Wireshark-dev: [Wireshark-dev] Using plugin support to handle pcap-ng block types
From: Paul Offord <[email protected]>
Date: Mon, 29 Jan 2018 22:40:17 +0000

Hi,

 

I am writing a plugin that handles two new PCAP-NG block types – details here: https://wiki.wireshark.org/Adding%20Support%20for%20a%20New%20Block%20Type

 

I’m using Guy’s code that adds support for plugins to handle pcap-ng block types, described in https://code.wireshark.org/review/#/c/1775/

 

I have written a read block handler and registered it with Wireshark using register_pcapng_block_type_handler() and the handler gets called correctly.  Once the block has been read by my handler, I want Wireshark to continue to process it, but currently the block is ignored and Wireshark just moves on to the next block.

 

The blocks are read via a while loop in the pcapng_read(…) function of pcapng.c.  After calling a function to read the block there is a “switch (wblock.type)” section of code that decides what next to do with the block.  My new block is not defined as a switch case (not surprising), but I believe the default case should take into account the fact that a block type may have been registered.  The code in this area currently looks like this:

 

        switch (wblock.type) {

 

            case(BLOCK_TYPE_SHB):

                pcapng_debug("pcapng_read: another section header block");

                g_array_append_val(wth->shb_hdrs, wblock.block);

                break;

 

            case(BLOCK_TYPE_PB):

            case(BLOCK_TYPE_SPB):

            case(BLOCK_TYPE_EPB):

            case(BLOCK_TYPE_SYSDIG_EVENT):

            case(BLOCK_TYPE_SYSDIG_EVF):

                /* packet block - we've found a packet */

                goto got_packet;

 

            case(BLOCK_TYPE_IDB):

                /* A new interface */

                pcapng_debug("pcapng_read: block type BLOCK_TYPE_IDB");

                pcapng_process_idb(wth, pcapng, &wblock);

                wtap_block_free(wblock.block);

                break;

 

            case(BLOCK_TYPE_NRB):

                /* More name resolution entries */

                pcapng_debug("pcapng_read: block type BLOCK_TYPE_NRB");

                if (wth->nrb_hdrs == NULL) {

                    wth->nrb_hdrs = g_array_new(FALSE, FALSE, sizeof(wtap_block_t));

                }

                g_array_append_val(wth->nrb_hdrs, wblock.block);

                break;

 

            case(BLOCK_TYPE_ISB):

                /* Another interface statistics report */

                pcapng_debug("pcapng_read: block type BLOCK_TYPE_ISB");

                if_stats_mand_block = (wtapng_if_stats_mandatory_t*)wtap_block_get_mandatory_data(wblock.block);

 

                … lines deleted …

 

                }

                wtap_block_free(wblock.block);

                break;

 

            default:

                /* XXX - improve handling of "unknown" blocks */

                pcapng_debug("pcapng_read: Unknown block type 0x%08x", wblock.type);

                break;

        }

    }

 

got_packet:

 

    /*pcapng_debug("Read length: %u Packet length: %u", bytes_read, wth->phdr.caplen);*/

    pcapng_debug("pcapng_read: data_offset is finally %" G_GINT64_MODIFIER "d", *data_offset);

 

    return TRUE;

 

 

I believe that the default condition should check for a plugin registered block type and then goto got_packet.  Also, as the code stands we never free wblock.block.

 

Is this a bug, or is my understanding of what’s intended wrong?

 

Thanks and regards…Paul


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