core: add KExecWatchdogSec option

Rather than always enabling the shutdown WD on kexec, which might be
dangerous in case the kernel driver and/or the hardware implementation
does not reset the wd on kexec, add a new timer, disabled by default,
to let users optionally enable the shutdown WD on kexec separately
from the runtime and reboot ones. Advise in the documentation to
also use the runtime WD in conjunction with it.

Fixes: a637d0f9ec ("core: set shutdown watchdog on kexec too")

man/systemd-system.conf.xml

@@ -128,6 +128,7 @@
       <varlistentry>
         <term><varname>RuntimeWatchdogSec=</varname></term>
         <term><varname>ShutdownWatchdogSec=</varname></term>
+        <term><varname>KExecWatchdogSec=</varname></term>
 
         <listitem><para>Configure the hardware watchdog at runtime and at reboot. Takes a timeout value in seconds (or
         in other time units if suffixed with <literal>ms</literal>, <literal>min</literal>, <literal>h</literal>,
@@ -149,7 +150,13 @@
         phase of system shutdown, configure <varname>JobTimeoutSec=</varname> and <varname>JobTimeoutAction=</varname>
         in the <literal>[Unit]</literal> section of the <filename>shutdown.target</filename> unit. By default
         <varname>RuntimeWatchdogSec=</varname> defaults to 0 (off), and <varname>ShutdownWatchdogSec=</varname> to
-        10min. These settings have no effect if a hardware watchdog is not available.</para></listitem>
+        10min. <varname>KExecWatchdogSec=</varname> may be used to additionally enable the watchdog when kexec
+        is being executed rather than when rebooting. Note that if the kernel does not reset the watchdog on kexec (depending
+        on the specific hardware and/or driver), in this case the watchdog might not get disabled after kexec succeeds
+        and thus the system might get rebooted, unless <varname>RuntimeWatchdogSec=</varname> is also enabled at the same time.
+        For this reason it is recommended to enable <varname>KExecWatchdogSec=</varname> only if
+        <varname>RuntimeWatchdogSec=</varname> is also enabled.
+        These settings have no effect if a hardware watchdog is not available.</para></listitem>
       </varlistentry>
 
       <varlistentry>

src/core/dbus-manager.c

@@ -2410,6 +2410,7 @@ const sd_bus_vtable bus_manager_vtable[] = {
         SD_BUS_PROPERTY("DefaultStandardError", "s", bus_property_get_exec_output, offsetof(Manager, default_std_output), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_WRITABLE_PROPERTY("RuntimeWatchdogUSec", "t", bus_property_get_usec, property_set_runtime_watchdog, offsetof(Manager, runtime_watchdog), 0),
         SD_BUS_WRITABLE_PROPERTY("ShutdownWatchdogUSec", "t", bus_property_get_usec, bus_property_set_usec, offsetof(Manager, shutdown_watchdog), 0),
+        SD_BUS_WRITABLE_PROPERTY("KExecWatchdogUSec", "t", bus_property_get_usec, bus_property_set_usec, offsetof(Manager, kexec_watchdog), 0),
         SD_BUS_WRITABLE_PROPERTY("ServiceWatchdogs", "b", bus_property_get_bool, bus_property_set_bool, offsetof(Manager, service_watchdogs), 0),
         SD_BUS_PROPERTY("ControlGroup", "s", NULL, offsetof(Manager, cgroup_root), 0),
         SD_BUS_PROPERTY("SystemState", "s", property_get_system_state, 0, 0),

src/core/main.c

@@ -122,6 +122,7 @@ static usec_t arg_default_start_limit_interval;
 static unsigned arg_default_start_limit_burst;
 static usec_t arg_runtime_watchdog;
 static usec_t arg_shutdown_watchdog;
+static usec_t arg_kexec_watchdog;
 static char *arg_early_core_pattern;
 static char *arg_watchdog_device;
 static char **arg_default_environment;
@@ -555,6 +556,7 @@ static int parse_config_file(void) {
                 { "Manager", "JoinControllers",              config_parse_warn_compat,        DISABLED_CONFIGURATION, NULL              },
                 { "Manager", "RuntimeWatchdogSec",           config_parse_sec,                0, &arg_runtime_watchdog                  },
                 { "Manager", "ShutdownWatchdogSec",          config_parse_sec,                0, &arg_shutdown_watchdog                 },
+                { "Manager", "KExecWatchdogSec",             config_parse_sec,                0, &arg_kexec_watchdog                    },
                 { "Manager", "WatchdogDevice",               config_parse_path,               0, &arg_watchdog_device                   },
                 { "Manager", "CapabilityBoundingSet",        config_parse_capability_set,     0, &arg_capability_bounding_set           },
                 { "Manager", "NoNewPrivileges",              config_parse_bool,               0, &arg_no_new_privs                      },
@@ -674,6 +676,7 @@ static void set_manager_settings(Manager *m) {
         m->service_watchdogs = arg_service_watchdogs;
         m->runtime_watchdog = arg_runtime_watchdog;
         m->shutdown_watchdog = arg_shutdown_watchdog;
+        m->kexec_watchdog = arg_kexec_watchdog;
         m->cad_burst_action = arg_cad_burst_action;
 
         manager_set_show_status(m, arg_show_status);
@@ -1356,6 +1359,7 @@ static int become_shutdown(
         _cleanup_strv_free_ char **env_block = NULL;
         size_t pos = 7;
         int r;
+        usec_t watchdog_timer = 0;
 
         assert(shutdown_verb);
         assert(!command_line[pos]);
@@ -1396,20 +1400,23 @@ static int become_shutdown(
 
         assert(pos < ELEMENTSOF(command_line));
 
-        if (STR_IN_SET(shutdown_verb, "reboot", "kexec") &&
-            arg_shutdown_watchdog > 0 &&
-            arg_shutdown_watchdog != USEC_INFINITY) {
+        if (streq(shutdown_verb, "reboot"))
+                watchdog_timer = arg_shutdown_watchdog;
+        else if (streq(shutdown_verb, "kexec"))
+                watchdog_timer = arg_kexec_watchdog;
+
+        if (watchdog_timer > 0 && watchdog_timer != USEC_INFINITY) {
 
                 char *e;
 
-                /* If we reboot let's set the shutdown
+                /* If we reboot or kexec let's set the shutdown
                  * watchdog and tell the shutdown binary to
                  * repeatedly ping it */
-                r = watchdog_set_timeout(&arg_shutdown_watchdog);
+                r = watchdog_set_timeout(&watchdog_timer);
                 watchdog_close(r < 0);
 
                 /* Tell the binary how often to ping, ignore failure */
-                if (asprintf(&e, "WATCHDOG_USEC="USEC_FMT, arg_shutdown_watchdog) > 0)
+                if (asprintf(&e, "WATCHDOG_USEC="USEC_FMT, watchdog_timer) > 0)
                         (void) strv_consume(&env_block, e);
 
                 if (arg_watchdog_device &&
@@ -2100,6 +2107,7 @@ static void reset_arguments(void) {
         arg_default_start_limit_burst = DEFAULT_START_LIMIT_BURST;
         arg_runtime_watchdog = 0;
         arg_shutdown_watchdog = 10 * USEC_PER_MINUTE;
+        arg_kexec_watchdog = 0;
         arg_early_core_pattern = NULL;
         arg_watchdog_device = NULL;
 
@@ -2639,6 +2647,7 @@ finish:
 
         if (m) {
                 arg_shutdown_watchdog = m->shutdown_watchdog;
+                arg_kexec_watchdog = m->kexec_watchdog;
                 m = manager_free(m);
         }
 

src/core/manager.h

@@ -229,6 +229,7 @@ struct Manager {
 
         usec_t runtime_watchdog;
         usec_t shutdown_watchdog;
+        usec_t kexec_watchdog;
 
         dual_timestamp timestamps[_MANAGER_TIMESTAMP_MAX];
 

src/core/system.conf.in

@@ -27,6 +27,7 @@
 #NUMAMask=
 #RuntimeWatchdogSec=0
 #ShutdownWatchdogSec=10min
+#KExecWatchdogSec=0
 #WatchdogDevice=
 #CapabilityBoundingSet=
 #NoNewPrivileges=no

test/fuzz/fuzz-unit-file/directives.service

@@ -703,6 +703,7 @@ LogTarget=
 RuntimeWatchdogSec=
 ShowStatus=
 ShutdownWatchdogSec=
+KExecWatchdogSec=
 SuspendMode=
 SuspendState=
 SystemCallArchitectures=