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/*************************************************************************\
* Copyright (C) Michael Kerrisk, 2018. *
* *
* This program is free software. You may use, modify, and redistribute it *
* under the terms of the GNU General Public License as published by the *
* Free Software Foundation, either version 3 or (at your option) any *
* later version. This program is distributed without any warranty. See *
* the file COPYING.gpl-v3 for details. *
\*************************************************************************/
/* Supplementary program for Chapter 61 */
/* scm_cred_send.c
Used in conjunction with scm_cred_recv.c to demonstrate passing of
process credentials via a UNIX domain socket.
This program sends credentials to a UNIX domain socket.
Usage is as shown in the usageErr() call below.
Credentials can be exchanged over stream or datagram sockets. This program
uses stream sockets by default; the "-d" command-line option specifies
that datagram sockets should be used instead.
This program is Linux-specific.
See also scm_multi_send.c.
*/
#include "scm_cred.h"
int
main(int argc, char *argv[])
{
int data, sfd, opt;
ssize_t ns;
Boolean useDatagramSocket, noExplicitCreds;
struct msghdr msgh;
struct iovec iov;
/* Allocate a char array of suitable size to hold the ancillary data.
However, since this buffer is in reality a 'struct cmsghdr', use a
union to ensure that it is aligned as required for that structure.
Alternatively, we could allocate the buffer using malloc(), which
returns a buffer that satisfies the strictest alignment
requirements of any type */
union {
char buf[CMSG_SPACE(sizeof(struct ucred))];
/* Space large enough to hold a ucred structure */
struct cmsghdr align;
} controlMsg;
struct cmsghdr *cmsgp; /* Pointer used to iterate through
headers in ancillary data */
/* Parse command-line options */
useDatagramSocket = FALSE;
noExplicitCreds = FALSE;
while ((opt = getopt(argc, argv, "dn")) != -1) {
switch (opt) {
case 'd':
useDatagramSocket = TRUE;
break;
case 'n':
noExplicitCreds = TRUE;
break;
default:
usageErr("%s [-d] [-n] [data [PID [UID [GID]]]]\n"
" -d use datagram socket\n"
" -n don't construct explicit "
"credentials structure\n", argv[0]);
}
}
/* The 'msg_name' field can be used to specify the address of the
destination socket when sending a datagram. However, we do not
need to use this field because we use connect() below, which sets
a default outgoing address for datagrams. */
msgh.msg_name = NULL;
msgh.msg_namelen = 0;
/* On Linux, we must transmit at least 1 byte of real data in
order to send ancillary data */
msgh.msg_iov = &iov;
msgh.msg_iovlen = 1;
iov.iov_base = &data;
iov.iov_len = sizeof(int);
/* Data is optionally taken from command line */
data = (argc > optind) ? atoi(argv[optind]) : 12345;
fprintf(stderr, "Sending data = %d\n", data);
if (noExplicitCreds) {
/* Don't construct an explicit credentials structure. (It is not
necessary to do so, if we just want the receiver to receive
our real credentials.) */
printf("Not explicitly sending a credentials structure\n");
msgh.msg_control = NULL;
msgh.msg_controllen = 0;
} else {
struct ucred *ucredp;
/* Set 'msgh' fields to describe the ancillary data buffer */
msgh.msg_control = controlMsg.buf;
msgh.msg_controllen = sizeof(controlMsg.buf);
/* The control message buffer must be zero-initialized in order for the
CMSG_NXTHDR() macro to work correctly. Although we don't need to use
CMSG_NXTHDR() in this example (because there is only one block of
ancillary data), we show this step to demonstrate best practice */
memset(controlMsg.buf, 0, sizeof(controlMsg.buf));
/* Set message header to describe the ancillary data that
we want to send */
cmsgp = CMSG_FIRSTHDR(&msgh);
cmsgp->cmsg_len = CMSG_LEN(sizeof(struct ucred));
cmsgp->cmsg_level = SOL_SOCKET;
cmsgp->cmsg_type = SCM_CREDENTIALS;
/* Set 'ucredp' to point to the data area in the 'cmsghdr' */
ucredp = (struct ucred *) CMSG_DATA(cmsgp);
/* Use sender's own PID, real UID, and real GID, unless
alternate values were supplied on the command line */
ucredp->pid = getpid();
if (argc > optind + 1 && strcmp(argv[optind + 1], "-") != 0)
ucredp->pid = atoi(argv[optind + 1]);
ucredp->uid = getuid();
if (argc > optind + 2 && strcmp(argv[optind + 2], "-") != 0)
ucredp->uid = atoi(argv[optind + 2]);
ucredp->gid = getgid();
if (argc > optind + 3 && strcmp(argv[optind + 3], "-") != 0)
ucredp->gid = atoi(argv[optind + 3]);
printf("Send credentials pid=%ld, uid=%ld, gid=%ld\n",
(long) ucredp->pid, (long) ucredp->uid, (long) ucredp->gid);
}
/* Connect to the peer socket */
sfd = unixConnect(SOCK_PATH, useDatagramSocket ? SOCK_DGRAM : SOCK_STREAM);
if (sfd == -1)
errExit("unixConnect");
/* Send real plus ancillary data */
ns = sendmsg(sfd, &msgh, 0);
if (ns == -1)
errExit("sendmsg");
printf("sendmsg() returned %ld\n", (long) ns);
exit(EXIT_SUCCESS);
}
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