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

[Jens Axboe <axboe@kernel.dk>]

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.

Note that it was also broken before, just differently broken. The most
likely outcome here was a full retry and now getting -EFAULT.

-- 
Jens Axboe