linux-kernel - [PATCH] watchdog: make nowayout sysfs file writable

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Message-Id: <20191105123125.25985-1-linux@rasmusvillemoes.dk>
Date:   Tue,  5 Nov 2019 13:31:25 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Guenter Roeck <linux@...ck-us.net>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel@...r.kernel.org, linux-watchdog@...r.kernel.org
Subject: [PATCH] watchdog: make nowayout sysfs file writable

It can be useful to delay setting the nowayout feature for a watchdog
device. Moreover, not every driver (notably gpio_wdt) implements a
nowayout module parameter/otherwise respects CONFIG_WATCHDOG_NOWAYOUT,
and modifying those drivers carries a risk of causing a regression for
someone who has two watchdog devices, sets CONFIG_WATCHDOG_NOWAYOUT
and somehow relies on the gpio_wdt driver being ignorant of
that (i.e., allowing one to gracefully close a gpio_wdt but not the
other watchdog in the system).

So instead, simply make the nowayout sysfs file writable. Obviously,
setting nowayout is a one-way street.

Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
 .../ABI/testing/sysfs-class-watchdog          |  9 ++++++--
 drivers/watchdog/watchdog_dev.c               | 22 ++++++++++++++++++-
 2 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-class-watchdog b/Documentation/ABI/testing/sysfs-class-watchdog
index 675f9b537661..9860a8b2ba75 100644
--- a/Documentation/ABI/testing/sysfs-class-watchdog
+++ b/Documentation/ABI/testing/sysfs-class-watchdog
@@ -17,8 +17,13 @@ What:		/sys/class/watchdog/watchdogn/nowayout
 Date:		August 2015
 Contact:	Wim Van Sebroeck <wim@...ana.be>
 Description:
-		It is a read only file. While reading, it gives '1' if that
-		device supports nowayout feature else, it gives '0'.
+		It is a read/write file. While reading, it gives '1'
+		if the device has the nowayout feature set, otherwise
+		it gives '0'. Writing a '1' to the file enables the
+		nowayout feature. Once set, the nowayout feature
+		cannot be disabled, so writing a '0' either has no
+		effect (if the feature was already disabled) or
+		results in a permission error.
 
 What:		/sys/class/watchdog/watchdogn/state
 Date:		August 2015
diff --git a/drivers/watchdog/watchdog_dev.c b/drivers/watchdog/watchdog_dev.c
index dbd2ad4c9294..0c478b8f8d5a 100644
--- a/drivers/watchdog/watchdog_dev.c
+++ b/drivers/watchdog/watchdog_dev.c
@@ -452,7 +452,27 @@ static ssize_t nowayout_show(struct device *dev, struct device_attribute *attr,
 
 	return sprintf(buf, "%d\n", !!test_bit(WDOG_NO_WAY_OUT, &wdd->status));
 }
-static DEVICE_ATTR_RO(nowayout);
+
+static ssize_t nowayout_store(struct device *dev, struct device_attribute *attr,
+				const char *buf, size_t len)
+{
+	struct watchdog_device *wdd = dev_get_drvdata(dev);
+	unsigned int value, current;
+	int ret;
+
+	ret = kstrtouint(buf, 0, &value);
+	if (ret)
+		return ret;
+	if (value > 1)
+		return -EINVAL;
+	current = !!test_bit(WDOG_NO_WAY_OUT, &wdd->status);
+	/* nowayout cannot be disabled once set */
+	if (current && !value)
+		return -EPERM;
+	watchdog_set_nowayout(wdd, value);
+	return len;
+}
+static DEVICE_ATTR_RW(nowayout);
 
 static ssize_t status_show(struct device *dev, struct device_attribute *attr,
 				char *buf)
-- 
2.23.0