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Date: Sat, 15 Jun 2024 12:06:09 -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 v5 2/4] sock: support put_cmsg to
userspace in TX path
On 6/15/24 4:42 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 put_cmsg to userspace in TX path by storing user
>> msg_control address in a new field in struct msghdr, and adding a new bit
>> flag use_msg_control_user_tx to toggle the behavior of put_cmsg. Thus,
>> it's possible to piggyback information in the msg_control of sendmsg.
>>
>> Signed-off-by: Zijian Zhang <zijianzhang@...edance.com>
>> Signed-off-by: Xiaochun Lu <xiaochun.lu@...edance.com>
>> ---
>> include/linux/socket.h | 4 ++++
>> net/compat.c | 33 +++++++++++++++++++++++++--------
>> net/core/scm.c | 42 ++++++++++++++++++++++++++++++++----------
>> net/socket.c | 2 ++
>> 4 files changed, 63 insertions(+), 18 deletions(-)
>>
>> diff --git a/include/linux/socket.h b/include/linux/socket.h
>> index 89d16b90370b..8d3db04f4a39 100644
>> --- a/include/linux/socket.h
>> +++ b/include/linux/socket.h
>> @@ -71,9 +71,12 @@ struct msghdr {
>> void __user *msg_control_user;
>> };
>> bool msg_control_is_user : 1;
>> + bool use_msg_control_user_tx : 1;
>> bool msg_get_inq : 1;/* return INQ after receive */
>> unsigned int msg_flags; /* flags on received message */
>> + void __user *msg_control_user_tx; /* msg_control_user in TX piggyback path */
>> __kernel_size_t msg_controllen; /* ancillary data buffer length */
>> + __kernel_size_t msg_controllen_user_tx; /* msg_controllen in TX piggyback path */
>> struct kiocb *msg_iocb; /* ptr to iocb for async requests */
>> struct ubuf_info *msg_ubuf;
>> int (*sg_from_iter)(struct sock *sk, struct sk_buff *skb,
>> @@ -391,6 +394,7 @@ struct ucred {
>>
>> extern int move_addr_to_kernel(void __user *uaddr, int ulen, struct sockaddr_storage *kaddr);
>> extern int put_cmsg(struct msghdr*, int level, int type, int len, void *data);
>
>> diff --git a/net/core/scm.c b/net/core/scm.c
>> index 4f6a14babe5a..de70ff1981a1 100644
>> --- a/net/core/scm.c
>> +++ b/net/core/scm.c
>> @@ -228,25 +228,29 @@ int __scm_send(struct socket *sock, struct msghdr *msg, struct scm_cookie *p)
>> }
>> EXPORT_SYMBOL(__scm_send);
>>
>> -int put_cmsg(struct msghdr * msg, int level, int type, int len, void *data)
>> +static int __put_cmsg(struct msghdr *msg, int level, int type, int len, void *data)
>> {
>> int cmlen = CMSG_LEN(len);
>> + __kernel_size_t msg_controllen;
>>
>> + msg_controllen = msg->use_msg_control_user_tx ?
>> + msg->msg_controllen_user_tx : msg->msg_controllen;
>> if (msg->msg_flags & MSG_CMSG_COMPAT)
>> return put_cmsg_compat(msg, level, type, len, data);
>>
>> - if (!msg->msg_control || msg->msg_controllen < sizeof(struct cmsghdr)) {
>> + if (!msg->msg_control || msg_controllen < sizeof(struct cmsghdr)) {
>> msg->msg_flags |= MSG_CTRUNC;
>> return 0; /* XXX: return error? check spec. */
>> }
>> - if (msg->msg_controllen < cmlen) {
>> + if (msg_controllen < cmlen) {
>> msg->msg_flags |= MSG_CTRUNC;
>> - cmlen = msg->msg_controllen;
>> + cmlen = msg_controllen;
>> }
>>
>> - if (msg->msg_control_is_user) {
>> - struct cmsghdr __user *cm = msg->msg_control_user;
>> + if (msg->use_msg_control_user_tx || msg->msg_control_is_user) {
>> + struct cmsghdr __user *cm;
>>
>> + cm = msg->msg_control_is_user ? msg->msg_control_user : msg->msg_control_user_tx;
>> check_object_size(data, cmlen - sizeof(*cm), true);
>>
>> if (!user_write_access_begin(cm, cmlen))
>> @@ -267,12 +271,17 @@ int put_cmsg(struct msghdr * msg, int level, int type, int len, void *data)
>> memcpy(CMSG_DATA(cm), data, cmlen - sizeof(*cm));
>> }
>>
>> - cmlen = min(CMSG_SPACE(len), msg->msg_controllen);
>> - if (msg->msg_control_is_user)
>> + cmlen = min(CMSG_SPACE(len), msg_controllen);
>> + if (msg->msg_control_is_user) {
>> msg->msg_control_user += cmlen;
>> - else
>> + msg->msg_controllen -= cmlen;
>> + } else if (msg->use_msg_control_user_tx) {
>> + msg->msg_control_user_tx += cmlen;
>> + msg->msg_controllen_user_tx -= cmlen;
>> + } else {
>> msg->msg_control += cmlen;
>> - msg->msg_controllen -= cmlen;
>> + msg->msg_controllen -= cmlen;
>> + }
>> return 0;
>>
>> efault_end:
>> @@ -280,8 +289,21 @@ int put_cmsg(struct msghdr * msg, int level, int type, int len, void *data)
>> efault:
>> return -EFAULT;
>> }
>> +
>> +int put_cmsg(struct msghdr *msg, int level, int type, int len, void *data)
>> +{
>> + msg->use_msg_control_user_tx = false;
>> + return __put_cmsg(msg, level, type, len, data);
>> +}
>> EXPORT_SYMBOL(put_cmsg);
>>
>> +int put_cmsg_user_tx(struct msghdr *msg, int level, int type, int len, void *data)
>> +{
>> + msg->use_msg_control_user_tx = true;
>> + return __put_cmsg(msg, level, type, len, data);
>> +}
>> +EXPORT_SYMBOL(put_cmsg_user_tx);
>> +
>> void put_cmsg_scm_timestamping64(struct msghdr *msg, struct scm_timestamping_internal *tss_internal)
>> {
>> struct scm_timestamping64 tss;
>> diff --git a/net/socket.c b/net/socket.c
>> index e416920e9399..2755bc7bef9c 100644
>> --- a/net/socket.c
>> +++ b/net/socket.c
>> @@ -2561,6 +2561,8 @@ static int ____sys_sendmsg(struct socket *sock, struct msghdr *msg_sys,
>> err = -EFAULT;
>> if (copy_from_user(ctl_buf, msg_sys->msg_control_user, ctl_len))
>> goto out_freectl;
>> + msg_sys->msg_control_user_tx = msg_sys->msg_control_user;
>> + msg_sys->msg_controllen_user_tx = msg_sys->msg_controllen;
>
> No need for this separate user_tx pointer and put_cmsg_user_tx.
>
> ___sys_sendmsg copies the user data to a stack allocated kernel
> buffer. All subsequent operations are on this buffer. __put_cmsg
> already supports writing to this kernel buffer.
>
> All that is needed is to copy_to_user the buffer on return from
> __sock_sendmsg. And only if it should be copied, which the bit in
> msghdr can signal.
>
copy_to_user upon returning from __sock_sendmsg is clean, but we may
need to take compat into account.
put_cmsg has already handled compat cleanly, I am trying to reuse it.
Since msg_control_user is overwritten in ____sys_sendmsg to a kernel
stack buffer, I piggyback user_tx pointer for further use by
put_cmsg_user_tx.
Or, upon returning of ____sys_sendmsg, we can set msg_control_user back
to user addr. And, for_each_cmsghdr, if cmsg_type == SCM_ZC_... we can
do put_cmsg?
>> msg_sys->msg_control = ctl_buf;
>> msg_sys->msg_control_is_user = false;
>> }
>> --
>> 2.20.1
>>
>
>
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