Re: [PATCH v3 3/3] namei: aggressively check for nd->root escape on ".." resolution

[Al Viro]

On Wed, Oct 10, 2018 at 02:37:28AM +1100, Aleksa Sarai wrote:
> > > +static inline int nd_alloc_dpathbuf(struct nameidata *nd)
> > > +{
> > > +       if (unlikely(!nd->dpathbuf)) {
> > > +               if (nd->flags & LOOKUP_RCU) {
> > > +                       nd->dpathbuf = kmalloc(PATH_MAX, GFP_ATOMIC);
> > > +                       if (unlikely(!nd->dpathbuf))
> > > +                               return -ECHILD;
> > > +               } else {
> > > +                       nd->dpathbuf = kmalloc(PATH_MAX, GFP_KERNEL);
> > > +                       if (unlikely(!nd->dpathbuf))
> > > +                               return -ENOMEM;
> > > +               }
> > > +       }
> > > +       return 0;
> > > +}
> > 
> > Note that a fixed-size path buffer means that if the path is very
> > long, e.g. because you followed long symlinks on the way down, this
> > can cause lookup failures.
> 
> This is already an issue with __d_path (even if the buffer was larger)
> because it will not output a path longer than PATH_MAX. I imagine this
> is a pretty strong argument for why we should refactor __d_path so that
> we can *just* use the escape checking to avoid -ENAMETOOLONG.

Let me get it straight - the whole point of that buffer is to check
if __d_path() returns NULL?  So you allocate it so that you would have
place to copy the path components into... only to have them completely
ignored?

How is that different from path_is_under()?