netdev - Re: [PATCH] net: Convert skb->csum_(start|offset) integrity BUG_ON() to WARN

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Date:	Thu, 14 Feb 2013 11:21:55 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Thomas Graf <tgraf@...g.ch>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] net: Convert skb->csum_(start|offset) integrity
 BUG_ON() to WARN_ON() & drop

On Thu, 2013-02-14 at 18:50 +0000, Thomas Graf wrote:
> On 02/14/13 at 08:22am, Eric Dumazet wrote:
> > It seems not possible to avoid bugs, being a BUG_ON() or a out of bound
> > memory access or whatever. We must fix them eventually.
> 
> Of course, I never intended to not fix this but I still think
> leaving the BUG_ON() is wrong ;-)
> 
> > In this case, it seems we must limit payload to 
> > 
> > 65535 - MAX_TCP_HEADER 
> > 
> > It would make tcp_xmit_size_goal() a bit shorter.
> > 
> > diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> > index 2c7e596..2f6c8e5 100644
> > --- a/net/ipv4/tcp.c
> > +++ b/net/ipv4/tcp.c
> > @@ -793,10 +793,7 @@ static unsigned int tcp_xmit_size_goal(struct sock *sk, u32 mss_now,
> >  	xmit_size_goal = mss_now;
> >  
> >  	if (large_allowed && sk_can_gso(sk)) {
> > -		xmit_size_goal = ((sk->sk_gso_max_size - 1) -
> > -				  inet_csk(sk)->icsk_af_ops->net_header_len -
> > -				  inet_csk(sk)->icsk_ext_hdr_len -
> > -				  tp->tcp_header_len);
> > +		xmit_size_goal = sk->sk_gso_max_size - 1 - MAX_TCP_HEADER;
> >  
> >  		/* TSQ : try to have two TSO segments in flight */
> >  		xmit_size_goal = min_t(u32, xmit_size_goal,
> 
> I don't think this would help. The allocated skb data will still
> exceed 64K and thus after trimming the acked data and collapsing
> the header might be stored after the 64K mark.
> 

OK, I now understand your issue.

> We would have to limit the skb tailroom to 64K upon allocation.
> That would mean we would waste some of the additional space that
> kmalloc() might have given us:
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 32443eb..c8f9850 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -241,6 +241,8 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mas
>          * to allow max possible filling before reallocation.
>          */
>         size = SKB_WITH_OVERHEAD(ksize(data));
> +       /* ensure that all offsets based on skb->head fit into 16bits */
> +       size = min_t(int, size, 65535);
>         prefetchw(data + size);
>  

This part is certainly not good. 

alloc_skb() is generic and some users really want more than 64K

>         /*
> 
> 
> Or if that is not ideal, avoiding the collapse that causes the overflow
> would also help:
> 
> 
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 5d45159..e9111b4 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -2301,6 +2301,12 @@ static void tcp_retrans_try_collapse(struct sock *sk, struct sk_buff *to,
>                 if (after(TCP_SKB_CB(skb)->end_seq, tcp_wnd_end(tp)))
>                         break;
>  
> +               /* Never collapse if the resulting headroom + data exceeds
> +                * 64K as that is the maximum csum_start can cover.
> +                */
> +               if (skb_headroom(to) + to->len + skb->len > 65535)
> +                       break;
> +
>                 tcp_collapse_retrans(sk, to);
>    

Definitely better.



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