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

[Jürgen Groß <jgross@xxxxxxxx>]

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:
>
>  	case XDP_TX:
> -		get_page(pdata);
>  		xdpf = xdp_convert_buff_to_frame(xdp);
> +		if (unlikely(!xdpf)) {
> +			trace_xdp_exception(queue->info->netdev, prog, act);
> +			break;
> +		}
> +		get_page(pdata);
>  		err = xennet_xdp_xmit(queue->info->netdev, 1, &xdpf, 0);
>  		if (unlikely(!err))
>  			xdp_return_frame_rx_napi(xdpf);
> -		else if (unlikely(err < 0))
> +		else if (unlikely(err < 0)) {
>  			trace_xdp_exception(queue->info->netdev, prog, act);
> +			xdp_return_frame_rx_napi(xdpf);
> +		}

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);
}

>  		break;
>  	case XDP_REDIRECT:
>  		get_page(pdata);
>  		err = xdp_do_redirect(queue->info->netdev, xdp, prog);
>  		*need_xdp_flush = true;
> -		if (unlikely(err))
> +		if (unlikely(err)) {
>  			trace_xdp_exception(queue->info->netdev, prog, act);
> +			__xdp_return(page_address(pdata), &xdp->mem, true, xdp);
> +		}
>  		break;


Juergen

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