Re: Crash in tap pmd when using more than 8 rx queues

[Ferruh Yigit <ferruh.yigit@amd.com>]

On 9/10/2024 5:58 PM, Stephen Hemminger wrote:
> On Fri, 6 Sep 2024 12:16:47 +0100
> Ferruh Yigit <ferruh.yigit@amd.com> wrote:
> 
>> On 9/5/2024 1:55 PM, Edwin Brossette wrote:
>>> Hello,
>>>
>>> I have recently stumbled into an issue with my DPDK-based application
>>> running the failsafe pmd. This pmd uses a tap device, with which my
>>> application fails to start if more than 8 rx queues are used. This issue
>>> appears to be related to this patch:
>>> https://git.dpdk.org/dpdk/commit/?
>>> id=c36ce7099c2187926cd62cff7ebd479823554929 <https://git.dpdk.org/dpdk/  
>>> commit/?id=c36ce7099c2187926cd62cff7ebd479823554929>  
>>>
>>> I have seen in the documentation that there was a limitation to 8 max
>>> queues shared when using a tap device shared between multiple processes.
>>> However, my application uses a single primary process, with no secondary
>>> process, but it appears that I am still running into this limitation.
>>>
>>> Now if we look at this small chunk of code:
>>>
>>> memset(&msg, 0, sizeof(msg));
>>> strlcpy(msg.name <http://msg.name>, TAP_MP_REQ_START_RXTX,
>>> sizeof(msg.name <http://msg.name>));
>>> strlcpy(request_param->port_name, dev->data->name, sizeof(request_param-  
>>>> port_name));  
>>> msg.len_param = sizeof(*request_param);
>>> for (i = 0; i < dev->data->nb_tx_queues; i++) {
>>>     msg.fds[fd_iterator++] = process_private->txq_fds[i];
>>>     msg.num_fds++;
>>>     request_param->txq_count++;
>>> }
>>> for (i = 0; i < dev->data->nb_rx_queues; i++) {
>>>     msg.fds[fd_iterator++] = process_private->rxq_fds[i];
>>>     msg.num_fds++;
>>>     request_param->rxq_count++;
>>> }
>>> (Note that I am not using the latest DPDK version, but stable v23.11.1.
>>> But I believe the issue is still present on latest.)
>>>
>>> There are no checks on the maximum value i can take in the for loops.
>>> Since the size of msg.fds is limited by the maximum of 8 queues shared
>>> between process because of the IPC API, there is a potential buffer
>>> overflow which can happen here.
>>>
>>> See the struct declaration:
>>> struct rte_mp_msg {
>>>      char name[RTE_MP_MAX_NAME_LEN];
>>>      int len_param;
>>>      int num_fds;
>>>      uint8_t param[RTE_MP_MAX_PARAM_LEN];
>>>      int fds[RTE_MP_MAX_FD_NUM];
>>> };
>>>
>>> This means that if the number of queues used is more than 8, the program
>>> will crash. This is what happens on my end as I get the following log:
>>> *** stack smashing detected ***: terminated
>>>
>>> Reverting the commit mentionned above fixes my issue. Also setting a
>>> check like this works for me:
>>>
>>> if (dev->data->nb_tx_queues + dev->data->nb_rx_queues > RTE_MP_MAX_FD_NUM)
>>>      return -1;
>>>
>>> I've made the changes on my local branch to fix my issue. This mail is
>>> just to bring attention on this problem.
>>> Thank you in advance for considering it.
>>>   
>>
>> Hi Edwin,
>>
>> Thanks for the report, I confirm issue is valid, although that code
>> changed a little (to increase 8 limit) [3].
>>
>> And in this release Stephen put another patch [1] to increase the limit
>> even more, but irrelevant from the limit, tap code needs to be fixed.
>>
>> To fix:
>> 1. We need to add "nb_rx_queues > RTE_MP_MAX_FD_NUM" check you
>> mentioned, to not blindly update the 'msg.fds[]'
>> 2. We should prevent this to be a limit for tap PMD when there is only
>> primary process, this seems was oversight in our end.
>>
> 
> It is not clear what the error handling should be if the user requests
> 10 queues but RTE_MP_MAX_FD_NUM is 8. Ideally, it should work if no secondary
> process is used. But there is no good way to know that in the driver.
> 
> That is why it is best to just set TAP max queues to be less than
> or equal to RTE_MP_MAX_FD_NUM, and enforce that with a static assertion
> at compile time.
>

We can limit queue number to RTE_MP_MAX_FD_NUM when there is secondary
process, but when there is only primary process this restriction is
artificial, and I think we should try to remove it.

One solution can be to provide a devarg if tap pmd will be used only in
primary process, and in this case it skips all code required for
secondary process support, this removes the queue number limit.
Also this can be backward compatible, without devarg default behavior
can be secondary support and queue number is limited, with devarg limit
removed etc..

By spending some time on it probably we can come with even better
solution. Like improving MP socket communication to remove this
restriction completely independent from RTE_MP_MAX_FD_NUM size...