[PATCH] watchdog: core: add option to make timeouts read-only

[Marcus Folkesson <marcus.folkesson@gmail.com>]

Some systems may not allow applications to configure the watchdog
timer at all. This restriction is not limited to stop the watchdog,
but also change timeouts as well.

This adds a kernel parameter to disable the ability to change the
watchdog timouts from userspace.

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
---
 drivers/watchdog/Kconfig        |  9 +++++++++
 drivers/watchdog/watchdog_dev.c | 12 ++++++++++--
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/drivers/watchdog/Kconfig b/drivers/watchdog/Kconfig
index aff773bcebdb..bcba48b5c88b 100644
--- a/drivers/watchdog/Kconfig
+++ b/drivers/watchdog/Kconfig
@@ -46,6 +46,15 @@ config WATCHDOG_NOWAYOUT
 	  get killed. If you say Y here, the watchdog cannot be stopped once
 	  it has been started.
 
+config WATCHDOG_TIMEOUT_READONLY
+	bool "Make timeouts read-only from userspace"
+	help
+	  Say Y here if you want the watchdog timeout/pretimeout to be read-only
+	  from userspace. This requires that the timeout is configured before
+	  userspace takes over.
+
+	  Say N if you are unsure.
+
 config WATCHDOG_HANDLE_BOOT_ENABLED
 	bool "Update boot-enabled watchdog until userspace takes over"
 	default y
diff --git a/drivers/watchdog/watchdog_dev.c b/drivers/watchdog/watchdog_dev.c
index ffbdc4642ea5..6064806a2a8d 100644
--- a/drivers/watchdog/watchdog_dev.c
+++ b/drivers/watchdog/watchdog_dev.c
@@ -87,6 +87,9 @@ static struct kthread_worker *watchdog_kworker;
 static bool handle_boot_enabled =
 	IS_ENABLED(CONFIG_WATCHDOG_HANDLE_BOOT_ENABLED);
 
+static bool timeout_is_readonly =
+	IS_ENABLED(CONFIG_WATCHDOG_TIMEOUT_READONLY);
+
 static inline bool watchdog_need_worker(struct watchdog_device *wdd)
 {
 	/* All variables in milli-seconds */
@@ -359,7 +362,7 @@ static int watchdog_set_timeout(struct watchdog_device *wdd,
 {
 	int err = 0;
 
-	if (!(wdd->info->options & WDIOF_SETTIMEOUT))
+	if (!(wdd->info->options & WDIOF_SETTIMEOUT) || timeout_is_readonly)
 		return -EOPNOTSUPP;
 
 	if (watchdog_timeout_invalid(wdd, timeout))
@@ -390,7 +393,7 @@ static int watchdog_set_pretimeout(struct watchdog_device *wdd,
 {
 	int err = 0;
 
-	if (!(wdd->info->options & WDIOF_PRETIMEOUT))
+	if (!(wdd->info->options & WDIOF_PRETIMEOUT) || timeout_is_readonly)
 		return -EOPNOTSUPP;
 
 	if (watchdog_pretimeout_invalid(wdd, timeout))
@@ -1181,3 +1184,8 @@ module_param(handle_boot_enabled, bool, 0444);
 MODULE_PARM_DESC(handle_boot_enabled,
 	"Watchdog core auto-updates boot enabled watchdogs before userspace takes over (default="
 	__MODULE_STRING(IS_ENABLED(CONFIG_WATCHDOG_HANDLE_BOOT_ENABLED)) ")");
+
+module_param(timeout_is_readonly, bool, 0444);
+MODULE_PARM_DESC(timeout_is_readonly,
+	"Watchdog timeouts is readonly from userspace (default="
+	__MODULE_STRING(IS_ENABLED(CONFIG_WATCHDOG_TIMEOUT_READONLY)) ")");
-- 
2.16.2