File: dumb-init.c

package info (click to toggle)
dumb-init 1.2.2-1.1
  • links: PTS, VCS
  • area: main
  • in suites: buster, sid
  • size: 280 kB
  • sloc: python: 653; ansic: 253; makefile: 92; sh: 50
file content (333 lines) | stat: -rw-r--r-- 10,901 bytes parent folder | download
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
/*
 * 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);
        }
    }
}