Spacing/style nits.

format.c

@@ -2597,7 +2597,7 @@ format_cb_user(__unused struct format_tree *ft)
 
 	if ((pw = getpwuid(getuid())) != NULL)
 		return (xstrdup(pw->pw_name));
-	return NULL;
+	return (NULL);
 }
 
 /* Format table type. */

screen.c

@@ -662,9 +662,9 @@ screen_mode_to_string(int mode)
 	static char	tmp[1024];
 
 	if (mode == 0)
-		return "NONE";
+		return ("NONE");
 	if (mode == ALL_MODES)
-		return "ALL";
+		return ("ALL");
 
 	*tmp = '\0';
 	if (mode & MODE_CURSOR)

server-acl.c

@@ -0,0 +1,186 @@
+/* $OpenBSD$ */
+
+/*
+ * Copyright (c) 2021 Holland Schutte, Jayson Morberg
+ * Copyright (c) 2021 Dallas Lyons <dallasdlyons@gmail.com>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF MIND, USE, DATA OR PROFITS, WHETHER
+ * IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING
+ * OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/socket.h>
+
+#include <ctype.h>
+#include <pwd.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "tmux.h"
+
+struct server_acl_user {
+	uid_t				uid;
+
+	int				flags;
+#define SERVER_ACL_READONLY 0x1
+
+	RB_ENTRY(server_acl_user)	entry;
+};
+
+static int
+server_acl_cmp(struct server_acl_user *user1, struct server_acl_user *user2)
+{
+	if (user1->uid < user2->uid)
+		return (-1);
+	return (user1->uid > user2->uid);
+}
+
+RB_HEAD(server_acl_entries, server_acl_user) server_acl_entries;
+RB_GENERATE_STATIC(server_acl_entries, server_acl_user, entry, server_acl_cmp);
+
+/* Initialize server_acl tree. */
+void
+server_acl_init(void)
+{
+	RB_INIT(&server_acl_entries);
+
+	if (getuid() != 0)
+		server_acl_user_allow(0);
+	server_acl_user_allow(getuid());
+}
+
+/* Find user entry. */
+struct server_acl_user*
+server_acl_user_find(uid_t uid)
+{
+	struct server_acl_user	find = { .uid = uid };
+
+	return (RB_FIND(server_acl_entries, &server_acl_entries, &find));
+}
+
+/* Display the tree. */
+void
+server_acl_display(struct cmdq_item *item)
+{
+	struct server_acl_user	*loop;
+	struct passwd		*pw;
+	const char		*name;
+
+	RB_FOREACH(loop, server_acl_entries, &server_acl_entries) {
+		if (loop->uid == 0)
+			continue;
+		if ((pw = getpwuid(loop->uid)) != NULL)
+			name = pw->pw_name;
+		else
+			name = "unknown";
+		if (loop->flags == SERVER_ACL_READONLY)
+			cmdq_print(item, "%s (R)", name);
+		else
+			cmdq_print(item, "%s (W)", name);
+	}
+}
+
+/* Allow a user. */
+void
+server_acl_user_allow(uid_t uid)
+{
+	struct server_acl_user	*user;
+
+	user = server_acl_user_find(uid);
+	if (user == NULL) {
+		user = xcalloc(1, sizeof *user);
+		user->uid = uid;
+		RB_INSERT(server_acl_entries, &server_acl_entries, user);
+	}
+}
+
+/* Deny a user (remove from the tree). */
+void
+server_acl_user_deny(uid_t uid)
+{
+	struct server_acl_user	*user;
+
+	user = server_acl_user_find(uid);
+	if (user != NULL) {
+		RB_REMOVE(server_acl_entries, &server_acl_entries, user);
+		free(user);
+	}
+}
+
+/* Allow this user write access. */
+void
+server_acl_user_allow_write(uid_t uid)
+{
+	struct server_acl_user	*user;
+	struct client		*c;
+
+	user = server_acl_user_find(uid);
+	if (user == NULL)
+		return;
+	user->flags &= ~SERVER_ACL_READONLY;
+
+	TAILQ_FOREACH(c, &clients, entry) {
+		uid = proc_get_peer_uid(c->peer);
+		if (uid != (uid_t)-1 && uid == user->uid)
+			c->flags &= ~CLIENT_READONLY;
+	}
+}
+
+/* Deny this user write access. */
+void
+server_acl_user_deny_write(uid_t uid)
+{
+	struct server_acl_user	*user;
+	struct client		*c;
+
+	user = server_acl_user_find(uid);
+	if (user == NULL)
+		return;
+	user->flags |= SERVER_ACL_READONLY;
+
+	TAILQ_FOREACH(c, &clients, entry) {
+		uid = proc_get_peer_uid(c->peer);
+		if (uid != (uid_t)-1 && uid == user->uid)
+			c->flags |= CLIENT_READONLY;
+	}
+}
+
+/*
+ * Check if the client's UID exists in the ACL list and if so, set as read only
+ * if needed. Return false if the user does not exist.
+ */
+int
+server_acl_join(struct client *c)
+{
+	struct server_acl_user	*user;
+	uid_t			 uid;
+
+	uid = proc_get_peer_uid(c->peer);
+	if (uid == (uid_t)-1)
+		return (0);
+
+	user = server_acl_user_find(uid);
+	if (user == NULL)
+		return (0);
+	if (user->flags & SERVER_ACL_READONLY)
+		c->flags |= CLIENT_READONLY;
+	return (1);
+}
+
+/* Get UID for user entry. */
+uid_t
+server_acl_get_uid(struct server_acl_user *user)
+{
+	return (user->uid);
+}

server-client.c

@@ -2775,6 +2775,14 @@ server_client_dispatch(struct imsg *imsg, void *arg)
 	}
 }
 
+/* Callback when command is not allowed. */
+static enum cmd_retval
+server_client_read_only(struct cmdq_item *item, __unused void *data)
+{
+	cmdq_error(item, "client is read-only");
+	return (CMD_RETURN_ERROR);
+}
+
 /* Callback when command is done. */
 static enum cmd_retval
 server_client_command_done(struct cmdq_item *item, __unused void *data)
@@ -2799,6 +2807,7 @@ server_client_dispatch_command(struct client *c, struct imsg *imsg)
 	char			**argv, *cause;
 	struct cmd_parse_result	 *pr;
 	struct args_value	 *values;
+	struct cmdq_item	 *new_item;
 
 	if (c->flags & CLIENT_EXIT)
 		return;
@@ -2837,7 +2846,12 @@ server_client_dispatch_command(struct client *c, struct imsg *imsg)
 	free(values);
 	cmd_free_argv(argc, argv);
 
-	cmdq_append(c, cmdq_get_command(pr->cmdlist, NULL));
+	if ((c->flags & CLIENT_READONLY) &&
+	    !cmd_list_all_have(pr->cmdlist, CMD_READONLY))
+		new_item = cmdq_get_callback(server_client_read_only, NULL);
+	else
+		new_item = cmdq_get_command(pr->cmdlist, NULL);
+	cmdq_append(c, new_item);
 	cmdq_append(c, cmdq_get_callback(server_client_command_done, NULL));
 
 	cmd_list_free(pr->cmdlist);
@@ -3071,9 +3085,11 @@ server_client_set_flags(struct client *c, const char *flags)
 			continue;
 
 		log_debug("client %s set flag %s", c->name, next);
-		if (not)
+		if (not) {
+			if (c->flags & CLIENT_READONLY)
+				flag &= ~CLIENT_READONLY;
 			c->flags &= ~flag;
-		else
+		} else
 			c->flags |= flag;
 		if (flag == CLIENT_CONTROL_NOOUTPUT)
 			control_reset_offsets(c);
@@ -3141,7 +3157,7 @@ server_client_add_client_window(struct client *c, u_int id)
 		cw->window = id;
 		RB_INSERT(client_windows, &c->windows, cw);
 	}
-	return cw;
+	return (cw);
 }
 
 /* Get client active pane. */

window-buffer.c

@@ -308,7 +308,7 @@ window_buffer_get_key(void *modedata, void *itemdata, u_int line)
 	}
 	pb = paste_get_name(item->name);
 	if (pb == NULL)
-		return KEYC_NONE;
+		return (KEYC_NONE);
 
 	ft = format_create(NULL, NULL, FORMAT_NONE, 0);
 	format_defaults(ft, NULL, NULL, 0, NULL);
@@ -320,7 +320,7 @@ window_buffer_get_key(void *modedata, void *itemdata, u_int line)
 	key = key_string_lookup_string(expanded);
 	free(expanded);
 	format_free(ft);
-	return key;
+	return (key);
 }
 
 static struct screen *

window-client.c

@@ -281,7 +281,7 @@ window_client_get_key(void *modedata, void *itemdata, u_int line)
 	key = key_string_lookup_string(expanded);
 	free(expanded);
 	format_free(ft);
-	return key;
+	return (key);
 }
 
 static struct screen *

window-tree.c

@@ -895,7 +895,7 @@ window_tree_get_key(void *modedata, void *itemdata, u_int line)
 	key = key_string_lookup_string(expanded);
 	free(expanded);
 	format_free(ft);
-	return key;
+	return (key);
 }
 
 static struct screen *