tree-wide: add short comments for namespace_open() and namespace_enter()

Also use -EBADF when unspecified.

src/shared/logs-show.c

@@ -1688,7 +1688,7 @@ static int get_boot_id_for_machine(const char *machine, sd_id128_t *boot_id) {
         if (r < 0)
                 return r;
 
-        r = namespace_open(pid, &pidnsfd, &mntnsfd, NULL, NULL, &rootfd);
+        r = namespace_open(pid, &pidnsfd, &mntnsfd, /* ret_netns_fd = */ NULL, /* ret_userns_fd = */ NULL, &rootfd);
         if (r < 0)
                 return r;
 

src/shared/machine-id-setup.c

@@ -265,7 +265,12 @@ int machine_id_commit(const char *root) {
         fd = safe_close(fd);
 
         /* Store current mount namespace */
-        r = namespace_open(0, NULL, &initial_mntns_fd, NULL, NULL, NULL);
+        r = namespace_open(0,
+                           /* ret_pidns_fd = */ NULL,
+                           &initial_mntns_fd,
+                           /* ret_netns_fd = */ NULL,
+                           /* ret_userns_fd = */ NULL,
+                           /* ret_root_fd = */ NULL);
         if (r < 0)
                 return log_error_errno(r, "Can't fetch current mount namespace: %m");
 
@@ -284,7 +289,11 @@ int machine_id_commit(const char *root) {
                 return log_error_errno(r, "Cannot write %s. This is mandatory to get a persistent machine ID: %m", etc_machine_id);
 
         /* Return to initial namespace and proceed a lazy tmpfs unmount */
-        r = namespace_enter(-1, initial_mntns_fd, -1, -1, -1);
+        r = namespace_enter(/* pidns_fd = */ -EBADF,
+                            initial_mntns_fd,
+                            /* netns_fd = */ -EBADF,
+                            /* userns_fd = */ -EBADF,
+                            /* root_fd = */ -EBADF);
         if (r < 0)
                 return log_warning_errno(r, "Failed to switch back to initial mount namespace: %m.\nWe'll keep transient %s file until next reboot.", etc_machine_id);
 

src/shared/mount-util.c

@@ -1100,7 +1100,7 @@ static int mount_in_namespace(
         if (!pidref_is_set(target))
                 return -ESRCH;
 
-        r = namespace_open(target->pid, &pidns_fd, &mntns_fd, NULL, NULL, &root_fd);
+        r = namespace_open(target->pid, &pidns_fd, &mntns_fd, /* ret_netns_fd = */ NULL, /* ret_userns_fd = */ NULL, &root_fd);
         if (r < 0)
                 return log_debug_errno(r, "Failed to retrieve FDs of the target process' namespace: %m");
 

src/shared/socket-netlink.c

@@ -420,11 +420,11 @@ int netns_get_nsid(int netnsfd, uint32_t *ret) {
         if (netnsfd < 0) {
                 r = namespace_open(
                                 0,
-                                /* pidns_fd= */ NULL,
-                                /* mntns_fd= */ NULL,
+                                /* ret_pidns_fd = */ NULL,
+                                /* ret_mntns_fd = */ NULL,
                                 &_netns_fd,
-                                /* userns_fd= */ NULL,
-                                /* root_fd= */ NULL);
+                                /* ret_userns_fd = */ NULL,
+                                /* ret_root_fd = */ NULL);
                 if (r < 0)
                         return r;