netdev - Re: [PATCH net-next v7 1/3] sock: support copying cmsgs to the user space in sendmsg

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Message-ID: <6fca7568-6eef-479d-afdf-7960a4f6382c@bytedance.com>
Date: Tue, 9 Jul 2024 10:17:10 -0700
From: Zijian Zhang <zijianzhang@...edance.com>
To: Simon Horman <horms@...nel.org>
Cc: netdev@...r.kernel.org, edumazet@...gle.com,
 willemdebruijn.kernel@...il.com, cong.wang@...edance.com,
 xiaochun.lu@...edance.com, "David S. Miller" <davem@...emloft.net>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 David Ahern <dsahern@...nel.org>, Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH net-next v7 1/3] sock: support copying cmsgs to the user
 space in sendmsg

On 7/9/24 2:14 AM, Simon Horman wrote:
> + Dave Miller, Jakub Kicinski, Paolo Abeni, David Ahern, and Jens Axboe
> 
>    Please generate the CC list Networking for patches using
>    get_maintainer.pl --git-min-percent=25 this.patch
>    but omitting LKML.
> 
> 
> On Mon, Jul 08, 2024 at 09:04:03PM +0000, zijianzhang@...edance.com wrote:
>> From: Zijian Zhang <zijianzhang@...edance.com>
>>
>> Users can pass msg_control as a placeholder to recvmsg, and get some info
>> from the kernel upon returning of it, but it's not available for sendmsg.
>> Recvmsg uses put_cmsg to copy info back to the user, while ____sys_sendmsg
>> creates a kernel copy of msg_control and passes that to the callees,
>> put_cmsg in sendmsg path will write into this kernel buffer.
>>
>> If users want to get info after returning of sendmsg, they typically have
>> to call recvmsg on the ERRMSG_QUEUE of the socket, incurring extra system
>> call overhead. This commit supports copying cmsg from the kernel space to
>> the user space upon returning of sendmsg to mitigate this overhead.
>>
>> Signed-off-by: Zijian Zhang <zijianzhang@...edance.com>
>> Signed-off-by: Xiaochun Lu <xiaochun.lu@...edance.com>
> 
> ...
> 
>> diff --git a/net/socket.c b/net/socket.c
>> index e416920e9399..6a9c9e24d781 100644
>> --- a/net/socket.c
>> +++ b/net/socket.c
>> @@ -2525,8 +2525,43 @@ static int copy_msghdr_from_user(struct msghdr *kmsg,
>>   	return err < 0 ? err : 0;
>>   }
>>   
>> -static int ____sys_sendmsg(struct socket *sock, struct msghdr *msg_sys,
>> -			   unsigned int flags, struct used_address *used_address,
>> +static int sendmsg_copy_cmsg_to_user(struct msghdr *msg_sys,
>> +				     struct user_msghdr __user *umsg)
>> +{
>> +	struct compat_msghdr __user *umsg_compat =
>> +				(struct compat_msghdr __user *)umsg;
>> +	unsigned int flags = msg_sys->msg_flags;
>> +	struct msghdr msg_user = *msg_sys;
>> +	unsigned long cmsg_ptr;
>> +	struct cmsghdr *cmsg;
>> +	int err;
>> +
>> +	msg_user.msg_control_is_user = true;
>> +	msg_user.msg_control_user = umsg->msg_control;
> 
> nit: Sparse seems unhappy about the use of a __user pointer here.
> 
>       net/socket.c:2540:37: warning: dereference of noderef expression
> 
>> +	cmsg_ptr = (unsigned long)msg_user.msg_control;
>> +	for_each_cmsghdr(cmsg, msg_sys) {
>> +		if (!CMSG_OK(msg_sys, cmsg))
>> +			break;
>> +		if (cmsg_copy_to_user(cmsg))
>> +			put_cmsg(&msg_user, cmsg->cmsg_level, cmsg->cmsg_type,
>> +				 cmsg->cmsg_len - sizeof(*cmsg), CMSG_DATA(cmsg));
>> +	}
>> +
>> +	err = __put_user((msg_sys->msg_flags & ~MSG_CMSG_COMPAT), COMPAT_FLAGS(umsg));
> 
> nit: The line above could be trivially line-wrapped so that it is
>       no more than 80 columns wide, as is still preferred in Networking code.
> 
>       Flagged by: checkpatch.pl --max-line-length=80
> 
>> +	if (err)
>> +		return err;
>> +	if (MSG_CMSG_COMPAT & flags)
>> +		err = __put_user((unsigned long)msg_user.msg_control - cmsg_ptr,
>> +				 &umsg_compat->msg_controllen);
>> +	else
>> +		err = __put_user((unsigned long)msg_user.msg_control - cmsg_ptr,
>> +				 &umsg->msg_controllen);
>> +	return err;
>> +}
> 
> ...

Thanks for the suggestions, will update in the next version.