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[54.92.141.197]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-6b2a5eb4827sm34867286d6.93.2024.06.15.12.06.11 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 15 Jun 2024 12:06:13 -0700 (PDT) Message-ID: <2fcd2f6c-2c8b-4057-aaa2-e7cfcac3442b@bytedance.com> Date: Sat, 15 Jun 2024 12:06:09 -0700 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [External] Re: [PATCH net-next v5 2/4] sock: support put_cmsg to userspace in TX path To: Willem de Bruijn , netdev@vger.kernel.org Cc: edumazet@google.com, cong.wang@bytedance.com, xiaochun.lu@bytedance.com References: <20240613233133.2463193-1-zijianzhang@bytedance.com> <20240613233133.2463193-3-zijianzhang@bytedance.com> <666d7e1a7b72d_1ba35a294a5@willemb.c.googlers.com.notmuch> Content-Language: en-US From: Zijian Zhang In-Reply-To: <666d7e1a7b72d_1ba35a294a5@willemb.c.googlers.com.notmuch> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 6/15/24 4:42 AM, Willem de Bruijn wrote: > zijianzhang@ wrote: >> From: Zijian Zhang >> >> 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 >> Signed-off-by: Xiaochun Lu >> --- >> 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 >> > >