python and NDR blobs GC reference surprise

Dave Craft wimberosa at
Wed Nov 30 14:15:43 MST 2011

I'm neither a python or ndr guru but the following surprised me enough
to mention
in this email.   If its a bug I'd explore fixing at some point but if
its not a bug its documented
here for posterity.

The issue....

I had an instance in the samba_kcc python code where I needed to produce
a repsFromToBlob and attendant sub-structure repsFromTo1OtherInfo.   Here's
the way I initially attempted...

    reps = drsblobs.repsFromToBlob()
    reps.version = 0x1
    reps.ctr.other_info = drsblobs.repsFromTo1OtherInfo()

That reps.ctr.other_info piece has to be instantiated for the particular
version of the repsFromTo.  In this instance I am producing a
version 0x1 structure.

The surprise....

What happens with this code however is that python does not
have a reference count for that repsFromTo1OtherInfo and immediately
deletes it.   You thus get

    talloc: access after free error - first free may be at
    Bad talloc magic value - access after free

if you try to access reps.ctr.other_info subsequently.

To get a reference count on to stick around you have to do this.

    reps = drsblobs.repsFromToBlob()
    reps.version = 0x1
    other_info = drsblobs.repsFromTo1OtherInfo()
    reps.ctr.other_info = other_info

That standalone "other_info" has to be there to hold the python
reference count.

Regards, Dave Craft
Cut the headlights and put it in neutral.

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