tests: add smoke test for systemd-nspawn

Basically, this test runs:
```
    systemd-nspawn --register=no -D "$_root" -b
    systemd-nspawn --register=no -D "$_root" --private-network -b
    systemd-nspawn --register=no -D "$_root" -U -b
    systemd-nspawn --register=no -D "$_root" --private-network -U -b
```
and exports the `UNIFIED_CGROUP_HIERARCHY=[yes|no]`, `SYSTEMD_NSPAWN_USE_CGNS=[yes|no]`

Inspired by
* systemd#3589 (comment)
* systemd#4372 (comment)
* systemd#4223 (comment)
* systemd#1555

and so on :-)

1 files changed, 143 insertions, 0 deletions

diff --git a/test/TEST-13-NSPAWN-SMOKE/has-overflow.c b/test/TEST-13-NSPAWN-SMOKE/has-overflow.c
new file mode 100644
index 0000000000..1b3331fad7
--- /dev/null
+++ b/test/TEST-13-NSPAWN-SMOKE/has-overflow.c
@@ -0,0 +1,143 @@
+#define _GNU_SOURCE
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/mount.h>
+#include <sched.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <sys/wait.h>
+#include <signal.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <string.h>
+#include <limits.h>
+#include <errno.h>
+#include <grp.h>
+
+#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); } while (0)
+
+struct child_args {
+   int    pipe_fd[2];  /* Pipe used to synchronize parent and child */
+};
+
+static void usage(char *pname) {
+        fprintf(stderr, "Options can be:\n");
+        fprintf(stderr, "\t-M uid_map  Specify UID map for user namespace\n");
+        fprintf(stderr, "\t-G gid_map  Specify GID map for user namespace\n");
+
+        exit(EXIT_FAILURE);
+}
+
+static void update_map(char *mapping, char *map_file) {
+        int fd, j;
+        size_t map_len;
+
+        map_len = strlen(mapping);
+
+        fd = open(map_file, O_RDWR);
+        if (fd == -1) {
+                fprintf(stderr, "ERROR: open %s: %s\n", map_file, strerror(errno));
+                exit(EXIT_FAILURE);
+        }
+
+        if (write(fd, mapping, map_len) != map_len) {
+                fprintf(stderr, "ERROR: write %s: %s\n", map_file, strerror(errno));
+                exit(EXIT_FAILURE);
+        }
+
+        close(fd);
+}
+
+static void proc_setgroups_write(pid_t child_pid, char *str) {
+        char setgroups_path[PATH_MAX];
+        int fd;
+
+        snprintf(setgroups_path, PATH_MAX, "/proc/%ld/setgroups", (long) child_pid);
+
+        fd = open(setgroups_path, O_RDWR);
+        if (fd == -1) {
+                if (errno != ENOENT)
+                        fprintf(stderr, "ERROR: open %s: %s\n", setgroups_path, strerror(errno));
+                return;
+        }
+
+        if (write(fd, str, strlen(str)) == -1)
+                fprintf(stderr, "ERROR: write %s: %s\n", setgroups_path, strerror(errno));
+
+        close(fd);
+}
+
+static int child_func(void *arg) {
+        struct child_args *args = (struct child_args *) arg;
+        char ch;
+
+        close(args->pipe_fd[1]);
+        if (read(args->pipe_fd[0], &ch, 1) != 0) {
+                fprintf(stderr, "Failure in child: read from pipe returned != 0\n");
+                exit(EXIT_FAILURE);
+        }
+
+        mount("tmpfs", "/tmp", "tmpfs", MS_MGC_VAL, "mode=777,uid=0,gid=0");
+        if (mkdir("/tmp/hey", 0777) < 0)
+                exit(EXIT_FAILURE);
+
+        exit(EXIT_SUCCESS);
+}
+
+#define STACK_SIZE (1024 * 1024)
+
+static char child_stack[STACK_SIZE];
+
+int main(int argc, char *argv[]) {
+        int flags, opt;
+        pid_t child_pid;
+        struct child_args args;
+        char *uid_map, *gid_map;
+        const int MAP_BUF_SIZE = 100;
+        char map_buf[MAP_BUF_SIZE];
+        char map_path[PATH_MAX];
+        int status;
+
+        flags = 0;
+        gid_map = NULL;
+        uid_map = NULL;
+        while ((opt = getopt(argc, argv, "+M:G:")) != -1) {
+                switch (opt) {
+                        case 'M':
+                                uid_map = optarg;
+                                break;
+                        case 'G':
+                                gid_map = optarg;
+                                break;
+                        default:
+                                  usage(argv[0]);
+                }
+        }
+
+        if (!uid_map || !gid_map)
+                usage(argv[0]);
+
+        flags |= CLONE_NEWNS;
+        flags |= CLONE_NEWUSER;
+
+        if (pipe(args.pipe_fd) == -1)
+                errExit("pipe");
+
+        child_pid = clone(child_func, child_stack + STACK_SIZE, flags | SIGCHLD, &args);
+        if (child_pid == -1)
+                errExit("clone");
+
+        snprintf(map_path, PATH_MAX, "/proc/%ld/uid_map", (long) child_pid);
+        update_map(uid_map, map_path);
+
+        proc_setgroups_write(child_pid, "allow");
+        snprintf(map_path, PATH_MAX, "/proc/%ld/gid_map", (long) child_pid);
+        update_map(gid_map, map_path);
+
+        close(args.pipe_fd[1]);
+
+        if (waitpid(child_pid, &status, 0) == -1)
+                errExit("waitpid");
+
+        exit(WIFEXITED(status) && WEXITSTATUS(status) == EXIT_SUCCESS ? EXIT_FAILURE : EXIT_SUCCESS);
+}