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

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

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.

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