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

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Message-ID: <4679ca25-572b-44aa-bc00-cb9dc1c0080c@suse.com>
Date: Thu, 17 Apr 2025 12:23:56 +0200
From: Jürgen Groß <jgross@...e.com>
To: Alexey <sdl@...ct.ru>, 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 17.04.25 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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