On Tue, Dec 20, 2016 at 12:33:47PM +0100, Tobias Klauser wrote:
> On 2016-12-18 at 10:52:49 +0100, Vadim Kochan <vadi...@gmail.com> wrote:
> > Extend proto field expression to:
> > 
> >     proto_field[{index}:{len}] = {func}
> I like the idea behind this very much, but I'm not particularly happy
> with the syntax of this.
> First, I find it a bit confusing that the length is specified and not
> the end index (as at least I would assume it i.e. like array indexing in
> Python works).

Well, I do not thing we need to rely on Python's syntax, actually
I used such syntax from pcap filter (man pcap-filter):

        To access data inside the packet, use the following syntax:
        proto [ expr : size ] Proto  is  one  of  ether, fddi, tr, wlan, ppp,
        slip, link, ip, arp, rarp, tcp, udp, icmp, ip6 or radio, and indicates
        the protocol layer for the index operation.  (ether, fddi, wlan, tr,
        ppp, slip and link all  refer to  the  link  layer. radio refers to the
        "radio header" added to some 802.11 captures.)  Note that tcp, udp and
        other upper-layer protocol types only apply to IPv4, not IPv6 (this will
        be fixed in the  future).   The byte offset, relative to the indicated
        protocol layer, is given by expr.  Size is optional and indicates the
        number of bytes in the field of interest; it can be either one, two, or
        four,  and  defaults  to  one.   The length operator, indicated by the
        keyword len, gives the length of the packet.

> Second, the need to mention a field twice with
> something like:
>   eth(saddr=aa:bb:cc:dd:ee:ff, saddr[0]=dinc())
> isn't very intuitive to understand IMO. And moreover, from the existing
> syntax I'd assume some kind of function call semantics (like in C) with
> the fields being parameters to the function. But indexing a function
> parameter isn't really what I'm used to (but then again this is just
> personal taste).

Hm, to reduce using field assignment twice I think it is possible to
extend proto dynamic functions with default value as parameter then
it make possible to have a mix of index + function parameter like:


with min & max drnd parameters:

    eth(sa[0]=dinc(100, 200, 11:22:33:44:55:66))

If you like it then can we 1st introduce current solution and after
just extend this to the above's syntax ?

> And third, it's not immediately intuitive to which item the index
> refers, i.e. is the MAC address ab:bb:cc:dd:ee:ff afterwards or
> aa:bb:cc:dd:ee:00?

Indexing starts from 0 byte in the network order, please see a below examples of
trafgen + netsniff-ng outputs:

1) trafgen -o lo --cpus 1 -n 1 '{ eth(da[0]=dinc(), da=11:22:33:44:55:66), 
tcp() }'

        P lo 54 1482259546s.934331688ns #1 
         [ Eth MAC (00:00:00:00:00:00 => 00:22:33:44:55:66)

Hm, here you can see that if to specify 1st proto function and after a
field value then looks like the value will be overwritten by function, not
sure if it is a bug or feature ...

2) trafgen -o lo --cpus 1 -n 1 '{ eth(da=11:22:33:44:55:66, da[0]=dinc()), 
tcp() }'

        M lo 54 1482259682s.24357875ns #2 
         [ Eth MAC (00:00:00:00:00:00 => 11:22:33:44:55:66)

3) trafgen -o lo --cpus 1 -n 3 '{ eth(da=11:22:33:44:55:66, da[0]=dinc()), 
tcp() }'

        M lo 54 1482259851s.161018621ns #3 
         [ Eth MAC (00:00:00:00:00:00 => 11:22:33:44:55:66)

        P lo 54 1482259851s.161032201ns #4 
         [ Eth MAC (00:00:00:00:00:00 => 12:22:33:44:55:66)

        M lo 54 1482259851s.161033977ns #5 
         [ Eth MAC (00:00:00:00:00:00 => 13:22:33:44:55:66)

4) trafgen -o lo --cpus 1 -n 3 '{ ipv4(da=, da[0]=dinc()), tcp() }'

        < lo 54 1482265434s.453794790ns #1 
         [ IPv4 Addr ( =>

        < lo 54 1482265434s.453811528ns #2 
         [ IPv4 Addr ( =>

        < lo 54 1482265434s.453815331ns #3 
         [ IPv4 Addr ( =>

5) trafgen -o lo --cpus 1 -n 3 '{ ipv4(da=, da[1]=dinc()), tcp() }'

        < lo 54 1482265603s.917104425ns #4 
         [ IPv4 Addr ( =>

        < lo 54 1482265603s.917122777ns #5 
         [ IPv4 Addr ( =>

        < lo 54 1482265603s.917127151ns #6 
         [ IPv4 Addr ( =>
> But then I currently don't immediately see a better way to implement
> this behavior and I also see why you'd chosen the indexing to work with
> length rather than end index.
> One option might be to let the user specify multi-byte/-word fields as a
> C-array-like initializer list and support function calls therein, i.e.
> the example above could be written as:
>   eth(saddr={ 0xaa, 0xbb, 0xcc, 0xdd, 0xee, drnd() })

Hm, looks interesting, in such way it will possible to specify few
functions at 1 shot, need to think how it would be possible to specify
the value size.

> which would be a bit more in line with the traditional trafgen syntax.
> Let me think about this a bit more...

I think we can have 1 or 3 different solutions:

        1) Current

        2) Current + function param default value

        3) Array-like

Vadim Kochan

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