Date: Mon, 1 Jul 2024 12:46:05 -0700
From: Zijian Zhang <zijianzhang@...edance.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>, netdev@...r.kernel.org
Cc: edumazet@...gle.com, cong.wang@...edance.com, xiaochun.lu@...edance.com
Subject: Re: [External] Re: [PATCH net-next v6 2/4] sock: support copy cmsg to
 userspace in TX path



On 6/30/24 7:43 AM, Willem de Bruijn wrote:
> zijianzhang@ 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.
>>
>> Signed-off-by: Zijian Zhang <zijianzhang@...edance.com>
>> Signed-off-by: Xiaochun Lu <xiaochun.lu@...edance.com>
>> ---
>>   include/linux/socket.h |  6 ++++++
>>   net/core/sock.c        |  2 ++
>>   net/ipv4/ip_sockglue.c |  2 ++
>>   net/ipv6/datagram.c    |  3 +++
>>   net/socket.c           | 45 ++++++++++++++++++++++++++++++++++++++++++
>>   5 files changed, 58 insertions(+)
>>
>> diff --git a/include/linux/socket.h b/include/linux/socket.h
>> index 89d16b90370b..35adc30c9db6 100644
>> --- a/include/linux/socket.h
>> +++ b/include/linux/socket.h
>> @@ -168,6 +168,11 @@ static inline struct cmsghdr * cmsg_nxthdr (struct msghdr *__msg, struct cmsghdr
>>   	return __cmsg_nxthdr(__msg->msg_control, __msg->msg_controllen, __cmsg);
>>   }
>>   
>> +static inline bool cmsg_copy_to_user(struct cmsghdr *__cmsg)
>> +{
>> +	return 0;
>> +}
>> +
>>   static inline size_t msg_data_left(struct msghdr *msg)
>>   {
>>   	return iov_iter_count(&msg->msg_iter);
>> @@ -329,6 +334,7 @@ struct ucred {
>>   
>>   #define MSG_ZEROCOPY	0x4000000	/* Use user data in kernel path */
>>   #define MSG_SPLICE_PAGES 0x8000000	/* Splice the pages from the iterator in sendmsg() */
>> +#define MSG_CMSG_COPY_TO_USER	0x10000000	/* Copy cmsg to user space */
> 
> Careful that userspace must not be able to set this bit. See also
> MSG_INTERNAL_SENDMSG_FLAGS.
> 
> Perhaps better to define a bit like msg_control_is_user.
> 
>>   #define MSG_FASTOPEN	0x20000000	/* Send data in TCP SYN */
>>   #define MSG_CMSG_CLOEXEC 0x40000000	/* Set close_on_exec for file
>>   					   descriptor received through
>> diff --git a/net/core/sock.c b/net/core/sock.c
>> index 9abc4fe25953..4a766a91ff5c 100644
>> --- a/net/core/sock.c
>> +++ b/net/core/sock.c
>> @@ -2879,6 +2879,8 @@ int sock_cmsg_send(struct sock *sk, struct msghdr *msg,
>>   	for_each_cmsghdr(cmsg, msg) {
>>   		if (!CMSG_OK(msg, cmsg))
>>   			return -EINVAL;
>> +		if (cmsg_copy_to_user(cmsg))
>> +			msg->msg_flags |= MSG_CMSG_COPY_TO_USER;
> 
> Probably better to pass msg to __sock_cmsg_send and only set this
> field in the specific cmsg handler that uses it.
> 

Thanks for the above suggestions!

>>   		if (cmsg->cmsg_level != SOL_SOCKET)
>>   			continue;
>>   		ret = __sock_cmsg_send(sk, cmsg, sockc);
...
>> +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 long cmsg_ptr = (unsigned long)umsg->msg_control;
>> +	unsigned int flags = msg_sys->msg_flags;
>> +	struct msghdr msg_user = *msg_sys;
>> +	struct cmsghdr *cmsg;
>> +	int err;
>> +
>> +	msg_user.msg_control = umsg->msg_control;
>> +	msg_user.msg_control_is_user = true;
>> +	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));
>> +	}
> 
> Alternatively just copy the entire msg_control if any cmsg wants to
> be copied back. The others will be unmodified. No need to iterate
> then.
> 

Copy the entire msg_control via copy_to_user does not take
MSG_CMSG_COMPAT into account. I may have to use put_cmsg to deal
with the compat version, and thus have to keep the for loop?

If so, I may keep the function cmsg_copy_to_user to avoid extra copy?

>> +
>> +	err = __put_user((msg_sys->msg_flags & ~MSG_CMSG_COMPAT), COMPAT_FLAGS(umsg));
>> +	if (err)
>> +		return err;
> 
> Does this value need to be written?
> 

I did this according to ____sys_recvmsg, maybe it's useful to export
flag like MSG_CTRUNC to users?

>> +	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;
>> +}
>> +
>>   static int ___sys_sendmsg(struct socket *sock, struct user_msghdr __user *msg,
>>   			 struct msghdr *msg_sys, unsigned int flags,
>>   			 struct used_address *used_address,
>> @@ -2638,6 +2671,18 @@ static int ___sys_sendmsg(struct socket *sock, struct user_msghdr __user *msg,
>>   
>>   	err = ____sys_sendmsg(sock, msg_sys, flags, used_address,
>>   				allowed_msghdr_flags);
>> +	if (err < 0)
>> +		goto out;
>> +
>> +	if (msg_sys->msg_flags & MSG_CMSG_COPY_TO_USER) {
>> +		ssize_t len = err;
>> +
>> +		err = sendmsg_copy_cmsg_to_user(msg_sys, msg);
>> +		if (err)
>> +			goto out;
>> +		err = len;
>> +	}
>> +out:
>>   	kfree(iov);
>>   	return err;
>>   }
>> -- 
>> 2.20.1
>>
> 
> 

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