linux-kernel - Re: Regression, bisected: reference leak with IPSec since ~2.6.31

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Message-ID: <20100921123946.GB8986@ff.dom.local>
Date:	Tue, 21 Sep 2010 12:39:46 +0000
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Nick Bowler <nbowler@...iptictech.com>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: Regression, bisected: reference leak with IPSec since ~2.6.31

On Tue, Sep 21, 2010 at 01:58:30PM +0200, Eric Dumazet wrote:
> Le mardi 21 septembre 2010 ?? 10:48 +0000, Jarek Poplawski a écrit :
> 
> > But if you were honest about caches there is no reason to be smart
> > no more. ;-)
> > 
> > Below is your patch mostly as an example only that I didn't mean
> > nothing complex (ipv4 only, not compiled).
> > 
> > Jarek P.
> > ---
> > 
> > diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> > index 04b6989..049d61f 100644
> > --- a/net/ipv4/ip_output.c
> > +++ b/net/ipv4/ip_output.c
> > @@ -490,7 +490,6 @@ int ip_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
> >  	if (skb_has_frags(skb)) {
> >  		struct sk_buff *frag;
> >  		int first_len = skb_pagelen(skb);
> > -		int truesizes = 0;
> >  
> >  		if (first_len - hlen > mtu ||
> >  		    ((first_len - hlen) & 7) ||
> > @@ -503,18 +502,18 @@ int ip_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
> >  			if (frag->len > mtu ||
> >  			    ((frag->len & 7) && frag->next) ||
> >  			    skb_headroom(frag) < hlen)
> > -			    goto slow_path;
> > +			    goto slow_path_clean;
> >  
> >  			/* Partially cloned skb? */
> >  			if (skb_shared(frag))
> > -				goto slow_path;
> > +				goto slow_path_clean;
> >  
> >  			BUG_ON(frag->sk);
> >  			if (skb->sk) {
> >  				frag->sk = skb->sk;
> >  				frag->destructor = sock_wfree;
> > +				skb->truesize -= frag->truesize;
> >  			}
> > -			truesizes += frag->truesize;
> >  		}
> >  
> >  		/* Everything is OK. Generate! */
> > @@ -524,7 +523,6 @@ int ip_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
> >  		frag = skb_shinfo(skb)->frag_list;
> >  		skb_frag_list_init(skb);
> >  		skb->data_len = first_len - skb_headlen(skb);
> > -		skb->truesize -= truesizes;
> >  		skb->len = first_len;
> >  		iph->tot_len = htons(first_len);
> >  		iph->frag_off = htons(IP_MF);
> > @@ -576,6 +574,17 @@ int ip_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
> >  		}
> >  		IP_INC_STATS(dev_net(dev), IPSTATS_MIB_FRAGFAILS);
> >  		return err;
> > +
> > +slow_path_clean:
> > +		if (skb->sk) {
> > +			skb_walk_frags(skb, frag) {
> > +				if (!frag->sk)
> > +					break;
> > +				frag->sk = NULL;
> > +				frag->destructor = NULL;
> > +				skb->truesize += frag->truesize;
> > +			}
> > +		}
> >  	}
> >  
> >  slow_path:
> 
> Nice try, but this adds a bug unfortunately, or if you prefer makes the
> "BUG_ON(frag->sk);" being not fully covered.
> 
> If we hit slow path, and one or more frags had frag->sk set, we silently
> unset it, instead of a nice BUG_ON().

Which exactly BUG_ON() on the slow path do you mean?

> 
> If you really want to have a slow_path_clean, you should use a different
> frag2 iterator :
> 
> slow_path_clean:
>        if (skb->sk) {
>                 skb_walk_frags(skb, frag2) {
>                        if (frag2 == frag)
>                                break;
>                        frag2->sk = NULL;
>                        frag2->destructor = NULL;
>                        skb->truesize += frag2->truesize;
>                }
>        }
> 
> Face it, this is too complex for net-2.6, could we just fix the bug, and
> 'optimize' later ?

I thought it's clear I didn't intend to change anything with this
example, but in any case I admit it: I'm 100% OK with your patch.

Jarek P.
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