linux-kernel - Re: [PATCH] xen-netfront: handle NULL returned by xdp_convert_buff_to

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Message-ID: <ff6eed52-5f1f-4070-90dc-8cf057f35e41@nppct.ru>
Date: Thu, 17 Apr 2025 14:19:36 +0300
From: Alexey <sdl@...ct.ru>
To: Jürgen Groß <jgross@...e.com>,
 Jakub Kicinski <kuba@...nel.org>
Cc: Stefano Stabellini <sstabellini@...nel.org>,
 Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Paolo Abeni <pabeni@...hat.com>, Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>,
 Jesper Dangaard Brouer <hawk@...nel.org>,
 John Fastabend <john.fastabend@...il.com>, xen-devel@...ts.xenproject.org,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
 lvc-project@...uxtesting.org, stable@...r.kernel.org
Subject: Re: [PATCH] xen-netfront: handle NULL returned by
 xdp_convert_buff_to_frame()


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