Potential handle leaks in dup_worker

Ken Brown kbrown@cornell.edu
Tue Feb 9 17:13:33 GMT 2021

On 2/9/2021 11:12 AM, Corinna Vinschen via Cygwin-developers wrote:
> On Feb  9 10:31, Ken Brown via Cygwin-developers wrote:
>> On 2/9/2021 10:02 AM, Corinna Vinschen via Cygwin-developers wrote:
>>> On Feb  9 09:19, Ken Brown via Cygwin-developers wrote:
>>>> On 2/9/2021 4:47 AM, Corinna Vinschen via Cygwin-developers wrote:
>>>>> On Feb  8 12:39, Ken Brown via Cygwin-developers wrote:
>>>>>> I've had occasion to work through dtable::dup_worker, and I'm seeing the
>>>>>> potential for leaks of path_conv handles.  I haven't seen any evidence that
>>>>>> the leaks actually occur, but the code should probably be cleaned up if I'm
>>>>>> right.
>>>>>> dup_worker calls clone to create newfh from oldfh.  clone calls copyto,
>>>>>> which calls operator=, which calls path_conv::operator=, which duplicates
>>>>>> the path_conv handle from oldfh to newfh.  Then copyto calls reset, which
>>>>>> calls path_conv::operator<<, which again duplicates the path_conv handle
>>>>>> from oldfh to newfh without first closing the previous one.  That's the
>>>>>> first leak.
>>>>>> Further on, dup_worker calls newfh->pc.reset_conv_handle (), which sets the
>>>>>> path_conv handle of newfh to NULL without closing the existing handle.  So
>>>>>> that's a second leak.  This one is easily fixed by calling close_conv_handle
>>>>>> instead of reset_conv_handle.
>>>>> Nice detective work, you're right.  For fun, this is easily testable.
>>>>> Apply this patch to Cygwin:
>>>>> [...]
>>>>>> As a practical matter, I think the path_conv handle of oldfh is always NULL
>>>>>> when dup_worker is called, so there's no actual leak.
>>>>> Right, because conv_handle should only be non-NULL in calls to stat(2)
>>>>> and friends.
>>>>> Nevertheless, it's a bad idea to keep this code.  So the question is
>>>>> this:  Do we actually *need* to duplicate the conv_handle at all?
>>>>> It doesn't look like this is ever needed.  Perhaps the code should
>>>>> just never duplicate conv_handle and just always reset it to NULL
>>>>> instead?
>>>> I've come across one place where I think it's needed.  Suppose build_fh_name
>>>> is called with PC_KEEP_HANDLE.  It calls build_fh_pc, which calls set_name,
>>>> which calls path_conv::operator<<.  I think we need to duplicate conv_handle
>>>> here.
>>> Indeed, you're right.  I just found that the fhandler_base::reset method
>>> is only called from copyto.  Given that fhandler::operator= already
>>> calls path_conv::operator=, and that duplicates the conv handle, why
>>> call path_conv::operator<< from fhandler_base::reset at all?  It looks
>>> like this is only duplicating what already has been done.
>> I think that's right.  It looks like operator<< differs from operator= only
>> in being careful not to overwrite an existing path.  So I can't see that it
>> ever makes sense to call operator<< right after calling operator=.
> It might be helpful not only to move reset to a protected inline method,
> but also to rename it, making entirely clear that this is just a copyto
> helper and nothing else.  I. e., something like _copyto_reset_helper().
> Are you going to create the patch or shall I?

I'll do it.


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