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

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

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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.

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