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Date: Sun, 30 Jun 2024 10:43:14 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: zijianzhang@...edance.com,
netdev@...r.kernel.org
Cc: edumazet@...gle.com,
willemdebruijn.kernel@...il.com,
cong.wang@...edance.com,
xiaochun.lu@...edance.com,
Zijian Zhang <zijianzhang@...edance.com>
Subject: Re: [PATCH net-next v6 2/4] sock: support copy cmsg to userspace in
TX path
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.
> if (cmsg->cmsg_level != SOL_SOCKET)
> continue;
> ret = __sock_cmsg_send(sk, cmsg, sockc);
> diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
> index cf377377b52d..464d08b27fa8 100644
> --- a/net/ipv4/ip_sockglue.c
> +++ b/net/ipv4/ip_sockglue.c
> @@ -249,6 +249,8 @@ int ip_cmsg_send(struct sock *sk, struct msghdr *msg, struct ipcm_cookie *ipc,
> 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;
> #if IS_ENABLED(CONFIG_IPV6)
> if (allow_ipv6 &&
> cmsg->cmsg_level == SOL_IPV6 &&
> diff --git a/net/ipv6/datagram.c b/net/ipv6/datagram.c
> index fff78496803d..b0341faf7f83 100644
> --- a/net/ipv6/datagram.c
> +++ b/net/ipv6/datagram.c
> @@ -776,6 +776,9 @@ int ip6_datagram_send_ctl(struct net *net, struct sock *sk,
> goto exit_f;
> }
>
> + if (cmsg_copy_to_user(cmsg))
> + msg->msg_flags |= MSG_CMSG_COPY_TO_USER;
> +
> if (cmsg->cmsg_level == SOL_SOCKET) {
> err = __sock_cmsg_send(sk, cmsg, &ipc6->sockc);
> if (err)
> diff --git a/net/socket.c b/net/socket.c
> index e416920e9399..6523cf5a7f32 100644
> --- a/net/socket.c
> +++ b/net/socket.c
> @@ -2621,6 +2621,39 @@ static int sendmsg_copy_msghdr(struct msghdr *msg,
> return 0;
> }
>
> +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.
> +
> + err = __put_user((msg_sys->msg_flags & ~MSG_CMSG_COMPAT), COMPAT_FLAGS(umsg));
> + if (err)
> + return err;
Does this value need to be written?
> + 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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