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dumb-init.c
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dumb-init.c
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/*
* dumb-init is a simple wrapper program designed to run as PID 1 and pass
* signals to its children.
*
* Usage:
* ./dumb-init python -c 'while True: pass'
*
* To get debug output on stderr, run with '-v'.
*/
#include <assert.h>
#include <errno.h>
#include <getopt.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include "VERSION.h"
#define PRINTERR(...) do { \
fprintf(stderr, "[dumb-init] " __VA_ARGS__); \
} while (0)
#define DEBUG(...) do { \
if (debug) { \
PRINTERR(__VA_ARGS__); \
} \
} while (0)
// Signals we care about are numbered from 1 to 31, inclusive.
// (32 and above are real-time signals.)
// TODO: this is likely not portable outside of Linux, or on strange architectures
#define MAXSIG 31
// Indices are one-indexed (signal 1 is at index 1). Index zero is unused.
// User-specified signal rewriting.
int signal_rewrite[MAXSIG + 1] = {[0 ... MAXSIG] = -1};
// One-time ignores due to TTY quirks. 0 = no skip, 1 = skip the next-received signal.
char signal_temporary_ignores[MAXSIG + 1] = {[0 ... MAXSIG] = 0};
pid_t child_pid = -1;
char debug = 0;
char use_setsid = 1;
int translate_signal(int signum) {
if (signum <= 0 || signum > MAXSIG) {
return signum;
} else {
int translated = signal_rewrite[signum];
if (translated == -1) {
return signum;
} else {
DEBUG("Translating signal %d to %d.\n", signum, translated);
return translated;
}
}
}
void forward_signal(int signum) {
signum = translate_signal(signum);
if (signum != 0) {
kill(use_setsid ? -child_pid : child_pid, signum);
DEBUG("Forwarded signal %d to children.\n", signum);
} else {
DEBUG("Not forwarding signal %d to children (ignored).\n", signum);
}
}
/*
* The dumb-init signal handler.
*
* The main job of this signal handler is to forward signals along to our child
* process(es). In setsid mode, this means signaling the entire process group
* rooted at our child. In non-setsid mode, this is just signaling the primary
* child.
*
* In most cases, simply proxying the received signal is sufficient. If we
* receive a job control signal, however, we should not only forward it, but
* also sleep dumb-init itself.
*
* This allows users to run foreground processes using dumb-init and to
* control them using normal shell job control features (e.g. Ctrl-Z to
* generate a SIGTSTP and suspend the process).
*
* The libc manual is useful:
* https://www.gnu.org/software/libc/manual/html_node/Job-Control-Signals.html
*
*/
void handle_signal(int signum) {
DEBUG("Received signal %d.\n", signum);
if (signal_temporary_ignores[signum] == 1) {
DEBUG("Ignoring tty hand-off signal %d.\n", signum);
signal_temporary_ignores[signum] = 0;
} else if (signum == SIGCHLD) {
int status, exit_status;
pid_t killed_pid;
while ((killed_pid = waitpid(-1, &status, WNOHANG)) > 0) {
if (WIFEXITED(status)) {
exit_status = WEXITSTATUS(status);
DEBUG("A child with PID %d exited with exit status %d.\n", killed_pid, exit_status);
} else {
assert(WIFSIGNALED(status));
exit_status = 128 + WTERMSIG(status);
DEBUG("A child with PID %d was terminated by signal %d.\n", killed_pid, exit_status - 128);
}
if (killed_pid == child_pid) {
forward_signal(SIGTERM); // send SIGTERM to any remaining children
DEBUG("Child exited with status %d. Goodbye.\n", exit_status);
exit(exit_status);
}
}
} else {
forward_signal(signum);
if (signum == SIGTSTP || signum == SIGTTOU || signum == SIGTTIN) {
DEBUG("Suspending self due to TTY signal.\n");
kill(getpid(), SIGSTOP);
}
}
}
void print_help(char *argv[]) {
fprintf(stderr,
"dumb-init v%s"
"Usage: %s [option] command [[arg] ...]\n"
"\n"
"dumb-init is a simple process supervisor that forwards signals to children.\n"
"It is designed to run as PID1 in minimal container environments.\n"
"\n"
"Optional arguments:\n"
" -c, --single-child Run in single-child mode.\n"
" In this mode, signals are only proxied to the\n"
" direct child and not any of its descendants.\n"
" -r, --rewrite s:r Rewrite received signal s to new signal r before proxying.\n"
" To ignore (not proxy) a signal, rewrite it to 0.\n"
" This option can be specified multiple times.\n"
" -v, --verbose Print debugging information to stderr.\n"
" -h, --help Print this help message and exit.\n"
" -V, --version Print the current version and exit.\n"
"\n"
"Full help is available online at https://github.com/Yelp/dumb-init\n",
VERSION,
argv[0]
);
}
void print_rewrite_signum_help() {
fprintf(
stderr,
"Usage: -r option takes <signum>:<signum>, where <signum> "
"is between 1 and %d.\n"
"This option can be specified multiple times.\n"
"Use --help for full usage.\n",
MAXSIG
);
exit(1);
}
void parse_rewrite_signum(char *arg) {
int signum, replacement;
if (
sscanf(arg, "%d:%d", &signum, &replacement) == 2 &&
(signum >= 1 && signum <= MAXSIG) &&
(replacement >= 0 && replacement <= MAXSIG)
) {
signal_rewrite[signum] = replacement;
} else {
print_rewrite_signum_help();
}
}
void set_rewrite_to_sigstop_if_not_defined(int signum) {
if (signal_rewrite[signum] == -1) {
signal_rewrite[signum] = SIGSTOP;
}
}
char **parse_command(int argc, char *argv[]) {
int opt;
struct option long_options[] = {
{"help", no_argument, NULL, 'h'},
{"single-child", no_argument, NULL, 'c'},
{"rewrite", required_argument, NULL, 'r'},
{"verbose", no_argument, NULL, 'v'},
{"version", no_argument, NULL, 'V'},
{NULL, 0, NULL, 0},
};
while ((opt = getopt_long(argc, argv, "+hvVcr:", long_options, NULL)) != -1) {
switch (opt) {
case 'h':
print_help(argv);
exit(0);
case 'v':
debug = 1;
break;
case 'V':
fprintf(stderr, "dumb-init v%s", VERSION);
exit(0);
case 'c':
use_setsid = 0;
break;
case 'r':
parse_rewrite_signum(optarg);
break;
default:
exit(1);
}
}
if (optind >= argc) {
fprintf(
stderr,
"Usage: %s [option] program [args]\n"
"Try %s --help for full usage.\n",
argv[0], argv[0]
);
exit(1);
}
char *debug_env = getenv("DUMB_INIT_DEBUG");
if (debug_env && strcmp(debug_env, "1") == 0) {
debug = 1;
DEBUG("Running in debug mode.\n");
}
char *setsid_env = getenv("DUMB_INIT_SETSID");
if (setsid_env && strcmp(setsid_env, "0") == 0) {
use_setsid = 0;
DEBUG("Not running in setsid mode.\n");
}
if (use_setsid) {
set_rewrite_to_sigstop_if_not_defined(SIGTSTP);
set_rewrite_to_sigstop_if_not_defined(SIGTTOU);
set_rewrite_to_sigstop_if_not_defined(SIGTTIN);
}
return &argv[optind];
}
// A dummy signal handler used for signals we care about.
// On the FreeBSD kernel, ignored signals cannot be waited on by `sigwait` (but
// they can be on Linux). We must provide a dummy handler.
// https://lists.freebsd.org/pipermail/freebsd-ports/2009-October/057340.html
void dummy(int signum) {}
int main(int argc, char *argv[]) {
char **cmd = parse_command(argc, argv);
sigset_t all_signals;
sigfillset(&all_signals);
sigprocmask(SIG_BLOCK, &all_signals, NULL);
int i = 0;
for (i = 1; i <= MAXSIG; i++) {
signal(i, dummy);
}
/*
* Detach dumb-init from controlling tty, so that the child's session can
* attach to it instead.
*
* We want the child to be able to be the session leader of the TTY so that
* it can do normal job control.
*/
if (use_setsid) {
if (ioctl(STDIN_FILENO, TIOCNOTTY) == -1) {
DEBUG(
"Unable to detach from controlling tty (errno=%d %s).\n",
errno,
strerror(errno)
);
} else {
/*
* When the session leader detaches from its controlling tty via
* TIOCNOTTY, the kernel sends SIGHUP and SIGCONT to the process
* group. We need to be careful not to forward these on to the
* dumb-init child so that it doesn't receive a SIGHUP and
* terminate itself (#136).
*/
if (getsid(0) == getpid()) {
DEBUG("Detached from controlling tty, ignoring the first SIGHUP and SIGCONT we receive.\n");
signal_temporary_ignores[SIGHUP] = 1;
signal_temporary_ignores[SIGCONT] = 1;
} else {
DEBUG("Detached from controlling tty, but was not session leader.\n");
}
}
}
child_pid = fork();
if (child_pid < 0) {
PRINTERR("Unable to fork. Exiting.\n");
return 1;
} else if (child_pid == 0) {
/* child */
sigprocmask(SIG_UNBLOCK, &all_signals, NULL);
if (use_setsid) {
if (setsid() == -1) {
PRINTERR(
"Unable to setsid (errno=%d %s). Exiting.\n",
errno,
strerror(errno)
);
exit(1);
}
if (ioctl(STDIN_FILENO, TIOCSCTTY, 0) == -1) {
DEBUG(
"Unable to attach to controlling tty (errno=%d %s).\n",
errno,
strerror(errno)
);
}
DEBUG("setsid complete.\n");
}
execvp(cmd[0], &cmd[0]);
// if this point is reached, exec failed, so we should exit nonzero
PRINTERR("%s: %s\n", cmd[0], strerror(errno));
return 2;
} else {
/* parent */
DEBUG("Child spawned with PID %d.\n", child_pid);
for (;;) {
int signum;
sigwait(&all_signals, &signum);
handle_signal(signum);
}
}
}