playing with linux namespaces
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ns_hacking/simple_init.c

188 lines
5.1 KiB

/* simple_init.c
Copyright 2013, Michael Kerrisk
Licensed under GNU General Public License v2 or later
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.
*/
#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>
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} while (0)
static int verbose = 0;
/* Display wait status (from waitpid() or similar) given in 'status' */
/* SIGCHLD handler: reap child processes as they change state */
static void
child_handler(int sig)
{
pid_t pid;
int status;
/* WUNTRACED and WCONTINUED allow waitpid() to catch stopped and
continued children (in addition to terminated children) */
while ((pid = waitpid(-1, &status,
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)
{
char **arg_vec;
int s;
wordexp_t pwordexp;
s = wordexp(cmd, &pwordexp, 0);
if (s != 0) {
fprintf(stderr, "Word expansion failed\n");
return NULL;
}
arg_vec = calloc(pwordexp.we_wordc + 1, sizeof(char *));
if (arg_vec == NULL)
errExit("calloc");
for (s = 0; s < pwordexp.we_wordc; s++)
arg_vec[s] = pwordexp.we_wordv[s];
arg_vec[pwordexp.we_wordc] = NULL;
return arg_vec;
}
static void
usage(char *pname)
{
fprintf(stderr, "Usage: %s [-q]\n", pname);
fprintf(stderr, "\t-v\tProvide verbose logging\n");
exit(EXIT_FAILURE);
}
int
main(int argc, char *argv[])
{
struct sigaction sa;
#define CMD_SIZE 10000
char cmd[CMD_SIZE];
pid_t pid;
int opt;
while ((opt = getopt(argc, argv, "v")) != -1) {
switch (opt) {
case 'v': verbose = 1; break;
default: usage(argv[0]);
}
}
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");
while (1) {
/* Read a shell command; exit on end of file */
printf("init$ ");
if (fgets(cmd, CMD_SIZE, stdin) == NULL) {
if (verbose)
printf("\tinit: exiting");
printf("\n");
exit(EXIT_SUCCESS);
}
if (cmd[strlen(cmd) - 1] == '\n')
cmd[strlen(cmd) - 1] = '\0'; /* Strip trailing '\n' */
if (strlen(cmd) == 0)
continue; /* Ignore empty commands */
pid = fork(); /* Create child process */
if (pid == -1)
errExit("fork");
if (pid == 0) { /* Child */
char **arg_vec;
arg_vec = expand_words(cmd);
if (arg_vec == NULL) /* Word expansion failed */
continue;
/* 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");
}
}