Segmentation fault due to double free for archetype.

[Takashi Yano]

On Mon, 17 Jan 2022 13:11:46 +0100
Corinna Vinschen wrote:
> On Jan 17 20:41, Takashi Yano wrote:
> > On Mon, 17 Jan 2022 12:01:51 +0100
> > Corinna Vinschen wrote:
> > > On Jan 15 19:20, Takashi Yano wrote:
> > > > I also found the following patch fixes the issue. Is this the
> > > > right thing?
> > > > 
> > > > diff --git a/winsup/cygwin/fhandler.cc b/winsup/cygwin/fhandler.cc
> > > > index fc7c0422e..e51208117 100644
> > > > --- a/winsup/cygwin/fhandler.cc
> > > > +++ b/winsup/cygwin/fhandler.cc
> > > > @@ -441,7 +441,7 @@ fhandler_base::open_with_arch (int flags, mode_t mode)
> > > >  	|| open (flags, mode & 07777)))
> > > >      {
> > > >        if (archetype)
> > > > -	delete archetype;
> > > > +	cygheap->fdtab.delete_archetype (archetype);
> > > >      }
> > > >    else if (archetype)
> > > >      {
> > > 
> > > Good catch!  I think this is basically ok, but you have to check the
> > > usecount, i. e.
> > > 
> > >   if (archetype && archetype_usecount (-1) == 0)
> > >     cygheap->fdtab.delete_archetype (archetype);
> > > 
> > > Does that sound right?
> > 
> > Where is archetype->usecount is incremented?
> 
> In fhandler_base::open_with_arch.
> 
> > It seems that
> > archetype->usecount is zero here. archetype->usecount is not
> > incremented around line 672 in dtable.cc when archetype is
> > created by fh->clone().
> > 
> >   else
> >     {
> >       if (!fh->get_name ())
> >         fh->set_name (fh->dev ().native ());
> >       fh->archetype = fh->clone ();
> >       debug_printf ("created an archetype (%p) for %s(%d/%d)", fh->archetype, fh->get_name (), fh->dev ().get_major (), fh->dev ().get_minor ());
> >       fh->archetype->archetype = NULL;
> >       *cygheap->fdtab.add_archetype () = fh->archetype;
> >     }
> 
> Right, but if open isn't called, because you already have an archetype
> at this point:
> 
>   if (!(res = (archetype && archetype->io_handle)
>         || open (flags, mode & 07777)))
> 
> Then the archetype is one already created by a former open_with_arch
> call and then you delete an archetype which is still in use, no?

archetype->usecount is not incremented yet here.
archetype->usecount is incremented only when this 'if' clause
is not true. So, isn't the following code right?

  if (!(res = (archetype && archetype->io_handle)
        || open (flags, mode & 07777)))
    {
      if (archetype && archetype->usecount == 0)
        cygheap->fdtab.delete_archetype (archetype);
    }


-- 
Takashi Yano <takashi.yano@nifty.ne.jp>