netdev - Re: [PATCH net] fib_rules: don't break ECN with TOS rules

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Date:	Sun, 12 Jun 2016 12:28:26 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Julian Anastasov <ja@....bg>
Cc:	netdev@...r.kernel.org, Florian Westphal <fw@...len.de>
Subject: Re: [PATCH net] fib_rules: don't break ECN with TOS rules

On Sun, Jun 12, 2016, at 11:28, Julian Anastasov wrote:
> 
> 	Hello,
> 
> On Sun, 12 Jun 2016, Hannes Frederic Sowa wrote:
> 
> > On Sun, Jun 12, 2016, at 02:09, Julian Anastasov wrote:
> > > 
> > > 	Well, may be the confusion comes from commit 89aef8921bfb
> > > ("ipv4: Delete routing cache.") where the 'tos &= IPTOS_RT_MASK;'
> > > line is lost from ip_route_input_common. I think, we should
> > > add it back, so that we can properly match input routes with rules
> > > that specify tos value. Old kernels didn't stored ECN bits in
> > > flowi4_tos in the input path, so we should do the same.
> > 
> > I would love to have done that but was fearing problems with user space
> > compatibility. Also IPTOS_RT_MASK is not enough for filtering, we need
> > to check for the whole INET_ECN_MASK.
> 
> 	I checked even 2.4.33 for reference. Nobody sets bit 1
> in key->tos but using IPTOS_TOS_MASK on configuration allows
> rules with 'tos 2'. I guess, in this case such rule can not be
> matched. And the TOS matching was used only under
> CONFIG_IP_ROUTE_TOS, so the ip rules were supposed to match only
> bits 2..4, not full TOS values.

Thanks for looking it up! :)

> > > > +	    INET_ECN_ignore(r->tos) != INET_ECN_ignore(fl4->flowi4_tos))
> > > >  		return 0;
> > > 
> > > 	fib4_rule_configure already rejects ECN bits in r->tos,
> > > so no need to filter them again in fast path.
> > 
> > The problem is that IPTOS_TOS_MASK is just the masking of the 0x1 bit,
> > not both ECN bits. Thi stems from the pre-DSCP time where it was
> > forbidden to use bit 0x1 in TOS.
> 
> 	I see, I didn't noticed it. If we decide to reject
> rules with tos 2 we can break scripts. May be we should just
> filter it:
> 
> 	if (r->tos && ((r->tos & IPTOS_RT_MASK) != fl4->flowi4_tos))
> 
> 	This will allow the hidden feature 'ip rule add tos 2' to
> match properly bits2..4 == 0 :) But may be we should not spend
> extra cycles in fast path for such feature. As nobody complained
> for problems with 3.6+, it seems match by tos is not used often.

Not sure if people really noticed or it just quietly broke ecn. :/

> May be we just need to restore the missing 'tos &= IPTOS_RT_MASK;'
> line?

I fear we already use those lower bits for other flags, like RTO_ONLINK.
I will study this today and will report back.

My last proposal would be to simply enclose my block with a if (r->tos)
{ ... }. Not sure, yet.

Bye,
Hannes