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

Jürgen Groß
2025-04-17T10:23:59Z

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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

P.S.: please don't use HTML in emails

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