netdev - Re: [PATCH net-next] ipv4: fix a dst leak in tunnels

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Message-ID: <1389922142.31367.445.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Thu, 16 Jan 2014 17:29:02 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, therbert@...gle.com, maze@...gle.com,
	cwang@...pensource.com
Subject: Re: [PATCH net-next] ipv4: fix a dst leak in tunnels

On Thu, 2014-01-16 at 17:13 -0800, David Miller wrote:
> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Thu, 16 Jan 2014 16:41:19 -0800
> 
> > This patch :
> > 
> > 1) Remove a dst leak if DST_NOCACHE was set on dst
> >    Fix this by holding a reference only if dst really cached.
> 
> Can you explain this problem a little bit more?
> 
> Why do we have to handle DST_NOCACHE specially?  We hold a reference
> and dst_release() knows what to do with DST_NOCACHE routes.
> 
> Or is it semantically undesirable for tunnels to cache these routes?
> If so, why do we leave sockets caching DST_NOCACHE routes just fine?

Previous code was doing in the callers of __tunnel_dst_set() a
dst_clone(dst)

Then, we were doing in __tunnel_dst_set() :

       if (dst && (dst->flags & DST_NOCACHE))
               dst = NULL;

When setting dst to NULL here, we forgot to release the reference we
took in callers.

After my patch, we instead do the dst_clone(dst) only in the case we are
not clearing dst :

+       if (dst) {
+               if (dst->flags & DST_NOCACHE)
+                       dst = NULL;
+               else
+                       dst_clone(dst);
+       }



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