Proof of concept port of fork-test.py to C

tests/fork-test.c

@@ -0,0 +1,167 @@
+#define _GNU_SOURCE
+#include <arpa/inet.h>
+#include <fcntl.h>
+#include <netinet/in.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#define NS_PREFIX "/run/netns/"
+#define ALARM_TIMEOUT 4
+
+int done = 0;
+int fail = 0;
+char *child_ns;
+char *server_addr;
+int server_port;
+
+int switch_ns(char *ns) {
+	char *path;
+	int fd, result;
+	path = malloc(strlen(NS_PREFIX) + strlen(ns) + 1);
+	strncpy(path, NS_PREFIX, strlen(NS_PREFIX));
+	strncat(path, ns, strlen(ns));
+	fd = open(path, 0);
+	if (fd < 0) {
+		result = fd; }
+	else {
+		result = setns(fd, CLONE_NEWNET);
+		close(fd);
+	}
+	free(path);
+	return(result);
+};
+
+int parent(pid_t child_pid, char *ns){
+	struct sockaddr_in address;
+	char buf[128];
+	int sock;
+	int newsock;
+	int status;
+	int n;
+	int result;
+	
+	if (switch_ns(ns)){
+		printf("PARENT: failed to switch netns to %s, exiting!\n", ns);
+		exit(1);
+	} else {
+		printf("PARENT: switched netns to %s\n", ns);
+	}
+
+	/* socket stuff goes here */
+	sock = socket(AF_INET, SOCK_STREAM, 0);
+	address.sin_family = AF_INET;
+	inet_pton(AF_INET, server_addr, &address.sin_addr);
+	address.sin_port = htons(server_port); 
+	result = bind(sock, &address, sizeof(address));
+	if (result < 0) { perror("bind");
+		exit(1);
+	}
+	result = listen(sock, 1);
+	if (result < 0) { perror("listen");
+		exit(1);
+	}
+
+	kill(child_pid, SIGUSR1);
+	alarm(4);
+	newsock = accept(sock, NULL, NULL);
+	if (newsock < 0) { perror("accept");
+		exit(1);
+	}
+	n = read(newsock, buf, 100);
+	if (n < 0) {
+		perror("parent sock");
+		exit(1);
+	}
+	buf[n] = 0;
+	printf("Received %d bytes: %s\n", n, buf);
+
+	wait(&status);
+	/* exit with status of child */
+	exit (WEXITSTATUS(status));
+}
+
+void handler(int sig){
+	struct sockaddr_in address;
+	int sock;
+	int result;
+	char *buf = "hello, world";
+
+	if (switch_ns(child_ns)){
+		printf("CHILD: failed to switch netns to %s, exiting!\n", child_ns);
+		fail = 1;
+		done = 1;
+		return;
+	};
+
+	printf("CHILD:  switched netns to %s\n", child_ns);
+	
+	/* socket */
+	sock = socket(AF_INET, SOCK_STREAM, 0);
+	address.sin_family = AF_INET;
+	inet_pton(AF_INET, server_addr, &address.sin_addr);
+	address.sin_port = htons(server_port);
+	result = connect(sock, &address, sizeof(address));
+	if (result < 0) {
+		perror("connect");
+		fail = 1;
+	} else {
+		write(sock, buf, strlen(buf));
+		if (result < 0) {
+			perror("write");
+			fail = 1;
+		};
+	}
+	close(sock);
+
+	done = 1;
+}
+
+int child(pid_t parent_pid){
+	/* loop thing waiting for signal handler */
+
+	/* there is a deadlock in libc here related to printf in
+	 * multiple threads or something */
+	while (!done){
+		if (kill(parent_pid, 0) < 0){
+			printf("CHILD: parent exited?\n");
+			fail = 1;
+			break;
+		}
+	}
+	exit(fail);
+}
+
+int main(int argc, char* argv[]){
+	pid_t ppid, cpid;
+	if (argc < 5){
+		printf("usage: %s parentns childns address port\n", argv[0]);
+		exit(1); 
+	}
+	if (geteuid()){
+		printf("%s: probably need to be root\n", argv[0]);
+		exit(1); 
+	}
+
+	child_ns = argv[2];
+	server_addr = argv[3];
+	server_port = atoi(argv[4]);
+
+	/* to avoid race, this should be done before the fork! */
+	signal(SIGUSR1, handler);
+	printf("this printf avoids a deadlock\n");
+
+	ppid = getpid();
+	cpid = fork();
+	if (cpid){
+		parent(cpid, argv[1]); }
+	else {
+		child(ppid);
+	}
+}