callers have no way to resolve routes
> > +		 * when we're not caching.  Instead, just point *rp at rt, so
> > +		 * the caller gets a single use out of the route
> > +		 */
> > +		goto report_and_exit;
> >  	}
> >  
> >  	rthp = &rt_hash_table[hash].chain;
> > @@ -1217,6 +1225,8 @@ restart:
> >  	rcu_assign_pointer(rt_hash_table[hash].chain, rt);
> >  
> >  	spin_unlock_bh(rt_hash_lock_addr(hash));
> > +
> > +report_and_exit:
> >  	if (rp)
> >  		*rp = rt;
> >  	else
> > --
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> 
> 
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