Re: [PATCH v2] net/memif: fix buffer overflow in zero copy Rx

[Mihai Brodschi <mihai.brodschi@broadcom.com>]

Hi Ferruh,

Apologies for the late response.
I've run some performance tests for the two proposed solutions.
In the tables below, the rte_memcpy results correspond to this patch.
The 2xpktmbuf_alloc results correspond to the other proposed solution.

bash commands:
server# ./dpdk-testpmd --vdev=net_memif0,id=1,role=server,bsize=1024,rsize=8 --single -l<SERVER_CORES> --file=test1 -- --nb-cores <NB_CORES> --txq <NB_CORES> --rxq <NB_CORES> --burst <BURST> -i
client# ./dpdk-testpmd --vdev=net_memif0,id=1,role=client,bsize=1024,rsize=8,zero-copy=yes --single -l<CLIENT_CORES> --file=test2 -- --nb-cores <NB_CORES> --txq <NB_CORES> --rxq <NB_CORES> --burst <BURST> -i

testpmd commands:
client:
testpmd> start
server:
testpmd> start tx_first


CPU: AMD EPYC 7713P
RAM: DDR4-3200
OS: Debian 12
DPDK: 22.11.1
SERVER_CORES=72,8,9,10,11
CLIENT_CORES=76,12,13,14,15

Results:
==================================================================
|                          | 1 CORE     | 2 CORES    | 4 CORES   |
==================================================================
| unpatched burst=32       | 9.95 Gbps  | 19.24 Gbps | 36.4 Gbps |
------------------------------------------------------------------
| 2xpktmbuf_alloc burst=32 | 9.86 Gbps  | 18.88 Gbps | 36.6 Gbps |
------------------------------------------------------------------
| 2xpktmbuf_alloc burst=31 | 9.17 Gbps  | 18.69 Gbps | 35.1 Gbps |
------------------------------------------------------------------
| rte_memcpy burst=32      | 9.54 Gbps  | 19.10 Gbps | 36.6 Gbps |
------------------------------------------------------------------
| rte_memcpy burst=31      | 9.39 Gbps  | 18.53 Gbps | 35.5 Gbps |
==================================================================


CPU: Intel Core i7-14700HX
RAM: DDR5-5600
OS:  Ubuntu 24.04.1
DPDK: 23.11.1
SERVER_CORES=0,1,3,5,7
CLIENT_CORES=8,9,11,13,15

Results:
==================================================================
|                          | 1 CORE     | 2 CORES    | 4 CORES   |
==================================================================
| unpatched burst=32       | 15.52 Gbps | 27.35 Gbps | 46.8 Gbps |
------------------------------------------------------------------
| 2xpktmbuf_alloc burst=32 | 15.49 Gbps | 27.68 Gbps | 46.4 Gbps |
------------------------------------------------------------------
| 2xpktmbuf_alloc burst=31 | 14.98 Gbps | 26.75 Gbps | 45.2 Gbps |
------------------------------------------------------------------
| rte_memcpy burst=32      | 15.99 Gbps | 28.44 Gbps | 49.3 Gbps |
------------------------------------------------------------------
| rte_memcpy burst=31      | 14.85 Gbps | 27.32 Gbps | 46.3 Gbps |
==================================================================


On 19/07/2024 12:03, Ferruh Yigit wrote:
> On 7/8/2024 12:45 PM, Ferruh Yigit wrote:
>> On 7/8/2024 4:39 AM, Mihai Brodschi wrote:
>>>
>>>
>>> On 07/07/2024 21:46, Mihai Brodschi wrote:
>>>>
>>>>
>>>> On 07/07/2024 18:18, Mihai Brodschi wrote:
>>>>>
>>>>>
>>>>> On 07/07/2024 17:05, Ferruh Yigit wrote:
>>>>>>
>>>>>> My expectation is numbers should be like following:
>>>>>>
>>>>>> Initially:
>>>>>>  size = 256
>>>>>>  head = 0
>>>>>>  tail = 0
>>>>>>
>>>>>> In first refill:
>>>>>>  n_slots = 256
>>>>>>  head = 256
>>>>>>  tail = 0
>>>>>>
>>>>>> Subsequent run that 32 slots used:
>>>>>>  head = 256
>>>>>>  tail = 32
>>>>>>  n_slots = 32
>>>>>>  rte_pktmbuf_alloc_bulk(mq, buf[head & mask], n_slots);
>>>>>>   head & mask = 0
>>>>>>   // So it fills first 32 elements of buffer, which is inbound
>>>>>>
>>>>>> This will continue as above, combination of only gap filled and head
>>>>>> masked with 'mask' provides the wrapping required.
>>>>>
>>>>> If I understand correctly, this works only if eth_memif_rx_zc always processes
>>>>> a number of packets which is a power of 2, so that the ring's head always wraps
>>>>> around at the end of a refill loop, never in the middle of it.
>>>>> Is there any reason this should be the case?
>>>>> Maybe the tests don't trigger the crash because this condition holds true for them?
>>>>
>>>> Here's how to reproduce the crash on DPDK stable 23.11.1, using testpmd:
>>>>
>>>> Server:
>>>> # ./dpdk-testpmd --vdev=net_memif0,id=1,role=server,bsize=1024,rsize=8 --single-file-segments -l2,3 --file-prefix test1 -- -i
>>>>
>>>> Client:
>>>> # ./dpdk-testpmd --vdev=net_memif0,id=1,role=client,bsize=1024,rsize=8,zero-copy=yes --single-file-segments -l4,5 --file-prefix test2 -- -i
>>>> testpmd> start
>>>>
>>>> Server:
>>>> testpmd> start tx_first
>>>> testpmt> set burst 15
>>>>
>>>> At this point, the client crashes with a segmentation fault.
>>>> Before the burst is set to 15, its default value is 32.
>>>> If the receiver processes packets in bursts of size 2^N, the crash does not occur.
>>>> Setting the burst size to any power of 2 works, anything else crashes.
>>>> After applying this patch, the crashes are completely gone.
>>>
>>> Sorry, this might not crash with a segmentation fault. To confirm the mempool is
>>> corrupted, please compile DPDK with debug=true and the c_args -DRTE_LIBRTE_MEMPOOL_DEBUG.
>>> You should see the client panic when changing the burst size to not be a power of 2.
>>> This also works on the latest main branch.
>>>
>>
>> Hi Mihai,
>>
>> Right, if the buffer size is not multiple of burst size, issue is valid.
>> And as there is a requirement to have buffer size power of two, burst
>> should have the same.
>> I assume this issue is not caught before because default burst size is 32.
>>
>> Can you please share some performance impact of the change, with two
>> possible solutions we discussed above?
>>
>> Other option is to add this as a limitation to the memif zero copy, but
>> this won't be good for usability.
>>
>> We can decide based on performance numbers.
>>
>>
> 
> Hi Jakup,
> 
> Do you have any comment on this?
> 
> I think we should either document this as limitation of the driver, or
> fix it, and if so need to decide the fix.
> 


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