namespaces/ns_run.c

This is namespaces/ns_run.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)

/* ns_run.c

   Join one or more namespaces using setns() and execute a command in
   those namespaces, possibly inside a child process.

   This program is similar in concept to nsenter(1), but has a
   different command-line interface.

   See https://lwn.net/Articles/532748/
*/
#define _GNU_SOURCE
#include <fcntl.h>
#include <sched.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/wait.h>

/* 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 [-f] [-n /proc/PID/ns/FILE] cmd [arg...]\n",
            pname);
    fprintf(stderr, "\t-f     Execute command in child process\n");
    fprintf(stderr, "\t-n     Join specified namespace\n");

    exit(EXIT_FAILURE);
}
int
main(int argc, char *argv[])
{
    /* Parse command-line options. The initial '+' character in
       the final getopt() argument prevents GNU-style permutation
       of command-line options. That's useful, since sometimes
       the 'command' to be executed by this program itself
       has command-line options. We don't want getopt() to treat
       those as options to this program. */

    int do_fork = 0;
    int opt;
    int fd;
    while ((opt = getopt(argc, argv, "+fn:")) != -1) {
        switch (opt) {

        case 'n':       /* Join a namespace */

            fd = open(optarg, O_RDONLY | O_CLOEXEC);  /* Get FD for namespace */
            if (fd == -1)
                errExit("open");

            if (setns(fd, 0) == -1)      /* Join that namespace */
                errExit("setns");

            close(fd);

            break;

        case 'f':
            do_fork = 1;
            break;

        default:
            usage(argv[0]);
        }
    }

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

    /* If the "-f" option was specified, execute the supplied command
       in a child process. This is mainly useful when working with PID
       namespaces, since setns() to a PID namespace only places
       (subsequently created) child processes in the names, and
       does not affect the PID namespace membership of the caller. */

    if (do_fork) {
        pid_t pid = fork();
        if (pid == -1)
            errExit("fork");

        if (pid != 0) {                 /* Parent */
            if (waitpid(pid, NULL, 0) == -1)    /* Wait for child */
                errExit("waitpid");
            exit(EXIT_SUCCESS);
        }

        /* Child falls through to code below */
    }

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

 

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