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

[Stefan Metzmacher <metze@samba.org>]

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.

metze