namespaces/unshare.c

This is namespaces/unshare.c, an example to accompany the book, The Linux Programming Interface.

This file is not printed in the book; it demonstrates Linux features that are not described in the book (typically features that have appeared since the book was published).

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.

 

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  Cover of The Linux Programming Interface

Function list (Bold in this list means a function is not static)

/* unshare.c

   A simple implementation of the unshare(1) command: unshare
   namespaces and execute a command.

   See https://lwn.net/Articles/531381/
*/
#define _GNU_SOURCE
#include <sched.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/wait.h>

#ifndef CLONE_NEWCGROUP         /* Added in Linux 4.6 */
#define CLONE_NEWCGROUP         0x02000000
#endif

#ifndef CLONE_NEWTIME           /* Added in Linux 5.6 */
#define CLONE_NEWTIME           0x00000080
#endif

/* A simple error-handling function: print an error message based
   on the value in 'errno' and terminate the calling process */

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                        } while (0)
static void
usage(char *pname)
{
    fprintf(stderr, "Usage: %s [options] cmd [arg...]\n", pname);
    fprintf(stderr, "Options can be:\n");
    fprintf(stderr, "    -f   fork() before executing cmd "
            "(useful when unsharing PID namespace)\n");
    fprintf(stderr, "    -C   unshare cgroup namespace\n");
    fprintf(stderr, "    -i   unshare IPC namespace\n");
    fprintf(stderr, "    -m   unshare mount namespace\n");
    fprintf(stderr, "    -n   unshare network namespace\n");
    fprintf(stderr, "    -p   unshare PID namespace\n");
    fprintf(stderr, "    -T   unshare time namespace\n");
    fprintf(stderr, "    -u   unshare UTS namespace\n");
    fprintf(stderr, "    -U   unshare user namespace\n");
    exit(EXIT_FAILURE);
}
int
main(int argc, char *argv[])
{
    int flags = 0;
    int do_fork = 0;
    int opt;
    while ((opt = getopt(argc, argv, "+CfimnpTuU")) != -1) {
        switch (opt) {
        case 'f': do_fork = 1;                  break;
        case 'C': flags |= CLONE_NEWCGROUP;     break;
        case 'i': flags |= CLONE_NEWIPC;        break;
        case 'm': flags |= CLONE_NEWNS;         break;
        case 'n': flags |= CLONE_NEWNET;        break;
        case 'p': flags |= CLONE_NEWPID;        break;
        case 'T': flags |= CLONE_NEWTIME;       break;
        case 'u': flags |= CLONE_NEWUTS;        break;
        case 'U': flags |= CLONE_NEWUSER;       break;
        default:  usage(argv[0]);
        }
    }

    if (optind >= argc)
        usage(argv[0]);

    if (unshare(flags) == -1)
        errExit("unshare");

    /* If we are unsharing the PID namespace, then the caller is *not*
       moved into the new namespace. Instead, only the children are moved
       into the namespace. Therefore, we support an option that causes
       the program to call fork() before executing the specified program,
       in order to create a new child that will be created in a new PID
       namespace. */

    if (do_fork) {
        if (fork()) {
            wait(NULL);         /* Parent waits for child to complete */
            exit(EXIT_SUCCESS);
        }

        /* Child falls through to execute command */
    }

    execvp(argv[optind], &argv[optind]);
    errExit("execvp");
}

 

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