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;
 }

src/nspawn/nspawn.c

@@ -3761,10 +3761,12 @@ static int run(int master,
                         return log_error_errno(errno, "Cannot open file %s: %m", arg_network_namespace_path);
 
                 r = fd_is_network_ns(netns_fd);
-                if (r < 0 && r != -ENOTTY)
+                if (r == -EUCLEAN)
+                        log_debug_errno(r, "Cannot determine if passed network namespace path '%s' really refers to a network namespace, assuming it does.", arg_network_namespace_path);
+                else if (r < 0)
                         return log_error_errno(r, "Failed to check %s fs type: %m", arg_network_namespace_path);
-                if (r == 0) {
-                        log_error("Path %s doesn't refer to a network namespace", arg_network_namespace_path);
+                else if (r == 0) {
+                        log_error("Path %s doesn't refer to a network namespace, refusing.", arg_network_namespace_path);
                         return -EINVAL;
                 }
         }

src/test/test-stat-util.c

@@ -67,11 +67,26 @@ static void test_path_is_temporary_fs(void) {
         assert_se(path_is_temporary_fs("/i-dont-exist") == -ENOENT);
 }
 
+static void test_fd_is_network_ns(void) {
+        _cleanup_close_ int fd = -1;
+        assert_se(fd_is_network_ns(STDIN_FILENO) == 0);
+        assert_se(fd_is_network_ns(STDERR_FILENO) == 0);
+        assert_se(fd_is_network_ns(STDOUT_FILENO) == 0);
+
+        assert_se((fd = open("/proc/self/ns/mnt", O_CLOEXEC|O_RDONLY)) >= 0);
+        assert_se(IN_SET(fd_is_network_ns(fd), 0, -EUCLEAN));
+        fd = safe_close(fd);
+
+        assert_se((fd = open("/proc/self/ns/net", O_CLOEXEC|O_RDONLY)) >= 0);
+        assert_se(IN_SET(fd_is_network_ns(fd), 1, -EUCLEAN));
+}
+
 int main(int argc, char *argv[]) {
         test_files_same();
         test_is_symlink();
         test_path_is_fs_type();
         test_path_is_temporary_fs();
+        test_fd_is_network_ns();
 
         return 0;
 }