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

Stefan Metzmacher
2023-06-19T13:27:19Z

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);

metze