Potential handle leaks in dup_worker

[Corinna Vinschen]

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.


Corinna