networkctl: introduce verb mask and unmask

Suggested in https://github.com/systemd/systemd/pull/29928#discussion_r1386626565

src/network/networkctl-config-file.c

@@ -13,6 +13,7 @@
 #include "bus-wait-for-jobs.h"
 #include "conf-files.h"
 #include "edit-util.h"
+#include "mkdir-label.h"
 #include "netlink-util.h"
 #include "networkctl.h"
 #include "networkctl-config-file.h"
@@ -518,3 +519,110 @@ int verb_cat(int argc, char *argv[], void *userdata) {
 
         return ret;
 }
+
+int verb_mask(int argc, char *argv[], void *userdata) {
+        ReloadFlags flags = 0;
+        int r;
+
+        r = mac_selinux_init();
+        if (r < 0)
+                return r;
+
+        STRV_FOREACH(name, strv_skip(argv, 1)) {
+                _cleanup_free_ char *config_path = NULL, *symlink_path = NULL;
+                ReloadFlags reload;
+
+                /* We update the real 'flags' at last, since the operation can be skipped. */
+                if (ENDSWITH_SET(*name, ".network", ".netdev"))
+                        reload = RELOAD_NETWORKD;
+                else if (endswith(*name, ".link"))
+                        reload = RELOAD_UDEVD;
+                else
+                        return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Invalid network config name '%s'.", *name);
+
+                r = get_config_files_by_name(*name, /* allow_masked = */ true, &config_path, /* ret_dropins = */ NULL);
+                if (r == -ENOENT)
+                        log_warning("No existing network config '%s' found, proceeding anyway.", *name);
+                else if (r < 0)
+                        return log_error_errno(r, "Failed to get the path of network config '%s': %m", *name);
+                else if (!path_startswith(config_path, "/usr")) {
+                        r = null_or_empty_path(config_path);
+                        if (r < 0)
+                                return log_error_errno(r,
+                                                       "Failed to check if '%s' is masked: %m", config_path);
+                        if (r > 0) {
+                                log_debug("%s is already masked, skipping.", config_path);
+                                continue;
+                        }
+
+                        /* At this point, we have found a config under mutable dir (/run/ or /etc/),
+                         * so masking through /run/ (--runtime) is not possible. If it's under /etc/,
+                         * then it doesn't work without --runtime either. */
+                        if (arg_runtime || path_startswith(config_path, "/etc"))
+                                return log_error_errno(SYNTHETIC_ERRNO(EEXIST),
+                                                       "Cannot mask network config %s: %s exists",
+                                                       *name, config_path);
+                }
+
+                symlink_path = path_join(NETWORK_DIRS[arg_runtime ? 1 : 0], *name);
+                if (!symlink_path)
+                        return log_oom();
+
+                (void) mkdir_parents_label(symlink_path, 0755);
+
+                if (symlink("/dev/null", symlink_path) < 0)
+                        return log_error_errno(errno,
+                                               "Failed to create symlink '%s' to /dev/null: %m", symlink_path);
+
+                flags |= reload;
+                log_info("Successfully created symlink '%s' to /dev/null.", symlink_path);
+        }
+
+        return reload_daemons(flags);
+}
+
+int verb_unmask(int argc, char *argv[], void *userdata) {
+        ReloadFlags flags = 0;
+        int r;
+
+        STRV_FOREACH(name, strv_skip(argv, 1)) {
+                _cleanup_free_ char *path = NULL;
+                ReloadFlags reload;
+
+                if (ENDSWITH_SET(*name, ".network", ".netdev"))
+                        reload = RELOAD_NETWORKD;
+                else if (endswith(*name, ".link"))
+                        reload = RELOAD_UDEVD;
+                else
+                        return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Invalid network config name '%s'.", *name);
+
+                r = get_config_files_by_name(*name, /* allow_masked = */ true, &path, /* ret_dropins = */ NULL);
+                if (r == -ENOENT) {
+                        log_debug_errno(r, "Network configuration '%s' doesn't exist, skipping.", *name);
+                        continue;
+                }
+                if (r < 0)
+                        return log_error_errno(r, "Failed to get the path of network config '%s': %m", *name);
+
+                r = null_or_empty_path(path);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to check if '%s' is masked: %m", path);
+                if (r == 0)
+                        continue;
+
+                if (path_startswith(path, "/usr"))
+                        return log_error_errno(r, "Cannot unmask network config under /usr/: %s", path);
+
+                if (unlink(path) < 0) {
+                        if (errno == ENOENT)
+                                continue;
+
+                        return log_error_errno(errno, "Failed to remove '%s': %m", path);
+                }
+
+                flags |= reload;
+                log_info("Successfully removed masked network config '%s'.", path);
+        }
+
+        return reload_daemons(flags);
+}

src/network/networkctl-config-file.h

@@ -3,3 +3,6 @@
 
 int verb_edit(int argc, char *argv[], void *userdata);
 int verb_cat(int argc, char *argv[], void *userdata);
+
+int verb_mask(int argc, char *argv[], void *userdata);
+int verb_unmask(int argc, char *argv[], void *userdata);

src/network/networkctl.c

@@ -2877,6 +2877,8 @@ static int help(void) {
                "  reload                 Reload .network and .netdev files\n"
                "  edit FILES|DEVICES...  Edit network configuration files\n"
                "  cat FILES|DEVICES...   Show network configuration files\n"
+               "  mask FILES...          Mask network configuration files\n"
+               "  unmask FILES...        Unmask network configuration files\n"
                "\nOptions:\n"
                "  -h --help              Show this help\n"
                "     --version           Show package version\n"
@@ -3033,6 +3035,8 @@ static int networkctl_main(int argc, char *argv[]) {
                 { "reload",      1,        1,        VERB_ONLINE_ONLY,              verb_reload         },
                 { "edit",        2,        VERB_ANY, 0,                             verb_edit           },
                 { "cat",         2,        VERB_ANY, 0,                             verb_cat            },
+                { "mask",        2,        VERB_ANY, 0,                             verb_mask           },
+                { "unmask",      2,        VERB_ANY, 0,                             verb_unmask         },
                 {}
         };