Re: [PATCH v3 3/5] rtc: Fix the RTC time comparison issues adding cast

[Alexandre Belloni <alexandre.belloni@bootlin.com>]

On 11/04/2025 14:35:56+0200, Alexandre Mergnat wrote:
> The RTC subsystem was experiencing comparison issues between signed and
> unsigned time values. When comparing time64_t variables (signed) with
> potentially unsigned range values, incorrect results could occur leading
> to runtime errors.
> 
> Adds explicit type casts to time64_t for critical RTC time comparisons
> in both class.c and interface.c files. The changes ensure proper
> handling of negative time values during range validation and offset
> calculations, particularly when dealing with timestamps before 1970.
> 
> The previous implementation might incorrectly interpret negative values
> as extremely large positive values, causing unexpected behavior in the
> RTC hardware abstraction logic.
> 

range_max is explicitly unsigned, casting it to a signed value will
break drivers.

> Signed-off-by: Alexandre Mergnat <amergnat@baylibre.com>
> ---
>  drivers/rtc/class.c     | 6 +++---
>  drivers/rtc/interface.c | 8 ++++----
>  2 files changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/rtc/class.c b/drivers/rtc/class.c
> index e31fa0ad127e9..1ee3f609f92ea 100644
> --- a/drivers/rtc/class.c
> +++ b/drivers/rtc/class.c
> @@ -282,7 +282,7 @@ static void rtc_device_get_offset(struct rtc_device *rtc)
>  	 * then we can not expand the RTC range by adding or subtracting one
>  	 * offset.
>  	 */
> -	if (rtc->range_min == rtc->range_max)
> +	if (rtc->range_min == (time64_t)rtc->range_max)
>  		return;
>  
>  	ret = device_property_read_u32(rtc->dev.parent, "start-year",
> @@ -299,7 +299,7 @@ static void rtc_device_get_offset(struct rtc_device *rtc)
>  	if (!rtc->set_start_time)
>  		return;
>  
> -	range_secs = rtc->range_max - rtc->range_min + 1;
> +	range_secs = (time64_t)rtc->range_max - rtc->range_min + 1;
>  
>  	/*
>  	 * If the start_secs is larger than the maximum seconds (rtc->range_max)
> @@ -327,7 +327,7 @@ static void rtc_device_get_offset(struct rtc_device *rtc)
>  	 *
>  	 * Otherwise the offset seconds should be 0.
>  	 */
> -	if (rtc->start_secs > rtc->range_max ||
> +	if (rtc->start_secs > (time64_t)rtc->range_max ||
>  	    rtc->start_secs + range_secs - 1 < rtc->range_min)
>  		rtc->offset_secs = rtc->start_secs - rtc->range_min;
>  	else if (rtc->start_secs > rtc->range_min)
> diff --git a/drivers/rtc/interface.c b/drivers/rtc/interface.c
> index aaf76406cd7d7..93bdf06807f23 100644
> --- a/drivers/rtc/interface.c
> +++ b/drivers/rtc/interface.c
> @@ -37,7 +37,7 @@ static void rtc_add_offset(struct rtc_device *rtc, struct rtc_time *tm)
>  	 */
>  	if ((rtc->start_secs > rtc->range_min && secs >= rtc->start_secs) ||
>  	    (rtc->start_secs < rtc->range_min &&
> -	     secs <= (rtc->start_secs + rtc->range_max - rtc->range_min)))
> +	     secs <= (time64_t)(rtc->start_secs + rtc->range_max - rtc->range_min)))
>  		return;
>  
>  	rtc_time64_to_tm(secs + rtc->offset_secs, tm);
> @@ -58,7 +58,7 @@ static void rtc_subtract_offset(struct rtc_device *rtc, struct rtc_time *tm)
>  	 * device. Otherwise we need to subtract the offset to make the time
>  	 * values are valid for RTC hardware device.
>  	 */
> -	if (secs >= rtc->range_min && secs <= rtc->range_max)
> +	if (secs >= rtc->range_min && secs <= (time64_t)rtc->range_max)
>  		return;
>  
>  	rtc_time64_to_tm(secs - rtc->offset_secs, tm);
> @@ -66,7 +66,7 @@ static void rtc_subtract_offset(struct rtc_device *rtc, struct rtc_time *tm)
>  
>  static int rtc_valid_range(struct rtc_device *rtc, struct rtc_time *tm)
>  {
> -	if (rtc->range_min != rtc->range_max) {
> +	if (rtc->range_min != (time64_t)rtc->range_max) {
>  		time64_t time = rtc_tm_to_time64(tm);
>  		time64_t range_min = rtc->set_start_time ? rtc->start_secs :
>  			rtc->range_min;
> @@ -74,7 +74,7 @@ static int rtc_valid_range(struct rtc_device *rtc, struct rtc_time *tm)
>  			(rtc->start_secs + rtc->range_max - rtc->range_min) :
>  			rtc->range_max;
>  
> -		if (time < range_min || time > range_max)
> +		if (time < range_min || time > (time64_t)range_max)
>  			return -ERANGE;
>  	}
>  
> 
> -- 
> 2.25.1
> 

-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com