Initial commit

.gitignore

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+sinit.o
+sinit

LICENSE

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+The MIT License (MIT)
+
+Copyright (c) 2015 Thoams Orozco <thomas@orozco.fr>
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.

Makefile

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+include config.mk
+
+OBJ = sinit.o
+BIN = sinit
+
+all: $(BIN)
+
+$(BIN): $(OBJ)
+	$(CC) $(LDFLAGS) -o $@ $(OBJ) $(LDLIBS)
+
+$(OBJ):
+
+install: all
+	mkdir -p $(DESTDIR)$(PREFIX)/bin
+	cp -f $(BIN) $(DESTDIR)$(PREFIX)/bin
+
+uninstall:
+	rm -f $(DESTDIR)$(PREFIX)/bin/$(BIN)
+
+dist: clean
+	mkdir -p sinit-$(VERSION)
+	cp LICENSE Makefile README config.def.h config.mk sinit.c sinit-$(VERSION)
+	tar -cf sinit-$(VERSION).tar sinit-$(VERSION)
+	gzip sinit-$(VERSION).tar
+	rm -rf sinit-$(VERSION)
+
+clean:
+	rm -f $(BIN) $(OBJ) sinit-$(VERSION).tar.gz
+
+.PHONY:
+	all install uninstall dist clean

README

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+Simple but correct init for Docker containers

config.mk

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+# sinit version
+VERSION = 0.1.0
+
+# paths
+PREFIX = /usr/local
+MANPREFIX = $(PREFIX)/share/man
+
+CC = cc
+LD = $(CC)
+CPPFLAGS =
+CFLAGS   = -Wextra -Wall -Os
+LDFLAGS  = -s -static

sinit.c

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+/* See LICENSE file for copyright and license details. */
+#include <sys/types.h>
+#include <sys/wait.h>
+
+#include <errno.h>
+#include <signal.h>
+#include <string.h>
+#include <time.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+static sigset_t set;
+
+pid_t spawn(char *const argv[]) {
+	pid_t pid;
+
+	pid = fork();
+	if (pid < 0) {
+		perror("[FATAL] Fork failed");
+		_exit(1);
+	} else if (pid == 0) {
+		sigprocmask(SIG_UNBLOCK, &set, NULL);
+		execvp(argv[0], argv);
+		perror("[ERROR] Executing child process failed");
+		_exit(1);
+	} else {
+		return pid;
+	}
+}
+
+int main(int argc, char *argv[]) {
+	siginfo_t sig;
+
+	pid_t child_pid;
+
+	pid_t current_pid;
+	int current_status;
+
+	struct timespec ts;
+	ts.tv_sec = 1;
+	ts.tv_nsec = 0;
+
+	/* Prepare signals */
+	if (sigfillset(&set)) {
+		perror("sigfillset");
+		return 1;
+	}
+	if (sigprocmask(SIG_BLOCK, &set, NULL)) {
+		perror("sigprocmask");
+		return 1;
+	}
+
+	/* Spawn the main command */
+	child_pid = spawn(argv+1);
+	printf("[INFO ] Spawned child process\n");
+
+	/* Loop forever:
+	 * - Reap zombies
+	 * - Forward signals
+	 */
+	while (1) {
+		if (sigtimedwait(&set, &sig, &ts) == -1) {
+			switch (errno) {
+				case EAGAIN:
+					break;
+				case EINTR:
+					break;
+				case EINVAL:
+					perror("[ERROR] Fatal!");
+					return 2;
+			}
+		} else {
+			/* There is a signal to handle here */
+			switch (sig.si_signo) {
+				case SIGCHLD:
+					// Special-cased, as we don't forward SIGCHLD. Instead, we'll
+					// fallthrough to reaping processes.
+					printf("[INFO ] Received SIGCHLD\n");
+					break;
+				default:
+					printf("[INFO ] Passing signal: %s\n", strsignal(sig.si_signo));
+					// Forward anything else
+					kill(child_pid, sig.si_signo);
+					break;
+			}
+		}
+
+		/* Now, reap zombies */
+		while (1) {
+			current_pid = waitpid(-1, &current_status, WNOHANG);
+			switch (current_pid) {
+				case -1:
+					// An error occured. Print it and exit.
+					perror("waitpids returned -1");
+					return 1;
+				case 0:
+					// No child to reap. We'll break out of the loop here.
+					break;
+				default:
+					// A child was reaped. Check whether it's the main one,
+					// and go for another iteration otherwise.
+					if (current_pid == child_pid) {
+						printf("[INFO ] Main child has exited\n");
+						if (WIFEXITED(current_status)) {
+							// Our process exited normally.
+							printf("[DEBUG] Main child exited normally\n");
+							return WEXITSTATUS(current_status);
+						} else if (WIFSIGNALED(current_status)) {
+							// Our process was terminated. Emulate what sh / bash
+							// would do, which is to return 128 + signal number.
+							printf("[DEBUG] Main child exited with signal\n");
+							return 128 + WTERMSIG(current_status);
+						} else {
+							printf("[WARNING] Main child exited for unknown reason\n");
+							return 1;
+						}
+					}
+					continue;
+			}
+
+			// If we make it here, that's because we did not continue in
+			// the switch case.
+			break;
+		}
+
+	}
+	/* not reachable */
+	return 0;
+}
+