Wireshark-dev: Re: [Wireshark-dev] 1.12.0rc2 : tshark crash when message is on 3 packets
From: wsgd <[email protected]>
Date: Thu, 19 Jun 2014 21:22:26 +0200
Le 19/06/2014 00:14, Pascal Quantin a écrit :
Indeed the sample code in README.dissector was not updated to make use of the new style dissectors (while packet-PROTOABBREV.c sample code is updated).
As Graham stated, the new style (indicating the number of bytes consumed by the dissector) should be used. The old one is still here because not all dissectors were converted to new style (quite a long and painful task... :) ).


2014-06-18 23:59 GMT+02:00 Graham Bloice <[email protected]>:
On 18 June 2014 13:12, wsgd <[email protected]> wrote:
Le 18/06/2014 00:41, Pascal Quantin a écrit :
2014-06-18 0:11 GMT+02:00 Pascal Quantin <[email protected]>:
2014-06-16 22:44 GMT+02:00 wsgd <[email protected]>:


My protocol (only to test this problem) specifications :
tcp port 20640
message is 5 bytes long

command line : tshark -r pb.cap  -T text -V
--> crash (see pb.1.12.0.txt)
ERROR:print.c:838:get_field_data: code should not be reached

This application has requested the Runtime to terminate it in an unusual way.
Please contact the application's support team for more information.

wireshark does not crash and display is ok
tshark 1.10.6 does not crash and display is ok (see pb.1.10.6.txt)

Plugin dissector code is into packet-tcp-5-bytes.c


Hi Olivier,

thanks for the report.
This is a regression introduced by g21e0a63b2 commit for bug 9169. I proposed a fix (not calling the data dissector when a subdissector claims that the current TCP fragment needs more desegmentation) here: https://code.wireshark.org/review/2350


Hi Olivier,

as Evan noted in the review of my patch, the data dissector should not even be called as your dissector accepted the packet. It appears that there is a small bug in your current code. In function dissect_tcp_5_bytes(), replacing the line 30:
            return  offset;
            return  offset + available;
does not trigger the crash.

With the previous code, your dissector was returning the value 0 for frame 4, like if the packet was rejected. But at the same time you were considering the packet as acceptable and changing the pinfo->desegment_len, leading to an inconsistent state that should have been caught by a missing check in packet-tcp.c


Hi Pascal,

Ok, my fault.
Sorry for the inconvenience.

Question : the dissect function must return void or int ?
I know both versions exist.
Is there one deprecated or one better ?

Only void dissect function into http://www.wireshark.org/docs/wsdg_html_chunked/ChDissectAdd.html.

void dissect function into https://code.wireshark.org/review/gitweb?p=wireshark.git;a=blob_plain;f=doc/README.dissector :
static void dissect_cstr(tvbuff_t * tvb, packet_info * pinfo, proto_tree * tree)
I can't see it in the docs anywhere (which is an omission that should be corrected), but epan\packet.h holds a little information about the types of dissector functions.  New code should be using the new dissector type.


Thank you both.