Re: [PATCH] watchdog: Add interface to config timeout and pretimeout in sysfs

[Guenter Roeck <linux@roeck-us.net>]

On Thu, Aug 20, 2020 at 02:38:58AM +0000, Wang Wensheng wrote:
> Those interfaces exist already in sysfs of watchdog driver core, but
> they are readonly. This patch add write hook so we can config timeout
> and pretimeout by writing those files.
> 
> Signed-off-by: Wang Wensheng <wangwensheng4@huawei.com>

This is problematic. For example, if a user changes the timeout on
a running watchdog, the application pinging the watchdog would not
know about it. Set the timeout to 1 second using the sysfs attribute,
and the system will trigger the watchdog almost immediately.

The only somewhat "safe" means to use this attribute would be to set
the timeout before a userspace application opens the watchdog device.
But then this application could just as well update the timeout
after opening the device. What is the use case ?

Thanks,
Guenter

> ---
>  drivers/watchdog/watchdog_dev.c | 48 +++++++++++++++++++++++++++++++--
>  1 file changed, 46 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/watchdog/watchdog_dev.c b/drivers/watchdog/watchdog_dev.c
> index 10b2090f3e5e..bb8ddc71d4ea 100644
> --- a/drivers/watchdog/watchdog_dev.c
> +++ b/drivers/watchdog/watchdog_dev.c
> @@ -485,7 +485,29 @@ static ssize_t timeout_show(struct device *dev, struct device_attribute *attr,
>  
>  	return sprintf(buf, "%u\n", wdd->timeout);
>  }
> -static DEVICE_ATTR_RO(timeout);
> +
> +static ssize_t timeout_store(struct device *dev, struct device_attribute *attr,
> +				const char *buf, size_t count)
> +{
> +	int ret;
> +	unsigned int val;
> +	struct watchdog_device *wdd = dev_get_drvdata(dev);
> +	struct watchdog_core_data *wd_data = wdd->wd_data;
> +
> +	ret = kstrtouint(buf, 0, &val);
> +	if (ret)
> +		return ret;
> +
> +	mutex_lock(&wd_data->lock);
> +	ret = watchdog_set_timeout(wdd, val);
> +	mutex_unlock(&wd_data->lock);
> +
> +	if (!ret)
> +		ret = count;
> +
> +	return ret;
> +}
> +static DEVICE_ATTR_RW(timeout);
>  
>  static ssize_t pretimeout_show(struct device *dev,
>  			       struct device_attribute *attr, char *buf)
> @@ -494,7 +516,29 @@ static ssize_t pretimeout_show(struct device *dev,
>  
>  	return sprintf(buf, "%u\n", wdd->pretimeout);
>  }
> -static DEVICE_ATTR_RO(pretimeout);
> +
> +static ssize_t pretimeout_store(struct device *dev,
> +		struct device_attribute *attr, const char *buf, size_t count)
> +{
> +	int ret;
> +	unsigned int val;
> +	struct watchdog_device *wdd = dev_get_drvdata(dev);
> +	struct watchdog_core_data *wd_data = wdd->wd_data;
> +
> +	ret = kstrtouint(buf, 0, &val);
> +	if (ret)
> +		return ret;
> +
> +	mutex_lock(&wd_data->lock);
> +	ret = watchdog_set_pretimeout(wdd, val);
> +	mutex_unlock(&wd_data->lock);
> +
> +	if (!ret)
> +		ret = count;
> +
> +	return ret;
> +}
> +static DEVICE_ATTR_RW(pretimeout);
>  
>  static ssize_t identity_show(struct device *dev, struct device_attribute *attr,
>  				char *buf)
> -- 
> 2.25.0
>