cmsg: add cmsg {send,recv}fd wrappers
This adds C wrappers for sendmsg and recvmsg, specifically used for passing around file descriptors in Go. The wrappers (sendfd, recvfd) expect to be called in a context where it makes sense (where the other side is carrying out the corresponding action). This patch is part of the console rewrite patchset. Signed-off-by: Aleksa Sarai <asarai@suse.de>
This commit is contained in:
Aleksa Sarai
parent
8893fa693b
commit
2055115566
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@ -0,0 +1,148 @@
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/*
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* Copyright 2016 SUSE LLC
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#include <errno.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#include <sys/socket.h>
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#include <sys/types.h>
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#include <unistd.h>
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#include "cmsg.h"
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#define error(fmt, ...) \
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({ \
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fprintf(stderr, "nsenter: " fmt ": %m\n", ##__VA_ARGS__); \
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errno = ECOMM; \
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goto err; /* return value */ \
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})
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/*
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* Sends a file descriptor along the sockfd provided. Returns the return
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* value of sendmsg(2). Any synchronisation and preparation of state
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* should be done external to this (we expect the other side to be in
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* recvfd() in the code).
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*/
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ssize_t sendfd(int sockfd, struct file_t file)
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{
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struct msghdr msg = {0};
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struct iovec iov[1] = {0};
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struct cmsghdr *cmsg;
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int *fdptr;
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int ret;
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union {
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char buf[CMSG_SPACE(sizeof(file.fd))];
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struct cmsghdr align;
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} u;
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/*
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* We need to send some other data along with the ancillary data,
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* otherwise the other side won't recieve any data. This is very
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* well-hidden in the documentation (and only applies to
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* SOCK_STREAM). See the bottom part of unix(7).
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*/
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iov[0].iov_base = file.name;
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iov[0].iov_len = strlen(file.name) + 1;
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msg.msg_name = NULL;
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msg.msg_namelen = 0;
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msg.msg_iov = iov;
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msg.msg_iovlen = 1;
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msg.msg_control = u.buf;
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msg.msg_controllen = sizeof(u.buf);
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cmsg = CMSG_FIRSTHDR(&msg);
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cmsg->cmsg_level = SOL_SOCKET;
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cmsg->cmsg_type = SCM_RIGHTS;
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cmsg->cmsg_len = CMSG_LEN(sizeof(int));
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fdptr = (int *) CMSG_DATA(cmsg);
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memcpy(fdptr, &file.fd, sizeof(int));
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return sendmsg(sockfd, &msg, 0);
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}
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/*
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* Receives a file descriptor from the sockfd provided. Returns the file
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* descriptor as sent from sendfd(). It will return the file descriptor
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* or die (literally) trying. Any synchronisation and preparation of
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* state should be done external to this (we expect the other side to be
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* in sendfd() in the code).
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*/
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struct file_t recvfd(int sockfd)
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{
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struct msghdr msg = {0};
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struct iovec iov[1] = {0};
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struct cmsghdr *cmsg;
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struct file_t file = {0};
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int *fdptr;
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int olderrno;
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union {
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char buf[CMSG_SPACE(sizeof(file.fd))];
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struct cmsghdr align;
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} u;
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/* Allocate a buffer. */
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/* TODO: Make this dynamic with MSG_PEEK. */
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file.name = malloc(TAG_BUFFER);
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if (!file.name)
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error("recvfd: failed to allocate file.tag buffer\n");
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/*
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* We need to "recieve" the non-ancillary data even though we don't
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* plan to use it at all. Otherwise, things won't work as expected.
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* See unix(7) and other well-hidden documentation.
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*/
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iov[0].iov_base = file.name;
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iov[0].iov_len = TAG_BUFFER;
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msg.msg_name = NULL;
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msg.msg_namelen = 0;
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msg.msg_iov = iov;
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msg.msg_iovlen = 1;
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msg.msg_control = u.buf;
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msg.msg_controllen = sizeof(u.buf);
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ssize_t ret = recvmsg(sockfd, &msg, 0);
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if (ret < 0)
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goto err;
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cmsg = CMSG_FIRSTHDR(&msg);
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if (!cmsg)
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error("recvfd: got NULL from CMSG_FIRSTHDR");
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if (cmsg->cmsg_level != SOL_SOCKET)
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error("recvfd: expected SOL_SOCKET in cmsg: %d", cmsg->cmsg_level);
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if (cmsg->cmsg_type != SCM_RIGHTS)
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error("recvfd: expected SCM_RIGHTS in cmsg: %d", cmsg->cmsg_type);
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if (cmsg->cmsg_len != CMSG_LEN(sizeof(int)))
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error("recvfd: expected correct CMSG_LEN in cmsg: %lu", cmsg->cmsg_len);
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fdptr = (int *) CMSG_DATA(cmsg);
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if (!fdptr || *fdptr < 0)
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error("recvfd: recieved invalid pointer");
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file.fd = *fdptr;
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return file;
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err:
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olderrno = errno;
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free(file.name);
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errno = olderrno;
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return (struct file_t){0};
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}
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@ -0,0 +1,57 @@
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// +build linux
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package utils
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/*
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* Copyright 2016 SUSE LLC
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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/*
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#include <errno.h>
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#include <stdlib.h>
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#include "cmsg.h"
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*/
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import "C"
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import (
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"os"
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"unsafe"
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)
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// RecvFd waits for a file descriptor to be sent over the given AF_UNIX
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// socket. The file name of the remote file descriptor will be recreated
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// locally (it is sent as non-auxilliary data in the same payload).
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func RecvFd(socket *os.File) (*os.File, error) {
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file, err := C.recvfd(C.int(socket.Fd()))
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if err != nil {
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return nil, err
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}
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defer C.free(unsafe.Pointer(file.name))
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return os.NewFile(uintptr(file.fd), C.GoString(file.name)), nil
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}
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// SendFd sends a file descriptor over the given AF_UNIX socket. In
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// addition, the file.Name() of the given file will also be sent as
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// non-auxilliary data in the same payload (allowing to send contextual
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// information for a file descriptor).
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func SendFd(socket, file *os.File) error {
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var cfile C.struct_file_t
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cfile.fd = C.int(file.Fd())
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cfile.name = C.CString(file.Name())
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defer C.free(unsafe.Pointer(cfile.name))
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_, err := C.sendfd(C.int(socket.Fd()), cfile)
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return err
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}
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@ -0,0 +1,36 @@
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/*
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* Copyright 2016 SUSE LLC
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#pragma once
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#if !defined(CMSG_H)
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#define CMSG_H
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#include <sys/types.h>
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/* TODO: Implement this properly with MSG_PEEK. */
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#define TAG_BUFFER 4096
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/* This mirrors Go's (*os.File). */
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struct file_t {
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char *name;
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int fd;
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};
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struct file_t recvfd(int sockfd);
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ssize_t sendfd(int sockfd, struct file_t file);
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#endif /* !defined(CMSG_H) */
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