Potential handle leaks in dup_worker

Ken Brown kbrown@cornell.edu
Tue Feb 9 15:31:16 GMT 2021

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=.


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