netdev - Re: [PATCH net-next v6 2/4] sock: support copy cmsg to userspace in TX path

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Message-ID: <20240627145140.675bbf86@kernel.org>
Date: Thu, 27 Jun 2024 14:51:40 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: zijianzhang@...edance.com
Cc: netdev@...r.kernel.org, edumazet@...gle.com,
 willemdebruijn.kernel@...il.com, cong.wang@...edance.com,
 xiaochun.lu@...edance.com
Subject: Re: [PATCH net-next v6 2/4] sock: support copy cmsg to userspace in
 TX path

On Wed, 26 Jun 2024 19:34:01 +0000 zijianzhang@...edance.com wrote:
> From: Zijian Zhang <zijianzhang@...edance.com>
> 
> Since ____sys_sendmsg creates a kernel copy of msg_control and passes
> that to the callees, put_cmsg will write into this kernel buffer. If
> people want to piggyback some information like timestamps upon returning
> of sendmsg. ____sys_sendmsg will have to copy_to_user to the original buf,
> which is not supported. As a result, users typically have to call recvmsg
> on the ERRMSG_QUEUE of the socket, incurring extra system call overhead.
> 
> This commit supports copying cmsg to userspace in TX path by introducing
> a flag MSG_CMSG_COPY_TO_USER in struct msghdr to guide the copy logic
> upon returning of ___sys_sendmsg.

sparse complains about the annotations:

net/socket.c:2635:30: warning: incorrect type in assignment (different address spaces)
net/socket.c:2635:30:    expected void *msg_control
net/socket.c:2635:30:    got void [noderef] __user *[noderef] __user msg_control
net/socket.c:2629:49: warning: dereference of noderef expression
-- 
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