Wireshark · Wireshark-dev: Re: [Wireshark-dev] allocator->in_scope

[Paul Offord <Paul.Offord@xxxxxxxxxxxx>]

Even bigger aha.  Although I only have one new block in the pcapng file, when I open the file via the file explorer dialogue, my block reader is called twice; once to allow WS to enrich the dialogue box (I think) and a second time to read the block before dissection.  I never noticed this before, and this probably explains a few weird problems I have been grappling with.

 

I know you are gripped by this drama, so I’ll let you know what I find.

 

From: Wireshark-dev <wireshark-dev-bounces@xxxxxxxxxxxxx> On Behalf Of Paul Offord
Sent: 25 March 2018 17:20
To: Developer support list for Wireshark <wireshark-dev@xxxxxxxxxxxxx>
Subject: Re: [Wireshark-dev] allocator->in_scope

 

Aha – whilst what I’ve written below is true, it doesn’t accurately reflect the issue.

 

If I start Wireshark and double click on a file in the recently opened list, part of the processing is this:

 

  cf_open() calls

    ws_epan_new() calls

      epan_new() calls

        init_dissection() calls

          wmem_enter_file_scope() which sets

            file_scope->in_scope = TRUE;

 

All of the above takes place before my new block reader is called.

 

If I start Wireshark, use Ctl-O and then double click on a file in the file explorer dialogue, there is no call to wmem_enter_file_scope() before my block reader is called.

 

I believe this is a bug.  The two file opens should be consistent in the setting of file_scope->in_scope = TRUE, and hence the scope of wmem_file_scope().

 

• Am I wrong?
• Have I missed something?
• Should I create a bug report?

 

Thanks and regards…Paul

 

From: Paul Offord
Sent: 25 March 2018 10:31
To: Developer support list for Wireshark <wireshark-dev@xxxxxxxxxxxxx>
Subject: allocator->in_scope

 

Hi,

 

Still working on my new block reader.  To recap, I’ve defined a new pcapng block type and written a dissector.  The first thing I have to do is read the new block type, and Wireshark provides a framework to do this.  In the new block reader I define some space like this:

 

            tdb_namespace = wmem_strdup_printf(wmem_file_scope(), "%s", option_block->option_data);

 

Eventually the wmem_strdup_printf(…) execution calls this function:

 

void *

wmem_alloc(wmem_allocator_t *allocator, const size_t size)

{

    if (allocator == NULL) {

        return g_malloc(size);

    }

 

    if (!allocator->in_scope) // debug code

        while (FALSE); // debug code

 

    g_assert(allocator->in_scope);

 

    if (size == 0) {

        return NULL;

    }

 

    return allocator->walloc(allocator->private_data, size);

}

 

The g_assert intermittently fails.  If I open one file containing the new block, allocator->in_scope is true.  If I call another it’s false.

 

The block read is called before we start dissecting the contents with the dissector code.

 

• At what point is wmem_file_scope in scope?
• Should it be when my block reader is called or is it only guaranteed when the dissector code is called?

 

Thanks and regards…Paul

 

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