Re: "not vlan" filter expression broken catastrophically!

[Paul Pearce <pearce () cs berkeley edu>]

I'd like to point out that vlan filtering in general is completely
broken under Linux 3x (as discussed several times on this list).

In Linux 3x they began stripping the vlan headers off of RX packets
and setting BPF ancillary flags, but not doing the same on TX packets.
Since the vlan tags are missing when RX packets reach the kernel filter it
means that stock libpcap plus any linux 3x kernel can only see TX
vlan tagged packets.

A recent (3.8 I believe) patch added the ability to use BPF to poke at
the vlan ancillary fields, and Ani RFC'd a patch to on this list to
shift vlan filtering to using the ancillary fields rather than offsetting into
the header. But even with that patch since RX and TX paths are
different, it's still not fixed.

You could imagine extending Ani's patch to check for the vlan
ancillary fields and if not set then look at the headers, but that
would mean the filter:

vlan X or vlan Y

would have different behavior on RX vs TX packets because of the
pointer into the header advancing when it encounters a vlan tag
on TX, but not RX.

In my humble (uneducated) opinion the correct fix is to get linux to
move to setting the vlan ancillary fields on TX packets as they do now
on RX packets, which would simplify things a lot for libpcap. But that
idea got a lot of pushback on the net-dev list. I didn't fully understand
their distinction as to why it was ok on RX vs TX, and they never
answered when I asked.
-Paul

On Fri, Feb 1, 2013 at 8:51 AM, Gianluca Varenni
<Gianluca.Varenni () riverbed com> wrote:
> The problem is that if you change the behavior of the vlan keyword, you potentially break a lot of applications that 
> are based on the old buggy behavior :-(
>
> -----Original Message-----
> From: fenner () gmail com [mailto:fenner () gmail com] On Behalf Of Bill Fenner
> Sent: Friday, February 01, 2013 4:49 AM
> To: Gianluca Varenni
> Cc: Ani Sinha; tcpdump-workers () lists tcpdump org; Michael Richardson; Francesco Ruggeri
> Subject: Re: [tcpdump-workers] "not vlan" filter expression broken catastrophically!
>
> On Thu, Jan 31, 2013 at 7:20 PM, Gianluca Varenni <Gianluca.Varenni () riverbed com> wrote:
> > To be totally honest, I think the whole way in which vlans are managed
> > in the filters is quite nonsense. The underlying problem is that
> > normally a BPF filter is an "or" or "and" combination of disjoint
> > filters, so if I write "filterA" or "filterB" I assume that the two
> > filters are disjoints, so
> >
> > "filterA or filterB" should be equivalent to "filterB or filterA"
> >
> > This is not true when using the "vlan" keyword. Vlan sticks globally and increments the offset of the L3 header 
> > unconditionally of two bytes, no turning back.
> >
> > For example "ip or vlan 14" is different than "vlan 14 or ip"
>
> We have wanted to fix the vlan support ever since it was added.  If I remember right we even talked about not adding 
> it and waiting to do it right.  It's definitely a hack, the vlan offset info should be associative and only apply to 
> anything that is "and"ed with the vlan keyword.  Sadly, the current structure of the parser / code generator do not 
> lend themselves to that.
>
> The global nature of the vlan offset is something that nobody is happy with.  All it will take to fix it is to 
> rewrite the grammar parser and filter generation code.
>
>   Bill
>
>
> > -----Original Message-----
> > From: tcpdump-workers-bounces () lists tcpdump org
> > [mailto:tcpdump-workers-bounces () lists tcpdump org] On Behalf Of Ani
> > Sinha
> > Sent: Thursday, January 31, 2013 3:42 PM
> > To: tcpdump-workers () lists tcpdump org
> > Cc: Bill Fenner; Michael Richardson; Francesco Ruggeri
> > Subject: [tcpdump-workers] "not vlan" filter expression broken catastrophically!
> >
> > hello folks :
> >
> > As you guys have been aware, I am hacking libpcap for a while. Me and Bill noticed something seriously broken for 
> > any filter expression that has a "not vlan" in it. For example, take a look at the filter code generated by libpcap 
> > with an expression like "not vlan and tcp port 80" :
> >
> > BpfExpression '(not vlan and tcp port 80)'
> >       { 0x28,  0,  0, 0x0000000c }, //(000) ldh  [12]
> >       { 0x15, 19,  0, 0x00008100 }, //(001) jeq  #0x8100     jt 21      jf 2
> >       { 0x28,  0,  0, 0x00000010 }, //(002) ldh  [16]
> >       { 0x15,  0,  6, 0x000086dd }, //(003) jeq  #0x86dd     jt 4       jf 10
> >       { 0x30,  0,  0, 0x00000018 }, //(004) ldb  [24]
> >       { 0x15,  0, 15, 0x00000006 }, //(005) jeq  #0x6        jt 6       jf 21
> >       { 0x28,  0,  0, 0x0000003a }, //(006) ldh  [58]
> >       { 0x15, 12,  0, 0x00000050 }, //(007) jeq  #0x50       jt 20      jf 8
> >       { 0x28,  0,  0, 0x0000003c }, //(008) ldh  [60]
> >       { 0x15, 10, 11, 0x00000050 }, //(009) jeq  #0x50       jt 20      jf 21
> >       { 0x15,  0, 10, 0x00000800 }, //(010) jeq  #0x800      jt 11      jf 21
> >       { 0x30,  0,  0, 0x0000001b }, //(011) ldb  [27]
> >       { 0x15,  0,  8, 0x00000006 }, //(012) jeq  #0x6        jt 13      jf 21
> >       { 0x28,  0,  0, 0x00000018 }, //(013) ldh  [24]
> >       { 0x45,  6,  0, 0x00001fff }, //(014) jset #0x1fff     jt 21      jf 15
> >       { 0xb1,  0,  0, 0x00000012 }, //(015) ldxb 4*([18]&0xf)
> >       { 0x48,  0,  0, 0x00000012 }, //(016) ldh  [x + 18]
> >       { 0x15,  2,  0, 0x00000050 }, //(017) jeq  #0x50       jt 20      jf 18
> >       { 0x48,  0,  0, 0x00000014 }, //(018) ldh  [x + 20]
> >       { 0x15,  0,  1, 0x00000050 }, //(019) jeq  #0x50       jt 20      jf 21
> >       {  0x6,  0,  0, 0x0000ffff }, //(020) ret  #65535
> >       {  0x6,  0,  0, 0x00000000 }, //(021) ret  #0
> >
> >
> > As you can see, it loads offset 12 (ethertype). For vlan packets, it jumps to #21 and returns false right away. 
> > However, for packets that are not vlan tagged, it goes to #2 which loads offset 16 in the packet. Notice that this 
> > is wrong! The offsets should be incremented by 4 only for vlan tagged packets and not for non-vlan packets. The 
> > problem is that in gencode.c, the off_linktype increments by 4 unconditionally whether or not the packet actually 
> > contains a vlan tag. We do not want to increment this offset if "not vlan" is true. So the above filter code is 
> > generated wrong.
> >
> > I just wanted to point this out to folks who wishes to dig in and fix it. I do not have time right now to think of a 
> > proper solution. It would seem using unconditional increments of offsets like off_linktype below the parser is not 
> > going to work. How do you know if the parser is going to take your code generated from the "vlan" expression and 
> > just negate it? Or may be we can hack another rule in grammar.y. I don't know.
> >
> > cheers,
> > ani
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