Re: [uml-devel] uml "ip header error" for large (fragmented) udp packets

[Lars Ellenberg <Lars.Ellenberg@linbit.com>]

/ 2004-10-07 20:41:51 +0200
\ BlaisorBlade:
> On Wednesday 06 October 2004 22:35, Andi Kleen wrote:
> > On Wed, Oct 06, 2004 at 08:48:23PM +0200, Lars Ellenberg wrote:

> > > FYI, full thread can be found for example at
> > > http://thread.gmane.org/gmane.linux.uml.devel/4607
> 
> > Paolo's analysis is basically correct. loopback sets this flag
> > for better performance.  Actually in 2.6 it probably doesn't help
> > very much anymore because TCP can do checksum copy RX now, and that
> > would get the checksum basically for free. But it's still there
> > and may still make things slightly faster.
> First thing: thanks a lot for your quick answer.
> 
> My discussion about "lo" was slightly unrelated to the exact problem, and a 
> bit confusing...
> 
> I was at first surprised from Ethereal complaining about the host kernel, so I 
> thought I could have a buggy Ethereal, and then went checking that it's a 
> Linux optimization, indeed.
> 
> > If UML taps the packets from lo it will see incorrect checksums.
> > Using a tun or ethertap device would avoid this.
> 
> > In the worst 
> > case you could also just delete the flag from the loopback
> > interface, it's only an optimization.
> 
> No, inside the Uml kernel they go through a virtual "ethN" interface, which 
> uses special code. That driver, in turn, will use either ethertap, or TAP (it 
> sends Ethernet frames), or even other mechanism.
> 
> You can find it (in 2.6.9-rc2 at least) in arch/um/drivers/net_*.c and 
> arch/um/os-Linux/drivers/*tap*.c. The code in *_kern.c files links against 
> the kernel API and includes, *_user.c against the host userspace includes.
> 
> And the problem is, probably, that the UML network drivers never declare their 
> checksumming status, as I said in the previous mail:
> 
> [quote]
> include/linux/skbuff.h describes the Checksum flags, and UML does not use 
> them: these two commands return no (relevant) output.
> 
> find arch/um/ -name '*.[ch]'|xargs grep NETIF
> find arch/um/ -name '*.[ch]'|xargs grep CHECKSUM
> [/quote]
> 
> And not even these ones:
> 
> find arch/um/ -name '*.[ch]'|xargs grep NETIF
> find arch/um/ -name '*.[ch]'|xargs grep CHECKSUM
> 
> Also, it's possible that there are even other bugs...

now, what I found:
arch/um/drivers/net_kern.c:
struct sk_buff *ether_adjust_skb(struct sk_buff *skb, int extra)
{
	if((skb != NULL) && (skb_tailroom(skb) < extra)){
	  	struct sk_buff *skb2;

		skb2 = skb_copy_expand(skb, 0, extra, GFP_ATOMIC);
		dev_kfree_skb(skb);
		skb = skb2;
	}
	if(skb != NULL) skb_put(skb, extra);
	return(skb);
}

net/core/skbuff.c:
 *	BUG ALERT: ip_summed is not copied. Why does this work? Is it used
 *	only by netfilter in the cases when checksum is recalculated? --ANK
 */
struct sk_buff *skb_copy_expand(const struct sk_buff *skb,
				int newheadroom, int newtailroom, int gfp_mask)
{


does that trigger something in someones brain maybe?
someone "sees" it?

otherwise I keep poking around...

	lge



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