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

[Alexey <sdl@nppct.ru>]

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