Re: [PATCH] xen-netfront: handle NULL returned by xdp_convert_buff_to_frame()

[Alexey <sdl@xxxxxxxx>]

On 17.04.2025 13:23, Jürgen Groß wrote:
> On 17.04.25 12:06, Alexey wrote:
> > On 17.04.2025 11:51, Juergen Gross wrote:
> > > On 17.04.25 10:45, Alexey wrote:
> > > > On 17.04.2025 10:12, Jürgen Groß wrote:
> > > > > On 17.04.25 09:00, Alexey wrote:
> > > > > > On 17.04.2025 03:58, Jakub Kicinski wrote:
> > > > > > > On Mon, 14 Apr 2025 18:34:01 +0000 Alexey Nepomnyashih wrote:
> > > > > > > >           get_page(pdata);
> > > > > > > Please notice this get_page() here.
> > > > > > >
> > > > > > > >           xdpf = xdp_convert_buff_to_frame(xdp);
> > > > > > > > +        if (unlikely(!xdpf)) {
> > > > > > > > + trace_xdp_exception(queue->info->netdev, prog, act);
> > > > > > > > +            break;
> > > > > > > > +        }
> > > > > > Do you mean that it would be better to move the get_page(pdata) 
> > > > > > call lower,
> > > > > > after checking for NULL in xdpf, so that the reference count is 
> > > > > > only increased
> > > > > > after a successful conversion?
> > > > > I think the error handling here is generally broken (or at least very
> > > > > questionable).
> > > > >
> > > > > I suspect that in case of at least some errors the get_page() is 
> > > > > leaking
> > > > > even without this new patch.
> > > > >
> > > > > In case I'm wrong a comment reasoning why there is no leak should be
> > > > > added.
> > > > >
> > > > >
> > > > > Juergen
> > > > I think pdata is freed in xdp_return_frame_rx_napi() -> __xdp_return()
> > > Agreed. But what if xennet_xdp_xmit() returns an error < 0?
> > >
> > > In this case xdp_return_frame_rx_napi() won't be called.
> > >
> > >
> > > Juergen
> > Agreed. There is no explicit freed pdata in the calling function
> > xennet_get_responses(). Without this, the page referenced by pdata
> > could be leaked.
> >
> > I suggest:
> Could you please merge the two if () blocks, as they share the
> call of xdp_return_frame_rx_napi() now? Something like:
>
> if (unlikely(err <= 0)) {
>     if (err < 0)
>         trace_xdp_exception(queue->info->netdev, prog, act);
>     xdp_return_frame_rx_napi(xdpf);
> }
>
> Juergen
>
> P.S.: please don't use HTML in emails
I can't do this because xennet_xdp_xmit() can return a value > 0