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libcontainer: rewrite cmsg to use sys/unix 

The original implementation is in C, which increases cognitive load and
possibly might cause us problems in the future. Since sys/unix is better
maintained than the syscall standard library switching makes more sense.

Signed-off-by: Aleksa Sarai <asarai@suse.de>
This commit is contained in:
Aleksa Sarai 2017-03-29 22:39:05 +11:00
parent 85de7ec363
commit cbc4f9865a
No known key found for this signature in database
GPG Key ID: 9E18AA267DDB8DB4
3 changed files with 56 additions and 202 deletions
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148
libcontainer/utils/cmsg.c
View File

@ -1,148 +0,0 @@
/*
* 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.
*/
#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;
int ret;
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", (unsigned long)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};
}

74
libcontainer/utils/cmsg.go
View File

@ -3,7 +3,7 @@
package utils package utils
/* /*
* Copyright 2016 SUSE LLC * Copyright 2016, 2017 SUSE LLC
* *
* Licensed under the Apache License, Version 2.0 (the "License"); * Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License. * you may not use this file except in compliance with the License.
@ -18,28 +18,66 @@ package utils
* limitations under the License. * limitations under the License.
*/ */
/*
#include <errno.h>
#include <stdlib.h>
#include "cmsg.h"
*/
import "C"
import ( import (
"fmt"
"os" "os"
"unsafe"
"golang.org/x/sys/unix"
) )
// MaxSendfdLen is the maximum length of the name of a file descriptor being
// sent using SendFd. The name of the file handle returned by RecvFd will never
// be larger than this value.
const MaxNameLen = 4096
// oobSpace is the size of the oob slice required to store a single FD. Note
// that unix.UnixRights appears to make the assumption that fd is always int32,
// so sizeof(fd) = 4.
var oobSpace = unix.CmsgSpace(4)
// RecvFd waits for a file descriptor to be sent over the given AF_UNIX // RecvFd waits for a file descriptor to be sent over the given AF_UNIX
// socket. The file name of the remote file descriptor will be recreated // socket. The file name of the remote file descriptor will be recreated
// locally (it is sent as non-auxiliary data in the same payload). // locally (it is sent as non-auxiliary data in the same payload).
func RecvFd(socket *os.File) (*os.File, error) { func RecvFd(socket *os.File) (*os.File, error) {
file, err := C.recvfd(C.int(socket.Fd())) // For some reason, unix.Recvmsg uses the length rather than the capacity
// when passing the msg_controllen and other attributes to recvmsg. So we
// have to actually set the length.
name := make([]byte, MaxNameLen)
oob := make([]byte, oobSpace)
sockfd := socket.Fd()
n, oobn, _, _, err := unix.Recvmsg(int(sockfd), name, oob, 0)
if err != nil { if err != nil {
return nil, err return nil, err
} }
defer C.free(unsafe.Pointer(file.name))
return os.NewFile(uintptr(file.fd), C.GoString(file.name)), nil if n >= MaxNameLen || oobn != oobSpace {
return nil, fmt.Errorf("recvfd: incorrect number of bytes read (n=%d oobn=%d)", n, oobn)
}
// Truncate.
name = name[:n]
oob = oob[:oobn]
scms, err := unix.ParseSocketControlMessage(oob)
if err != nil {
return nil, err
}
if len(scms) != 1 {
return nil, fmt.Errorf("recvfd: number of SCMs is not 1: %d", len(scms))
}
scm := scms[0]
fds, err := unix.ParseUnixRights(&scm)
if err != nil {
return nil, err
}
if len(fds) != 1 {
return nil, fmt.Errorf("recvfd: number of fds is not 1: %d", len(fds))
}
fd := uintptr(fds[0])
return os.NewFile(fd, string(name)), nil
} }
// SendFd sends a file descriptor over the given AF_UNIX socket. In // SendFd sends a file descriptor over the given AF_UNIX socket. In
@ -47,11 +85,11 @@ func RecvFd(socket *os.File) (*os.File, error) {
// non-auxiliary data in the same payload (allowing to send contextual // non-auxiliary data in the same payload (allowing to send contextual
// information for a file descriptor). // information for a file descriptor).
func SendFd(socket, file *os.File) error { func SendFd(socket, file *os.File) error {
var cfile C.struct_file_t name := []byte(file.Name())
cfile.fd = C.int(file.Fd()) if len(name) >= MaxNameLen {
cfile.name = C.CString(file.Name()) return fmt.Errorf("sendfd: filename too long: %s", file.Name())
defer C.free(unsafe.Pointer(cfile.name)) }
oob := unix.UnixRights(int(file.Fd()))
_, err := C.sendfd(C.int(socket.Fd()), cfile) return unix.Sendmsg(int(socket.Fd()), name, oob, nil, 0)
return err
} }

36
libcontainer/utils/cmsg.h
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@ -1,36 +0,0 @@
/*
* 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.
*/
#pragma once
#if !defined(CMSG_H)
#define CMSG_H
#include <sys/types.h>
/* TODO: Implement this properly with MSG_PEEK. */
#define TAG_BUFFER 4096
/* This mirrors Go's (*os.File). */
struct file_t {
char *name;
int fd;
};
struct file_t recvfd(int sockfd);
ssize_t sendfd(int sockfd, struct file_t file);
#endif /* !defined(CMSG_H) */