core/execute: always set $USER and introduce SetLoginEnvironment=

Before this commit, $USER, $HOME, $LOGNAME and $SHELL are only
set when User= is set for the unit. For system service, this
results in different behaviors depending on whether User=root is set.

$USER always makes sense on its own, so let's set it unconditionally.
Ideally $HOME should be set too, but it causes trouble when e.g. getty
passes '-p' to login(1), which then doesn't override $HOME. $LOGNAME and
$SHELL are more like "login environments", and are generally not
suitable for system services. Therefore, a new option SetLoginEnvironment=
is also added to control the latter three variables.

Fixes #23438

Replaces #8227

src/core/dbus-execute.c

@@ -1273,6 +1273,7 @@ const sd_bus_vtable bus_exec_vtable[] = {
         SD_BUS_PROPERTY("User", "s", NULL, offsetof(ExecContext, user), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("Group", "s", NULL, offsetof(ExecContext, group), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("DynamicUser", "b", bus_property_get_bool, offsetof(ExecContext, dynamic_user), SD_BUS_VTABLE_PROPERTY_CONST),
+        SD_BUS_PROPERTY("SetLoginEnvironment", "b", bus_property_get_tristate, offsetof(ExecContext, set_login_environment), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("RemoveIPC", "b", bus_property_get_bool, offsetof(ExecContext, remove_ipc), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("SetCredential", "a(say)", property_get_set_credential, 0, SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("SetCredentialEncrypted", "a(say)", property_get_set_credential, 0, SD_BUS_VTABLE_PROPERTY_CONST),
@@ -1730,6 +1731,9 @@ int bus_exec_context_set_transient_property(
         if (streq(name, "Group"))
                 return bus_set_transient_user_relaxed(u, name, &c->group, message, flags, error);
 
+        if (streq(name, "SetLoginEnvironment"))
+                return bus_set_transient_tristate(u, name, &c->set_login_environment, message, flags, error);
+
         if (streq(name, "TTYPath"))
                 return bus_set_transient_path(u, name, &c->tty_path, message, flags, error);
 

src/core/execute.c

@@ -1923,7 +1923,33 @@ static int build_environment(
                 our_env[n_env++] = x;
         }
 
-        if (home) {
+        /* We query "root" if this is a system unit and User= is not specified. $USER is always set. $HOME
+         * could cause problem for e.g. getty, since login doesn't override $HOME, and $LOGNAME and $SHELL don't
+         * really make much sense since we're not logged in. Hence we conditionalize the three based on
+         * SetLoginEnvironment= switch. */
+        if (!c->user && !c->dynamic_user && p->runtime_scope == RUNTIME_SCOPE_SYSTEM) {
+                r = get_fixed_user("root", &username, NULL, NULL, &home, &shell);
+                if (r < 0)
+                        return log_unit_error_errno(u, r, "Failed to determine user credentials for root: %m");
+        }
+
+        bool set_user_login_env = c->set_login_environment >= 0 ? c->set_login_environment : (c->user || c->dynamic_user);
+
+        if (username) {
+                x = strjoin("USER=", username);
+                if (!x)
+                        return -ENOMEM;
+                our_env[n_env++] = x;
+
+                if (set_user_login_env) {
+                        x = strjoin("LOGNAME=", username);
+                        if (!x)
+                                return -ENOMEM;
+                        our_env[n_env++] = x;
+                }
+        }
+
+        if (home && set_user_login_env) {
                 x = strjoin("HOME=", home);
                 if (!x)
                         return -ENOMEM;
@@ -1932,19 +1958,7 @@ static int build_environment(
                 our_env[n_env++] = x;
         }
 
-        if (username) {
-                x = strjoin("LOGNAME=", username);
-                if (!x)
-                        return -ENOMEM;
-                our_env[n_env++] = x;
-
-                x = strjoin("USER=", username);
-                if (!x)
-                        return -ENOMEM;
-                our_env[n_env++] = x;
-        }
-
-        if (shell) {
+        if (shell && set_user_login_env) {
                 x = strjoin("SHELL=", shell);
                 if (!x)
                         return -ENOMEM;
@@ -5262,6 +5276,7 @@ void exec_context_init(ExecContext *c) {
                 .tty_cols = UINT_MAX,
                 .private_mounts = -1,
                 .memory_ksm = -1,
+                .set_login_environment = -1,
         };
 
         FOREACH_ARRAY(d, c->directories, _EXEC_DIRECTORY_TYPE_MAX)

src/core/execute.h

@@ -254,6 +254,8 @@ struct ExecContext {
         char *group;
         char **supplementary_groups;
 
+        int set_login_environment;
+
         char *pam_name;
 
         char *utmp_id;

src/core/load-fragment-gperf.gperf.in

@@ -19,6 +19,7 @@
 {{type}}.User,                             config_parse_user_group_compat,              0,                                  offsetof({{type}}, exec_context.user)
 {{type}}.Group,                            config_parse_user_group_compat,              0,                                  offsetof({{type}}, exec_context.group)
 {{type}}.SupplementaryGroups,              config_parse_user_group_strv_compat,         0,                                  offsetof({{type}}, exec_context.supplementary_groups)
+{{type}}.SetLoginEnvironment,              config_parse_tristate,                       0,                                  offsetof({{type}}, exec_context.set_login_environment)
 {{type}}.Nice,                             config_parse_exec_nice,                      0,                                  offsetof({{type}}, exec_context)
 {{type}}.OOMScoreAdjust,                   config_parse_exec_oom_score_adjust,          0,                                  offsetof({{type}}, exec_context)
 {{type}}.CoredumpFilter,                   config_parse_exec_coredump_filter,           0,                                  offsetof({{type}}, exec_context)

src/shared/bus-unit-util.c

@@ -1078,7 +1078,8 @@ static int bus_append_execute_property(sd_bus_message *m, const char *field, con
                               "ProtectHostname",
                               "MemoryKSM",
                               "RestrictSUIDSGID",
-                              "RootEphemeral"))
+                              "RootEphemeral",
+                              "SetLoginEnvironment"))
                 return bus_append_parse_boolean(m, field, eq);
 
         if (STR_IN_SET(field, "ReadWriteDirectories",