nspawn: beef up netns checking a bit, for compat with old kernels

Fixes: #10544

src/basic/stat-util.c

@@ -213,15 +213,47 @@ int fd_is_network_fs(int fd) {
 }
 
 int fd_is_network_ns(int fd) {
+        struct statfs s;
         int r;
 
-        r = fd_is_fs_type(fd, NSFS_MAGIC);
-        if (r <= 0)
-                return r;
+        /* Checks whether the specified file descriptor refers to a network namespace. On old kernels there's no nice
+         * way to detect that, hence on those we'll return a recognizable error (EUCLEAN), so that callers can handle
+         * this somewhat nicely.
+         *
+         * This function returns > 0 if the fd definitely refers to a network namespace, 0 if it definitely does not
+         * refer to a network namespace, -EUCLEAN if we can't determine, and other negative error codes on error. */
+
+        if (fstatfs(fd, &s) < 0)
+                return -errno;
+
+        if (!is_fs_type(&s, NSFS_MAGIC)) {
+                /* On really old kernels, there was no "nsfs", and network namespace sockets belonged to procfs
+                 * instead. Handle that in a somewhat smart way. */
+
+                if (is_fs_type(&s, PROC_SUPER_MAGIC)) {
+                        struct statfs t;
+
+                        /* OK, so it is procfs. Let's see if our own network namespace is procfs, too. If so, then the
+                         * passed fd might refer to a network namespace, but we can't know for sure. In that case,
+                         * return a recognizable error. */
+
+                        if (statfs("/proc/self/ns/net", &t) < 0)
+                                return -errno;
+
+                        if (s.f_type == t.f_type)
+                                return -EUCLEAN; /* It's possible, we simply don't know */
+                }
+
+                return 0; /* No! */
+        }
 
         r = ioctl(fd, NS_GET_NSTYPE);
-        if (r < 0)
+        if (r < 0) {
+                if (errno == ENOTTY) /* Old kernels didn't know this ioctl, let's also return a recognizable error in that case */
+                        return -EUCLEAN;
+
                 return -errno;
+        }
 
         return r == CLONE_NEWNET;
 }