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

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Message-Id: <1465691799.3002631.634954313.3E40DB3C@webmail.messagingengine.com>
Date:	Sun, 12 Jun 2016 02:36:39 +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

Hi Julian,

On Sun, Jun 12, 2016, at 02:09, Julian Anastasov wrote:
> 
> 	Hello,
> 
> On Sat, 11 Jun 2016, Hannes Frederic Sowa wrote:
> 
> > Users of ToS rules could accidentally break ECN, this patch tries to
> > fix this in a way so we don't break shell scripts depending on the old
> > behavior while still being transparent to ECN. This quietly fixes ECN
> > behavior for old setups.
> > 
> > For IPv6 we have no check if we check for ECN bits, in IPv4 we only
> > check for the last bit, which is specified to be '0' from pre-DSCP times
> > (because of implementation confusion).
> > 
> > This patch changes fib rules in a way that matches only for ecn bits
> > will never match from now on (I consider them illegal), as we simply
> > ignore those rules (it was easier to explain in a pr_warn). Opinions?
> 
> 	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.

> 
> > Cc: Florian Westphal <fw@...len.de>
> > Signed-off-by: Hannes Frederic Sowa <hannes@...essinduktion.org>
> > ---
> >  include/net/inet_ecn.h | 5 +++++
> >  net/ipv4/fib_rules.c   | 6 +++++-
> >  net/ipv6/fib6_rules.c  | 7 ++++++-
> >  3 files changed, 16 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/net/inet_ecn.h b/include/net/inet_ecn.h
> > index 0dc0a51da38faa..be65d94c05ee01 100644
> > --- a/include/net/inet_ecn.h
> > +++ b/include/net/inet_ecn.h
> > @@ -17,6 +17,11 @@ enum {
> >  
> >  extern int sysctl_tunnel_ecn_log;
> >  
> > +static inline __u8 INET_ECN_ignore(__u8 dsfield)
> > +{
> > +	return dsfield & ~INET_ECN_MASK;
> > +}
> > +
> >  static inline int INET_ECN_is_ce(__u8 dsfield)
> >  {
> >  	return (dsfield & INET_ECN_MASK) == INET_ECN_CE;
> > diff --git a/net/ipv4/fib_rules.c b/net/ipv4/fib_rules.c
> > index 6e9ea69e5f751b..87efbc38589c76 100644
> > --- a/net/ipv4/fib_rules.c
> > +++ b/net/ipv4/fib_rules.c
> > @@ -145,7 +145,8 @@ static int fib4_rule_match(struct fib_rule *rule, struct flowi *fl, int flags)
> >  	    ((daddr ^ r->dst) & r->dstmask))
> >  		return 0;
> >  
> > -	if (r->tos && (r->tos != fl4->flowi4_tos))
> > +	if (!INET_ECN_ignore(r->tos) ||
> 
> 	Above check will succeed for any rule that does not use
> tos (r->tos = 0). So, rules without tos will return 0 (no match).

Oh yes, certainly. I switched back and forth between different schemas.
This obviously doesn't work, thanks! I actually only tested the cases
with tos rules. :(

> > +	    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.

> >  	return 1;
> > @@ -215,6 +216,9 @@ static int fib4_rule_configure(struct fib_rule *rule, struct sk_buff *skb,
> >  	rule4->dst_len = frh->dst_len;
> >  	rule4->dstmask = inet_make_mask(rule4->dst_len);
> >  	rule4->tos = frh->tos;
> > +	if (!INET_ECN_ignore(rule4->tos))
> > +		pr_warn("ipv4: never matching ipv4 rule with match for %x added\n",
> > +			rule4->tos);
> 
> 	You should see this warning for non-tos rules...

I added the warnings after testing, sorry...

Bye,
Hannes