aboutsummaryrefslogtreecommitdiff
path: root/conmon/cmsg.c
blob: c44db2ef1dd8f8e98941506f4e886965dc9b18c0 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
/*
 * Copyright 2016 SUSE LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/* NOTE: This code comes directly from runc/libcontainer/utils/cmsg.c. */

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>

#include "cmsg.h"

#define error(fmt, ...)							\
	({								\
		fprintf(stderr, "nsenter: " fmt ": %m\n", ##__VA_ARGS__); \
		errno = ECOMM;						\
		goto err; /* return value */				\
	})

/*
 * Sends a file descriptor along the sockfd provided. Returns the return
 * value of sendmsg(2). Any synchronisation and preparation of state
 * should be done external to this (we expect the other side to be in
 * recvfd() in the code).
 */
ssize_t sendfd(int sockfd, struct file_t file)
{
	struct msghdr msg = {0};
	struct iovec iov[1] = {0};
	struct cmsghdr *cmsg;
	int *fdptr;

	union {
		char buf[CMSG_SPACE(sizeof(file.fd))];
		struct cmsghdr align;
	} u;

	/*
	 * We need to send some other data along with the ancillary data,
	 * otherwise the other side won't recieve any data. This is very
	 * well-hidden in the documentation (and only applies to
	 * SOCK_STREAM). See the bottom part of unix(7).
	 */
	iov[0].iov_base = file.name;
	iov[0].iov_len = strlen(file.name) + 1;

	msg.msg_name = NULL;
	msg.msg_namelen = 0;
	msg.msg_iov = iov;
	msg.msg_iovlen = 1;
	msg.msg_control = u.buf;
	msg.msg_controllen = sizeof(u.buf);

	cmsg = CMSG_FIRSTHDR(&msg);
	cmsg->cmsg_level = SOL_SOCKET;
	cmsg->cmsg_type = SCM_RIGHTS;
	cmsg->cmsg_len = CMSG_LEN(sizeof(int));

	fdptr = (int *) CMSG_DATA(cmsg);
	memcpy(fdptr, &file.fd, sizeof(int));

	return sendmsg(sockfd, &msg, 0);
}

/*
 * Receives a file descriptor from the sockfd provided. Returns the file
 * descriptor as sent from sendfd(). It will return the file descriptor
 * or die (literally) trying. Any synchronisation and preparation of
 * state should be done external to this (we expect the other side to be
 * in sendfd() in the code).
 */
struct file_t recvfd(int sockfd)
{
	struct msghdr msg = {0};
	struct iovec iov[1] = {0};
	struct cmsghdr *cmsg;
	struct file_t file = {0};
	int *fdptr;
	int olderrno;

	union {
		char buf[CMSG_SPACE(sizeof(file.fd))];
		struct cmsghdr align;
	} u;

	/* Allocate a buffer. */
	/* TODO: Make this dynamic with MSG_PEEK. */
	file.name = malloc(TAG_BUFFER);
	if (!file.name)
		error("recvfd: failed to allocate file.tag buffer\n");

	/*
	 * We need to "recieve" the non-ancillary data even though we don't
	 * plan to use it at all. Otherwise, things won't work as expected.
	 * See unix(7) and other well-hidden documentation.
	 */
	iov[0].iov_base = file.name;
	iov[0].iov_len = TAG_BUFFER;

	msg.msg_name = NULL;
	msg.msg_namelen = 0;
	msg.msg_iov = iov;
	msg.msg_iovlen = 1;
	msg.msg_control = u.buf;
	msg.msg_controllen = sizeof(u.buf);

	ssize_t ret = recvmsg(sockfd, &msg, 0);
	if (ret < 0)
		goto err;

	cmsg = CMSG_FIRSTHDR(&msg);
	if (!cmsg)
		error("recvfd: got NULL from CMSG_FIRSTHDR");
	if (cmsg->cmsg_level != SOL_SOCKET)
		error("recvfd: expected SOL_SOCKET in cmsg: %d", cmsg->cmsg_level);
	if (cmsg->cmsg_type != SCM_RIGHTS)
		error("recvfd: expected SCM_RIGHTS in cmsg: %d", cmsg->cmsg_type);
	if (cmsg->cmsg_len != CMSG_LEN(sizeof(int)))
		error("recvfd: expected correct CMSG_LEN in cmsg: %lu", cmsg->cmsg_len);

	fdptr = (int *) CMSG_DATA(cmsg);
	if (!fdptr || *fdptr < 0)
		error("recvfd: recieved invalid pointer");

	file.fd = *fdptr;
	return file;

err:
	olderrno = errno;
	free(file.name);
	errno = olderrno;
	return (struct file_t){0};
}