implement a way for programs in the mount namespace (with glibc binaries) to launch processes in the original namespace (with musl binaries)

10 files changed, 876 insertions, 177 deletions

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..c0651ec
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,6 @@
+*.o
+.idea
+voidnsrun
+voidnsundo
+testclient
+testserver
diff --git a/Makefile b/Makefile
index 050d4ef..54945cb 100644
--- a/Makefile
+++ b/Makefile
@@ -1,27 +1,35 @@
 CC := gcc
 
-PROGRAM = voidnsrun
 CFLAGS  = -O2 -std=c99 -Wall -W
 LDFLAGS =
 
 INSTALL = /usr/bin/env install
 PREFIX	= /usr/local
 
-OBJS = voidnsrun.o
+all: voidnsrun voidnsundo
 
-all: $(PROGRAM)
+test: testserver testclient
 
-$(PROGRAM): $(OBJS)
+voidnsrun: voidnsrun.o utils.o
 	$(CC) $(CFLAGS) -o $@ $^ $(LDFLAGS)
 
-install: $(PROGRAM)
-	$(INSTALL) $(PROGRAM) $(PREFIX)/bin
-	chmod u+s $(PREFIX)/bin/${PROGRAM}
+voidnsundo: voidnsundo.o utils.o
+	$(CC) $(CFLAGS) -o $@ $^ $(LDFLAGS)
+
+testserver: testserver.o utils.o
+	$(CC) $(CFLAGS) -o $@ $^ $(LDFLAGS)
+
+testclient: testclient.o utils.o
+	$(CC) $(CFLAGS) -o $@ $^ $(LDFLAGS)
+
+install: voidnsrun voidnsundo
+	$(INSTALL) voidnsrun voidnsundo $(PREFIX)/bin
+	chmod u+s $(PREFIX)/bin/voidnsrun $(PREFIX)/bin/voidnsundo
 
 clean:
-	rm -f $(OBJS) $(PROGRAM)
+	rm -f *.o voidnsrun voidnsundo testserver testclient
 
 %.o: %.c
 	$(CC) $(CFLAGS) -c $^ -I. -o $@
 
-.PHONY: all install clean distclean
+.PHONY: all install clean distclean
\ No newline at end of file
diff --git a/config.h b/config.h
new file mode 100644
index 0000000..60f7fd4
--- /dev/null
+++ b/config.h
@@ -0,0 +1,13 @@
+#ifndef VOIDNSRUN_CONFIG_H
+#define VOIDNSRUN_CONFIG_H
+
+#define PROG_VERSION "1.2"
+#define USER_LISTS_MAX 50
+#define CONTAINER_DIR_VAR "VOIDNSRUN_DIR"
+#define UNDO_BIN_VAR "VOIDNSUNDO_BIN"
+#define SOCK_DIR_VAR "VOIDNSRUN_SOCK_DIR"
+#define SOCK_DIR_DEFAULT "/run/voidnsrun"
+#define SOCK_NAME "/sock"
+#define VOIDNSUNDO_NAME "voidnsundo"
+
+#endif //VOIDNSRUN_CONFIG_H
diff --git a/macros.h b/macros.h
new file mode 100644
index 0000000..973886c
--- /dev/null
+++ b/macros.h
@@ -0,0 +1,24 @@
+#ifndef VOIDNSRUN_MACROS_H
+#define VOIDNSRUN_MACROS_H
+
+#include <stdio.h>
+#include <string.h>
+
+extern bool g_verbose;
+
+#define ARRAY_SIZE(x)  (sizeof(x) / sizeof((x)[0]))
+#define UNUSED(x)      (void)(x)
+#define ERROR(f_, ...) fprintf(stderr, (f_), ##__VA_ARGS__)
+#define DEBUG(f_, ...) if (g_verbose) {       \
+        fprintf(stderr, "debug: ");           \
+        fprintf(stderr, (f_), ##__VA_ARGS__);  \
+    }
+
+#define ERROR_EXIT(f_, ...) { \
+        fprintf(stderr, (f_), ##__VA_ARGS__); \
+        goto end; \
+    }
+
+#define SOCK_DIR_PATH_MAX (108 - strlen(SOCK_NAME) - 1)
+
+#endif //VOIDNSRUN_MACROS_H
diff --git a/testclient.c b/testclient.c
new file mode 100644
index 0000000..19db1c1
--- /dev/null
+++ b/testclient.c
@@ -0,0 +1,44 @@
+#include <stdio.h>
+#include <errno.h>
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <sys/stat.h>
+#include <string.h>
+#include <unistd.h>
+#include <assert.h>
+#include "utils.h"
+
+#define ERROR_EXIT(f_, ...) { \
+		fprintf(stderr, (f_), ##__VA_ARGS__); \
+		return 1; \
+	}
+
+int main()
+{
+	struct sockaddr_un addr = {0};
+	int sock;
+
+	// Create and connect a unix domain socket
+	sock = socket(AF_UNIX, SOCK_STREAM, 0);
+	if (sock == -1)
+		ERROR_EXIT("socket: %s\n", strerror(errno));
+
+	addr.sun_family  = AF_UNIX;
+	strcpy(&addr.sun_path[1], "/tmp/voidnsrun-test.sock");
+
+	if (connect(sock, (struct sockaddr *)&addr, sizeof(addr)) == -1)
+		ERROR_EXIT("connect: %s\n", strerror(errno));
+
+	int fd = recv_fd(sock);
+	close(sock);
+
+	assert(fd != 0);
+
+	struct stat st;
+	if (fstat(fd, &st) == -1)
+		ERROR_EXIT("stat: %s\n", strerror(errno));
+
+	printf("st_ino: %lu\n", st.st_ino);
+
+	return 0;
+}
\ No newline at end of file
diff --git a/testserver.c b/testserver.c
new file mode 100644
index 0000000..091d580
--- /dev/null
+++ b/testserver.c
@@ -0,0 +1,96 @@
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <string.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <errno.h>
+#include <signal.h>
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <sys/prctl.h>
+#include "utils.h"
+
+#define ERROR(f_, ...) fprintf(stderr, (f_), ##__VA_ARGS__)
+#define UNUSED(x)      (void)(x)
+#define ERROR_EXIT(f_, ...) { \
+		fprintf(stderr, (f_), ##__VA_ARGS__); \
+		return 1; \
+	}
+
+volatile sig_atomic_t term_caught = 0;
+void onterm(int sig)
+{
+	printf("sigterm caught\n");
+	UNUSED(sig);
+	term_caught = 1;
+}
+
+int main()
+{
+	int result;
+	int sock_fd, sock_conn;
+	int nsfd;
+
+	/* Get current namespace's file descriptor. */
+	nsfd = open("/proc/self/ns/mnt", O_RDONLY);
+	if (nsfd == -1)
+		ERROR_EXIT("error: failed to acquire mount namespace's fd.%s\n",
+				   strerror(errno));
+
+	/* Fork. */
+	pid_t ppid_before_fork = getpid();
+	pid_t pid = fork();
+	if (pid == -1)
+		ERROR_EXIT("fork: %s\n", strerror(errno));
+
+	if (pid == 0) {
+		/* Catch SIGTERM. */
+		struct sigaction sa = {0};
+		sa.sa_handler = onterm;
+		sigaction(SIGTERM, &sa, NULL);
+
+        /* Ignore SIGINT. */
+        signal(SIGINT, SIG_IGN);
+
+		/* Set the child to die when parent thread dies. */
+		int r = prctl(PR_SET_PDEATHSIG, SIGTERM);
+		if (r == -1)
+			ERROR_EXIT("prctl: %s\n", strerror(errno));
+		if (getppid() != ppid_before_fork)
+			ERROR_EXIT("error: parent has died already.\n");
+
+		/* Create unix socket. */
+		sock_fd = socket(AF_UNIX, SOCK_STREAM, 0);
+		if (sock_fd == -1)
+			ERROR_EXIT("socket: %s.\n", strerror(errno));
+
+		struct sockaddr_un sock_addr = {0};
+		sock_addr.sun_family = AF_UNIX;
+		strcpy(&sock_addr.sun_path[1], "/tmp/voidnsrun-test.sock");
+
+		if (bind(sock_fd, (struct sockaddr *)&sock_addr, sizeof(sock_addr)) == -1)
+			ERROR_EXIT("bind: %s\n", strerror(errno));
+
+		listen(sock_fd, 1);
+
+		while (!term_caught) {
+			sock_conn = accept(sock_fd, NULL, 0);
+			if (sock_conn == -1) {
+				ERROR("accept: %s\n", strerror(errno));
+				continue;
+			}
+			printf("accepted\n");
+			send_fd(sock_conn, nsfd);
+		}
+		printf("exiting\n");
+	} else {
+		/* This is parent. Launch a program. */
+		char *argv[2] = {"/bin/sh", NULL};
+
+		result = execvp(argv[0], (char *const *)argv);
+		if (result == -1)
+			ERROR_EXIT("execvp: %s\n", strerror(errno));
+	}
+
+	return 0;
+}
\ No newline at end of file
diff --git a/utils.c b/utils.c
new file mode 100644
index 0000000..233a6e5
--- /dev/null
+++ b/utils.c
@@ -0,0 +1,154 @@
+#include <string.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <assert.h>
+#include <unistd.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include "macros.h"
+#include "utils.h"
+
+bool isdir(const char *s)
+{
+    struct stat st;
+    int result = stat(s, &st);
+    if (result == -1)
+        ERROR("stat(%s): %s\n", s, strerror(errno));
+    return result == 0 && S_ISDIR(st.st_mode);
+}
+
+bool isexe(const char *s)
+{
+    struct stat st;
+    int result = stat(s, &st);
+    if (result == -1)
+        ERROR("stat(%s): %s\n", s, strerror(errno));
+    return result == 0 && !S_ISDIR(st.st_mode) && st.st_mode & S_IXUSR;
+}
+
+bool exists(const char *s)
+{
+    struct stat st;
+    return stat(s, &st) == 0;
+}
+
+bool mkfile(const char *s)
+{
+    int fd;
+    if ((fd = creat(s, 0700)) == -1)
+        return false;
+    close(fd);
+    return true;
+}
+
+int send_fd(int sock, int fd)
+{
+    struct msghdr msg = {0};
+    struct iovec iov[1];
+    struct cmsghdr *cmsg = NULL;
+    char ctrl_buf[CMSG_SPACE(sizeof(int))];
+    char data[1];
+
+    memset(ctrl_buf, 0, CMSG_SPACE(sizeof(int)));
+
+    data[0] = ' ';
+    iov[0].iov_base = data;
+    iov[0].iov_len = sizeof(data);
+
+    msg.msg_name = NULL;
+    msg.msg_namelen = 0;
+    msg.msg_iov = iov;
+    msg.msg_iovlen = 1;
+    msg.msg_controllen =  CMSG_SPACE(sizeof(int));
+    msg.msg_control = ctrl_buf;
+
+    cmsg = CMSG_FIRSTHDR(&msg);
+    cmsg->cmsg_level = SOL_SOCKET;
+    cmsg->cmsg_type = SCM_RIGHTS;
+    cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+
+    *((int *)CMSG_DATA(cmsg)) = fd;
+
+    return sendmsg(sock, &msg, 0);
+}
+
+int recv_fd(int sock)
+{
+    struct msghdr msg = {0};
+    struct iovec iov[1];
+    struct cmsghdr *cmsg = NULL;
+    char ctrl_buf[CMSG_SPACE(sizeof(int))];
+    char data[1];
+
+    memset(ctrl_buf, 0, CMSG_SPACE(sizeof(int)));
+
+    iov[0].iov_base = data;
+    iov[0].iov_len = sizeof(data);
+
+    msg.msg_name = NULL;
+    msg.msg_namelen = 0;
+    msg.msg_control = ctrl_buf;
+    msg.msg_controllen = CMSG_SPACE(sizeof(int));
+    msg.msg_iov = iov;
+    msg.msg_iovlen = 1;
+
+    recvmsg(sock, &msg, 0);
+
+    cmsg = CMSG_FIRSTHDR(&msg);
+
+    return *((int *) CMSG_DATA(cmsg));
+}
+
+bool isxbpscommand(const char *s)
+{
+    const char *commands[] = {
+        "/xbps-install",
+        "/xbps-remove",
+        "/xbps-reconfigure"
+    };
+    for (size_t i = 0; i < ARRAY_SIZE(commands); i++) {
+        const char *command = commands[i];
+        if (!strcmp(s, command+1))
+            return true;
+        char *slash = strrchr(s, '/');
+        if (slash && !strcmp(slash, command))
+            return true;
+    }
+    return false;
+}
+
+bool strarray_append(struct strarray *a, char *s)
+{
+    if (a->end == a->size - 1)
+        return false;
+    else
+        a->list[a->end++] = s;
+    return true;
+}
+
+bool intarray_append(struct intarray *a, int i)
+{
+    if (a->end == a->size - 1)
+        return false;
+    else
+        a->list[a->end++] = i;
+    return true;
+}
+
+void strarray_alloc(struct strarray *a, size_t size)
+{
+    a->end = 0;
+    a->size = size;
+    a->list = malloc(sizeof(char *) * size);
+    assert(a->list != NULL);
+}
+
+void intarray_alloc(struct intarray *i, size_t size)
+{
+    i->end = 0;
+    i->size = size;
+    i->list = malloc(sizeof(int) * size);
+    assert(i->list != NULL);
+}
\ No newline at end of file
diff --git a/utils.h b/utils.h
new file mode 100644
index 0000000..cc9f610
--- /dev/null
+++ b/utils.h
@@ -0,0 +1,35 @@
+#ifndef VOIDNSRUN_UTILS_H
+#define VOIDNSRUN_UTILS_H
+
+#include <stdbool.h>
+#include "config.h"
+
+struct strarray {
+    size_t end;
+    size_t size;
+    char **list;
+};
+
+struct intarray {
+    size_t end;
+    size_t size;
+    int *list;
+};
+
+bool isdir(const char *s);
+bool isexe(const char *s);
+bool exists(const char *s);
+bool mkfile(const char *s);
+
+int send_fd(int sock, int fd);
+int recv_fd(int sock);
+
+bool isxbpscommand(const char *s);
+
+void strarray_alloc(struct strarray *a, size_t size);
+bool strarray_append(struct strarray *a, char *s);
+
+void intarray_alloc(struct intarray *i, size_t size);
+bool intarray_append(struct intarray *a, int i);
+
+#endif //VOIDNSRUN_UTILS_H
diff --git a/voidnsrun.c b/voidnsrun.c
index b1a34b3..ba48139 100644
--- a/voidnsrun.c
+++ b/voidnsrun.c
@@ -1,197 +1,362 @@
 #define _GNU_SOURCE
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <fcntl.h>
 #include <unistd.h>
 #include <errno.h>
 #include <sched.h>
 #include <stdbool.h>
+#include <dirent.h>
+#include <signal.h>
 #include <sys/mount.h>
 #include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/prctl.h>
+#include <sys/socket.h>
+#include <sys/un.h>
 #include <linux/limits.h>
 
-const char *var_name = "VOIDNSRUN_DIR";
-const char *prog_version = "1.0";
-
-#define USERMOUNTS_MAX 8
+#include "config.h"
+#include "utils.h"
+#include "macros.h"
 
-#define ARRAY_SIZE(x)  (sizeof(x) / sizeof((x)[0]))
-#define ERROR(f_, ...) fprintf(stderr, (f_), ##__VA_ARGS__)
+volatile sig_atomic_t term_caught = 0;
+bool g_verbose = false;
 
-bool isdir(const char *s)
+void usage(const char *progname)
 {
-	struct stat st;
-	int result = stat(s, &st);
-	if (result == -1)
-		ERROR("stat: %s\n", strerror(errno));
-	return result == 0 && S_ISDIR(st.st_mode);
+    printf("Usage: %s [OPTIONS] PROGRAM [ARGS]\n", progname);
+    printf("\n"
+            "Options:\n"
+            "    -r <path>: Container path. When this option is not present,\n"
+            "               " CONTAINER_DIR_VAR " environment variable is used.\n"
+            "    -m <path>: Add bind mount. You can add up to %d paths.\n"
+            "    -u <path>: Add undo utility bind mount. You can add up to %d paths.\n"
+            "    -U <path>: Undo program path. When this option is not present,\n"
+            "               " UNDO_BIN_VAR " environment variable is used.\n"
+            "    -i:        Don't treat missing source or target for an added mount\n"
+            "               as an error.\n"
+            "    -s:        Socket directory path. When this option is not present,\n"
+            "               " SOCK_DIR_VAR " environment variable is used. If both are\n"
+            "               missing, defaults to " SOCK_DIR_DEFAULT ".\n"
+            "    -V:        Verbose output.\n"
+            "    -h:        Print this help.\n"
+            "    -v:        Print version.\n",
+           USER_LISTS_MAX, USER_LISTS_MAX);
 }
 
-void usage(const char *progname)
+size_t mount_dirs(const char *source_prefix, size_t source_prefix_len, struct strarray *targets)
 {
-	printf("Usage: %s [OPTIONS] PROGRAM [ARGS]\n", progname);
-	printf("\n"
-			"Options:\n"
-			"	-m <path>: add bind mount\n"
-			"	-r <path>: altroot path. If this option is not present,\n"
-			"	           %s environment variable is used.\n"
-			"	-h:        print this help\n"
-			"	-v:        print version\n",
-			var_name);
+    char buf[PATH_MAX];
+    int successful = 0;
+    for (size_t i = 0; i < targets->end; i++) {
+        /* Check if it's safe to proceed. */
+        if (source_prefix_len + strlen(targets->list[i]) >= PATH_MAX) {
+            ERROR("error: path %s%s is too large.\n", source_prefix, targets->list[i]);
+            continue;
+        }
+
+        strcpy(buf, source_prefix);
+        strcat(buf, targets->list[i]);
+        if (!isdir(buf)) {
+            ERROR("error: source mount dir %s does not exists.\n", buf);
+            continue;
+        }
+
+        if (!isdir(targets->list[i])) {
+            ERROR("error: mount point %s does not exists.\n", targets->list[i]);
+            continue;
+        }
+
+        if (mount(buf, targets->list[i], NULL, MS_BIND|MS_REC, NULL) == -1)
+            ERROR("mount: failed to mount %s: %s\n", targets->list[i], strerror(errno));
+        else
+            successful++;
+    }
+    return successful;
 }
 
-bool mountlist(
-	const char *dirptr,
-	size_t dirlen,
-	const char **mountpoints,
-	size_t len,
-	bool ignore_missing)
+size_t mount_undo(const char *source, const struct strarray *targets, struct intarray *created)
 {
-	char buf[PATH_MAX];
-	for (size_t i = 0; i < len; i++) {
-		/* Check if it's safe to proceed. */
-		if (dirlen + strlen(mountpoints[i]) >= PATH_MAX) {
-			ERROR("error: path %s%s is too large.\n", dirptr, mountpoints[i]);
-			return false;
-		}
-
-		strcpy(buf, dirptr);
-		strcat(buf, mountpoints[i]);
-		if (!isdir(buf)) {
-			if (ignore_missing)
-				continue;
-			ERROR("error: source mount dir %s does not exists.\n", buf);
-			return false;
-		}
-		if (!isdir(mountpoints[i])) {
-			ERROR("error: mountpoint %s does not exists.\n", mountpoints[i]);
-			return false;
-		}
-		if (mount(buf, mountpoints[i], NULL, MS_BIND|MS_REC, NULL) == -1) {
-			ERROR("mount: failed to mount %s: %s\n",
-				mountpoints[i], strerror(errno));
-			return false;
-		}
-	}
-	return true;
+    int successful = 0;
+    for (size_t i = 0; i < targets->end; i++) {
+        if (!exists(targets->list[i])) {
+            if (mkfile(targets->list[i]))
+                intarray_append(created, i);
+            else
+                continue;
+        }
+
+        DEBUG("%s: source=%s, target=%s\n", __func__, source, targets->list[i]);
+        if (mount(source, targets->list[i], NULL, MS_BIND, NULL) == -1)
+            ERROR("mount: failed to mount %s to %s: %s",
+                 source, targets->list[i], strerror(errno));
+        else
+            successful++;
+    }
+    return successful;
 }
 
-bool isxbpscommand(const char *s)
+void onterm(int sig)
 {
-	const char *commands[] = {
-		"/xbps-install",
-		"/xbps-remove",
-		"/xbps-reconfigure"
-	};
-	for (size_t i = 0; i < ARRAY_SIZE(commands); i++) {
-		const char *command = commands[i];
-		if (!strcmp(s, command+1))
-			return true;
-		char *slash = strrchr(s, '/');
-		if (slash && !strcmp(slash, command))
-			return true;
-	}
-	return false;
+    UNUSED(sig);
+    term_caught = 1;
 }
 
 int main(int argc, char **argv)
 {
-	if (argc < 2) {
-		usage(argv[0]);
-		return 0;
-	}
-
-	int result;
-	char *dir = NULL;
-	size_t dirlen;
-	const char *usermounts[USERMOUNTS_MAX] = {0};
-	int usermounts_num = 0;
-	int c;
-	while ((c = getopt(argc, argv, "vhm:r:")) != -1) {
-		switch (c) {
-			case 'v':
-				printf("%s\n", prog_version);
-				return 0;
-			case 'h':
-				usage(argv[0]);
-				return 0;
-			case 'r':
-				dir = optarg;
-				break;
-			case 'm':
-				if (usermounts_num < USERMOUNTS_MAX) {
-					usermounts[usermounts_num++] = optarg;
-					break;
-				} else {
-					ERROR("error: only up to %d user mounts allowed.\n",
-						USERMOUNTS_MAX);
-					return 1;
-				}
-			case '?':
-				return 1;
-		}
-	}
-
-	/* Get alternative root dir. */
-	if (!dir)
-		dir = getenv(var_name);
-	if (!dir) {
-		ERROR("error: environment variable %s not found.\n", var_name);
-		return 1;
-	}
-
-	/* Validate it. */
-	if (!isdir(dir)) {
-		ERROR("error: %s is not a directory.\n", dir);
-		return 1;
-	}
-
-	dirlen = strlen(dir);
-
-	/* Do the unshare magic. */
-	result = unshare(CLONE_NEWNS);
-	if (result == -1) {
-		ERROR("unshare: %s\n", strerror(errno));
-		return 1;
-	}
-
-	/* Mount stuff from altroot to our private namespace. */
-	const char *mountpoints[3] = {"/usr", NULL, NULL};
-	if (isxbpscommand(argv[optind])) {
-		mountpoints[1] = "/var";
-		mountpoints[2] = "/etc";
-	} else {
-		mountpoints[1] = "/var/db/xbps";
-		mountpoints[2] = "/etc/xbps.d";
-	}
-	if (!mountlist(dir, dirlen, mountpoints, ARRAY_SIZE(mountpoints), true))
-		return 1;
-	if (usermounts_num > 0 &&
-		!mountlist(dir, dirlen, usermounts, usermounts_num, false))
-		return 1;
-
-	/* Drop root. */
-	uid_t uid = getuid();
-	gid_t gid = getgid();
-
-	result = setreuid(uid, uid);
-	if (result == -1) {
-		ERROR("setreuid: %s\n", strerror(errno));
-		return 1;
-	}
-
-	result = setregid(gid, gid);
-	if (result == -1) {
-		ERROR("setregid: %s\n", strerror(errno));
-		return 1;
-	}
-	
-	/* Launch program. */
-	result = execvp(argv[optind], (char *const *)argv+optind);
-	if (result == -1) {
-		ERROR("execvp(%s): %s\n", argv[optind], strerror(errno));
-		return 1;
-	}
-
-	return 0;
+    if (argc < 2) {
+        usage(argv[0]);
+        return 0;
+    }
+
+    int nsfd = -1;
+    char *dir = NULL;
+    char *undo_bin = NULL;
+    char *sock_dir = NULL;
+    int sock_fd = -1, sock_conn = -1;
+    size_t dirlen;
+    int c;
+    int exit_code = 1;
+    DIR *dirptr = NULL;
+    bool ignore_missing = false;
+    bool forked = false;
+    pid_t pid = 0;
+    char cwd[PATH_MAX];
+
+    struct strarray user_mounts;
+    strarray_alloc(&user_mounts, USER_LISTS_MAX);
+
+    struct strarray undo_mounts;
+    strarray_alloc(&undo_mounts, USER_LISTS_MAX);
+
+    struct intarray tounlink;
+    intarray_alloc(&tounlink, USER_LISTS_MAX);
+
+    while ((c = getopt(argc, argv, "vhm:r:u:U:is:V")) != -1) {
+        switch (c) {
+        case 'v':
+            printf("%s\n", PROG_VERSION);
+            return 0;
+        case 'h':
+            usage(argv[0]);
+            return 0;
+        case 'i':
+            ignore_missing = true;
+            break;
+        case 'r':
+            dir = optarg;
+            break;
+        case 'U':
+            undo_bin = optarg;
+            break;
+        case 's':
+            sock_dir = optarg;
+            break;
+        case 'V':
+            g_verbose = true;
+            break;
+        case 'm':
+            if (!strarray_append(&user_mounts, optarg))
+                ERROR_EXIT("error: only up to %lu user mounts allowed.\n",
+                           user_mounts.size);
+            break;
+        case 'u':
+            if (!strarray_append(&undo_mounts, optarg))
+                ERROR_EXIT("error: only up to %lu user mounts allowed.\n",
+                           undo_mounts.size);
+            break;
+        case '?':
+            return 1;
+        }
+    }
+
+    if (!argv[optind]) {
+        usage(argv[0]);
+        return 1;
+    }
+
+    /* Get container path. */
+    if (!dir)
+        dir = getenv(CONTAINER_DIR_VAR);
+    if (!dir)
+        ERROR_EXIT("error: environment variable %s not found.\n",
+             CONTAINER_DIR_VAR);
+
+    /* Validate it. */
+    if (!isdir(dir))
+        ERROR_EXIT("error: %s is not a directory.\n", dir);
+
+    dirlen = strlen(dir);
+
+    DEBUG("dir=%s\n", dir);
+
+    /* Get undo binary path, if needed. */
+    if (undo_mounts.end > 0) {
+        if (!undo_bin)
+            undo_bin = getenv(UNDO_BIN_VAR);
+        if (!undo_bin) {
+            ERROR_EXIT("error: environment variable %s not found.\n",
+                UNDO_BIN_VAR);
+        } else if (strlen(undo_bin) > PATH_MAX)
+            ERROR_EXIT("error: undo binary path is too long.\n");
+
+        /* Validate it. */
+        if (!isexe(undo_bin))
+            ERROR_EXIT("error: %s is not an executable.\n", undo_bin);
+
+        DEBUG("undo_bin=%s\n", undo_bin);
+    }
+
+    /* Get current namespace's file descriptor. */
+    nsfd = open("/proc/self/ns/mnt", O_RDONLY);
+    if (nsfd == -1)
+        ERROR_EXIT("error: failed to acquire mount namespace's fd.%s\n",
+                   strerror(errno));
+
+    /* Check socket directory. */
+    if (!sock_dir)
+        sock_dir = getenv(SOCK_DIR_VAR);
+    if (!sock_dir)
+        sock_dir = SOCK_DIR_DEFAULT;
+    if (strlen(sock_dir) > SOCK_DIR_PATH_MAX)
+        ERROR_EXIT("error: socket directory path is too long.\n");
+
+    if (access(sock_dir, F_OK) == -1) {
+        if (mkdir(sock_dir, 0700) == -1)
+            ERROR_EXIT("error: failed to create %s directory.\n", sock_dir);
+    } else {
+        if ((dirptr = opendir(sock_dir)) == NULL)
+            ERROR_EXIT("error: %s is not a directory.\n", sock_dir);
+    }
+    DEBUG("sock_dir=%s\n", sock_dir);
+
+    /* Get current working directory. */
+    getcwd(cwd, PATH_MAX);
+    DEBUG("cwd=%s\n", cwd);
+
+    /* Do the unshare magic. */
+    if (unshare(CLONE_NEWNS) == -1)
+        ERROR_EXIT("unshare: %s\n", strerror(errno));
+
+    /* Mount stuff from the container to the namespace. */
+    /* First, mount what user asked us to mount. */
+    if (mount_dirs(dir, dirlen, &user_mounts) < user_mounts.end && !ignore_missing)
+        ERROR_EXIT("error: some mounts failed.\n");
+
+    /* Then necessary stuff. */
+    struct strarray default_mounts;
+    strarray_alloc(&default_mounts, 3);
+    strarray_append(&default_mounts, "/usr");
+    if (isxbpscommand(argv[optind])) {
+        strarray_append(&default_mounts, "/var");
+        strarray_append(&default_mounts, "/etc");
+    }
+    if (mount_dirs(dir, dirlen, &default_mounts) < default_mounts.end)
+        ERROR_EXIT("error: some necessary mounts failed.\n");
+
+    /* Bind mount undo binary. */
+    if (mount_undo(undo_bin, &undo_mounts, &tounlink) < undo_mounts.end
+            && !ignore_missing)
+        ERROR_EXIT("error: some undo mounts failed.\n");
+
+    /* Mount sock_dir as tmpfs. It will only be visible in this namespace. */
+    if (mount("tmpfs", sock_dir, "tmpfs", 0, NULL) == -1)
+        ERROR_EXIT("mount: error mounting tmpfs in %s.\n", sock_dir);
+
+    /* Fork. */
+    pid_t ppid_before_fork = getpid();
+    pid = fork();
+    if (pid == -1)
+        ERROR_EXIT("fork: %s\n", strerror(errno));
+
+    forked = true;
+
+    if (pid == 0) {
+        /* Catch SIGTERM. */
+        struct sigaction sa = {0};
+        sa.sa_handler = onterm;
+        sigaction(SIGTERM, &sa, NULL);
+
+        /* Ignore SIGINT. */
+        signal(SIGINT, SIG_IGN);
+
+        /* Set the child to get SIGTERM when parent thread dies. */
+        int r = prctl(PR_SET_PDEATHSIG, SIGTERM);
+        if (r == -1)
+            ERROR_EXIT("prctl: %s\n", strerror(errno));
+        if (getppid() != ppid_before_fork)
+            ERROR_EXIT("error: parent has died already.\n");
+
+        /* Create unix socket. */
+        sock_fd = socket(AF_UNIX, SOCK_STREAM, 0);
+        if (sock_fd == -1)
+            ERROR_EXIT("socket: %s.\n", strerror(errno));
+
+        struct sockaddr_un sock_addr = {0};
+        sock_addr.sun_family = AF_UNIX;
+        strcpy(sock_addr.sun_path, sock_dir);
+        strcat(sock_addr.sun_path, SOCK_NAME);
+
+        if (bind(sock_fd, (struct sockaddr *)&sock_addr, sizeof(sock_addr)) == -1)
+            ERROR_EXIT("bind: %s\n", strerror(errno));
+
+        listen(sock_fd, 1);
+
+        while (!term_caught) {
+            sock_conn = accept(sock_fd, NULL, 0);
+            if (sock_conn == -1)
+                continue;
+            send_fd(sock_conn, nsfd);
+        }
+    } else {
+        /* Parent process. Dropping root rights. */
+        uid_t uid = getuid();
+        gid_t gid = getgid();
+
+        if (setreuid(uid, uid) == -1)
+            ERROR_EXIT("setreuid: %s\n", strerror(errno));
+
+        if (setregid(gid, gid) == -1)
+            ERROR_EXIT("setregid: %s\n", strerror(errno));
+
+        /* Restore working directory. */
+        if (chdir(cwd) == -1)
+            DEBUG("chdir: %s\n", strerror(errno));
+
+        /* Launching program. */
+        if (execvp(argv[optind], (char *const *)argv+optind) == -1)
+            ERROR_EXIT("execvp(%s): %s\n", argv[optind], strerror(errno));
+    }
+
+    exit_code = 0;
+
+end:
+    if (nsfd != -1)
+        close(nsfd);
+
+    if (sock_fd != -1)
+        close(sock_fd);
+
+    if (sock_conn != -1)
+        close(sock_conn);
+
+    if (dirptr != NULL)
+        closedir(dirptr);
+
+    if (tounlink.end > 0 && (!forked || pid == 0)) {
+        for (size_t i = 0; i < tounlink.end; i++) {
+            char *path = undo_mounts.list[tounlink.list[i]];
+            if (umount(path) == -1)
+                DEBUG("umount(%s): %s\n", path, strerror(errno));
+            if (unlink(path) == -1)
+                ERROR("unlink(%s): %s\n", path, strerror(errno));
+            else
+                DEBUG("unlink(%s)\n", path);
+        }
+    }
+
+    return exit_code;
 }
diff --git a/voidnsundo.c b/voidnsundo.c
new file mode 100644
index 0000000..fac27aa
--- /dev/null
+++ b/voidnsundo.c
@@ -0,0 +1,154 @@
+#define _GNU_SOURCE
+
+#include <string.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <libgen.h>
+#include <stdbool.h>
+#include <getopt.h>
+#include <dirent.h>
+#include <errno.h>
+#include <unistd.h>
+#include <sched.h>
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <linux/limits.h>
+
+#include "config.h"
+#include "utils.h"
+#include "macros.h"
+
+bool g_verbose = false;
+
+void usage(const char *progname)
+{
+    printf("Usage: %s [OPTIONS] PROGRAM [ARGS]\n", progname);
+    printf("\n"
+           "Options:\n"
+           "    -s:        Socket directory path. When this option is not present,\n"
+           "               " SOCK_DIR_VAR " environment variable is used. If both are\n"
+           "               missing, defaults to " SOCK_DIR_DEFAULT ".\n"
+           "    -V:        Verbose output.\n"
+           "    -h:        Print this help.\n"
+           "    -v:        Print version.\n");
+}
+
+int main(int argc, char **argv)
+{
+    bool binded = strcmp(basename(argv[0]), VOIDNSUNDO_NAME) != 0;
+    int c;
+    char *sock_dir = NULL;
+    int sock_fd = -1;
+    int exit_code = 1;
+    char realpath_buf[PATH_MAX];
+    char cwd[PATH_MAX];
+    if (!binded) {
+        if (argc < 2) {
+            usage(argv[0]);
+            return 0;
+        }
+
+        while ((c = getopt(argc, argv, "vhs:V")) != -1) {
+            switch (c) {
+            case 'v':
+                printf("%s\n", PROG_VERSION);
+                return 0;
+            case 'h':
+                usage(argv[0]);
+                return 0;
+            case 's':
+                sock_dir = optarg;
+                break;
+            case 'V':
+                g_verbose = true;
+                break;
+            case '?':
+                return 1;
+            }
+        }
+
+        if (!argv[optind]) {
+            usage(argv[0]);
+            return 1;
+        }
+    } else {
+        int bytes = readlink("/proc/self/exe", realpath_buf, PATH_MAX);
+        realpath_buf[bytes] = '\0';
+        /* DEBUG("/proc/self/exe points to %s\n", realpath_buf); */
+    }
+
+    /* Check socket directory. */
+    DIR *dirptr = NULL;
+    if (!sock_dir)
+        sock_dir = getenv(SOCK_DIR_VAR);
+    if (!sock_dir)
+        sock_dir = SOCK_DIR_DEFAULT;
+    if (strlen(sock_dir) > SOCK_DIR_PATH_MAX)
+        ERROR_EXIT("error: socket directory path is too long.\n");
+    if (!isdir(sock_dir))
+        ERROR_EXIT("error: %s is not a directory.\n", sock_dir);
+    if (access(sock_dir, F_OK) == -1) {
+        ERROR_EXIT("error: failed to access socket directory: %s.\n",
+                   strerror(errno));
+    } else {
+        if ((dirptr = opendir(sock_dir)) == NULL)
+            ERROR_EXIT("error: %s is not a directory.\n", sock_dir);
+    }
+    DEBUG("sock_dir=%s\n", sock_dir);
+
+    /* Get current working directory. */
+    getcwd(cwd, PATH_MAX);
+    DEBUG("cwd=%s\n", cwd);
+
+    /* Get namespace's fd. */
+    sock_fd = socket(AF_UNIX, SOCK_STREAM, 0);
+    if (sock_fd == -1)
+        ERROR_EXIT("socket: %s.\n", strerror(errno));
+
+    struct sockaddr_un sock_addr = {0};
+    sock_addr.sun_family  = AF_UNIX;
+    strcpy(sock_addr.sun_path, sock_dir);
+    strcat(sock_addr.sun_path, SOCK_NAME);
+
+    if (connect(sock_fd, (struct sockaddr *)&sock_addr, sizeof(sock_addr)) == -1)
+        ERROR_EXIT("connect: %s\n", strerror(errno));
+
+    int nsfd = recv_fd(sock_fd);
+    if (!nsfd)
+        ERROR_EXIT("error: failed to get nsfd.\n");
+
+    if (setns(nsfd, CLONE_NEWNS) == -1)
+        ERROR_EXIT("setns: %s.\n", strerror(errno));
+
+    /* Drop root. */
+    uid_t uid = getuid();
+    gid_t gid = getgid();
+
+    if (setreuid(uid, uid) == -1)
+        ERROR_EXIT("setreuid: %s\n", strerror(errno));
+
+    if (setregid(gid, gid) == -1)
+        ERROR_EXIT("setregid: %s\n", strerror(errno));
+
+    /* Restore working directory. */
+    if (chdir(cwd) == -1)
+        DEBUG("chdir: %s\n", strerror(errno));
+
+    /* Launch program. */
+    int argind = binded ? 0 : optind;
+    if (binded)
+        argv[0] = realpath_buf;
+    if (execvp(argv[argind], (char *const *)argv+argind) == -1)
+        ERROR_EXIT("execvp(%s): %s\n", argv[argind], strerror(errno));
+
+    exit_code = 0;
+
+end:
+    if (dirptr != NULL)
+        closedir(dirptr);
+
+    if (sock_fd != -1)
+        close(sock_fd);
+
+    return exit_code;
+}
\ No newline at end of file