Re: [PATCH] io_uring/net: save msghdr->msg_control for retries

Stefan Metzmacher
2023-06-19T14:40:03Z

Am 19.06.23 um 16:38 schrieb Jens Axboe:
> On 6/19/23 7:27?AM, Stefan Metzmacher wrote:
>> Am 19.06.23 um 15:09 schrieb Stefan Metzmacher:
>>> Am 19.06.23 um 15:05 schrieb Jens Axboe:
>>>> On 6/19/23 3:57?AM, Stefan Metzmacher wrote:
>>>>> Hi Jens,
>>>>>
>>>>>> If the application sets ->msg_control and we have to later retry this
>>>>>> command, or if it got queued with IOSQE_ASYNC to begin with, then we
>>>>>> need to retain the original msg_control value. This is due to the net
>>>>>> stack overwriting this field with an in-kernel pointer, to copy it
>>>>>> in. Hitting that path for the second time will now fail the copy from
>>>>>> user, as it's attempting to copy from a non-user address.
>>>>>
>>>>> I'm not 100% sure about the impact of this change.
>>>>>
>>>>> But I think the logic we need is that only the
>>>>> first __sys_sendmsg_sock() that returns > 0 should
>>>>> see msg_control. A retry because of MSG_WAITALL should
>>>>> clear msg_control[len] for a follow up __sys_sendmsg_sock().
>>>>> And I fear the patch below would not clear it...
>>>>>
>>>>> Otherwise the receiver/socket-layer will get the same msg_control twice,
>>>>> which is unexpected.
>>>>
>>>> Yes agree, if we do transfer some (but not all) data and WAITALL is set,
>>>> it should get cleared. I'll post a patch for that.
>>>
>>> Thanks!
>>>
>>>> Note that it was also broken before, just differently broken. The most
>>>> likely outcome here was a full retry and now getting -EFAULT.
>>>
>>> Yes, I can see that it was broken before...
>>
>> I haven't checked myself, but I'm wondering about the recvmsg case,
>> I guess we would need to advance the msg_control buffer after each
>> iteration, in order to avoid overwritting the already received messages
>> on retry.
>>
>> This all gets complicated with things like MSG_CTRUNC.
>>
>> I guess it's too late to reject MSG_WAITALL together with msg_control
>> for io_recvmsg() because of compat reasons,
>> but as MSG_WAITALL is also processed in the socket layer, we could keep it
>> simple for now and skip the this retry logic:
>>
>> if (flags & MSG_WAITALL)
>> min_ret = iov_iter_count(&kmsg->msg.msg_iter);
>>
>> This might become something similar to this,
>> but likely more complex, as would need to record kmsg->controllen == 0
>> condition already in io_recvmsg_prep:
>>
>> if (flags & MSG_WAITALL && kmsg->controllen == 0)
>> min_ret = iov_iter_count(&kmsg->msg.msg_iter);
>
> Yep agree, I think this is the best way - ensure that once we transfer
> data with cmsg, it's a one-shot kind of deal.
>
> Do you want to cut a patch for that one?

No, sorry I'm busy with other stuff and not able to to do any testing...

metze