namespaces/simple_init.c

This is namespaces/simple_init.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)

/* simple_init.c

   A simple init(1)-style program to be used as the init program in
   a PID namespace. The program reaps the status of its children and
   provides a simple shell facility for executing commands.

   See https://lwn.net/Articles/532748/
*/
#define _GNU_SOURCE
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>
#include <wordexp.h>
#include <errno.h>
#include <sys/wait.h>
#include <sys/mount.h>

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                        } while (0)

static int verbose = 0;

/* Display wait status (from waitpid() or similar) given in 'wstatus' */
/* SIGCHLD handler: reap child processes as they change state */

static void
child_handler(int sig)
{
    pid_t pid;
    int wstatus;

    /* WUNTRACED and WCONTINUED allow waitpid() to catch stopped and
       continued children (in addition to terminated children) */

    while ((pid = waitpid(-1, &wstatus,
                          WNOHANG | WUNTRACED | WCONTINUED)) != 0) {
        if (pid == -1) {
            if (errno == ECHILD)        /* No more children */
                break;
            else
                perror("waitpid");      /* Unexpected error */
        }

        if (verbose)
            printf("\tinit: SIGCHLD handler: PID %ld terminated\n",
                    (long) pid);
    }
}
/* Perform word expansion on string in 'cmd', allocating and
   returning a vector of words on success or NULL on failure */

static char **
expand_words(char *cmd)
{
    wordexp_t pwordexp;

    int s = wordexp(cmd, &pwordexp, 0);
    if (s != 0) {
        fprintf(stderr, "Word expansion failed.\n"
                        "\tNote that only simple "
                        "commands plus arguments are supported\n"
                        "\t(no pipelines, I/O redirection, and so on)\n");
        return NULL;
    }

    char **arg_vec = calloc(pwordexp.we_wordc + 1, sizeof(char *));
    if (arg_vec == NULL)
        errExit("calloc");

    for (int j = 0; j < pwordexp.we_wordc; j++)
        arg_vec[j] = pwordexp.we_wordv[j];

    arg_vec[pwordexp.we_wordc] = NULL;

    return arg_vec;
}
static void
usage(char *pname)
{
    fprintf(stderr, "Usage: %s [-v] [-p proc-mount]\n", pname);
    fprintf(stderr, "\t-v              Provide verbose logging\n");
    fprintf(stderr, "\t-p proc-mount   Mount a procfs at specified path\n");

    exit(EXIT_FAILURE);
}
int
main(int argc, char *argv[])
{
    char *proc_path = NULL;
    int opt;
    while ((opt = getopt(argc, argv, "p:v")) != -1) {
        switch (opt) {
        case 'p': proc_path = optarg;   break;
        case 'v': verbose = 1;          break;
        default:  usage(argv[0]);
        }
    }

    struct sigaction sa;
    sa.sa_flags = SA_RESTART | SA_NOCLDSTOP;
    sigemptyset(&sa.sa_mask);
    sa.sa_handler = child_handler;
    if (sigaction(SIGCHLD, &sa, NULL) == -1)
        errExit("sigaction");

    if (verbose)
        printf("\tinit: my PID is %ld\n", (long) getpid());

    /* Performing terminal operations while not being the foreground
       process group for the terminal generates a SIGTTOU that stops the
       process.  However our init "shell" needs to be able to perform
       such operations (just like a normal shell), so we ignore that
       signal, which allows the operations to proceed successfully. */

    signal(SIGTTOU, SIG_IGN);

    /* Become leader of a new process group and make that process
       group the foreground process group for the terminal */

    if (setpgid(0, 0) == -1)
        errExit("setpgid");;
    if (tcsetpgrp(STDIN_FILENO, getpgrp()) == -1)
        errExit("tcsetpgrp-child");

    /* If the user asked to mount a procfs, mount it at the specified path */

    if (proc_path != NULL) {

        /* Some distributions enable mount propagation (mount --make-shared)
           by default. This would cause the mount that we create here to
           propagate to other namespaces. If we were mounting the
           procfs for this new PID namespace at "/proc" (which is typical),
           then this would hide the original "/proc" mount point in the
           initial namespace, which we probably don't want, since it will
           confuse a lot of system tools. To prevent propagation from
           occurring, we need to mark the mount point either as a slave
           mount or as a private mount.

           For further information on this topic, see the kernel source
           file Documentation/filesystems/sharedsubtree.txt and the
           mount(8) man page */

        if (verbose)
            printf("Making %s a private mount\n", proc_path);

        /* EINVAL is the case that occurs if 'proc_path' exists but is
           not (yet) a mount point */

        if (mount("none", proc_path, NULL, MS_SLAVE, NULL) == -1 &&
                errno != EINVAL)
            perror("mount-make-slave-/");

        if (verbose)
            printf("Mounting procfs at %s\n", proc_path);

        if (mount("proc", proc_path, "proc", 0, NULL) == -1)
            errExit("mount-procfs");
    }

    /* Loop executing "shell" commands. Note that our shell facility is
       very simple: it handles simple commands with arguments, and
       performs wordexp() expansions (globbing, variable and command
       substitution, tilde expansion, and quote removal). Complex
       commands (pipelines, ||, &&) and I/O redirections, and
       standard shell features are not supported. */

    while (1) {

        /* Read a shell command; exit on end of file */

#define CMD_SIZE 10000
        char cmd[CMD_SIZE];

        printf("init$ ");
        if (fgets(cmd, CMD_SIZE, stdin) == NULL) {
            if (verbose)
                printf("\tinit: exiting");
            printf("\n");
            break;
        }

        if (cmd[strlen(cmd) - 1] == '\n')
            cmd[strlen(cmd) - 1] = '\0';        /* Strip trailing '\n' */

        if (strlen(cmd) == 0)
            continue;           /* Ignore empty commands */

        pid_t pid = fork();             /* Create child process */
        if (pid == -1) {
            perror("fork");
            break;
        }

        if (pid == 0) {         /* Child */
            char **arg_vec;

            arg_vec = expand_words(cmd);
            if (arg_vec == NULL)        /* Word expansion failed */
                exit(EXIT_FAILURE);

            /* Make child the leader of a new process group and
               make that process group the foreground process
               group for the terminal */

            if (setpgid(0, 0) == -1)
                errExit("setpgid");;
            if (tcsetpgrp(STDIN_FILENO, getpgrp()) == -1)
                errExit("tcsetpgrp-child");

            /* Child executes shell command and terminates */

            execvp(arg_vec[0], arg_vec);
            errExit("execvp");          /* Only reached if execvp() fails */
        }

        /* Parent falls through to here */

        if (verbose)
            printf("\tinit: created child %ld\n", (long) pid);

        pause();                /* Will be interrupted by signal handler */

        /* After child changes state, ensure that the 'init' program
           is the foreground process group for the terminal */

        if (tcsetpgrp(STDIN_FILENO, getpgrp()) == -1)
            errExit("tcsetpgrp-parent");
    }

    /* If we mounted a procfs earlier, unmount it before terminating */

    if (proc_path != NULL) {
        if (verbose)
            printf("Unmounting procfs at %s\n", proc_path);
        if (umount(proc_path) == -1)
            errExit("umount-procfs");
    }

    exit(EXIT_SUCCESS);
}

 

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Note that, in most cases, the programs rendered in these web pages are not free standing: you'll typically also need a few other source files (mostly in the lib/ subdirectory) as well. Generally, it's easier to just download the entire source tarball and build the programs with make(1). By hovering your mouse over the various hyperlinked include files and function calls above, you can see which other source files this file depends on.

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