[PATCH v2 3/3] rtc: ftrtc010: fix integer overflow in time calculation

[Dan Carpenter]

Commit 8ef8e9839a23 ("rtc: ftrtc010: fix integer overflow in time
calculation") from Aug 27, 2026 (linux-next), leads to the following
Smatch static checker warning:

	drivers/rtc/rtc-ftrtc010.c:75 ftrtc010_rtc_read_time()
	warn: cast after binop

On Thu, 27 Aug 2026 16:03:20 +0800 Liu Dalin <liudalin@xxxxxxxxxxxxxxx> wrote:
> In ftrtc010_rtc_read_time() and ftrtc010_rtc_set_time(), all
> variables (offset, days, hour, min, sec) are u32. The expression
> "days * 86400 + hour * 3600 + min * 60 + sec" is evaluated in
> 32-bit arithmetic, which silently wraps around for dates beyond
> approximately year 2106 (U32_MAX / 86400 ≈ 49710 days).
> 
> In ftrtc010_rtc_read_time(), perform the addition in 32-bit
> arithmetic first to preserve two's complement wrapping for negative
> offsets, then cast the result to timeu64_t. In
> ftrtc010_rtc_set_time(), cast individual u32 operands to timeu64_t
> before multiplication to prevent overflow in the right-hand side
> of the offset calculation.
> 
> Fixes: 1d61d2592c1f ("rtc: ftrtc010: Rename to Faraday FTRTC010")
> Assisted-by: Sashiko AI [static analysis]
> Signed-off-by: Liu Dalin <liudalin@xxxxxxxxxxxxxxx>
> ---
>  drivers/rtc/rtc-ftrtc010.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/rtc/rtc-ftrtc010.c b/drivers/rtc/rtc-ftrtc010.c
> index 19e6333bfbab..b29c96be40f4 100644
> --- a/drivers/rtc/rtc-ftrtc010.c
> +++ b/drivers/rtc/rtc-ftrtc010.c
> @@ -72,7 +72,7 @@ static int ftrtc010_rtc_read_time(struct device *dev, struct rtc_time *tm)
>  	days = readl(rtc->rtc_base + FTRTC010_RTC_DAYS);
>  	offset = readl(rtc->rtc_base + FTRTC010_RTC_RECORD);
>  
> -	time = offset + days * 86400 + hour * 3600 + min * 60 + sec;
> +	time = (timeu64_t)(offset + days * 86400 + hour * 3600 + min * 60 + sec);

There isn't any point in doing the cast outside the parentheses.  The
other chunk probably does it correctly.

regards,
dan carpenter

>  
>  	rtc_time64_to_tm(time, tm);
>  
> @@ -92,7 +92,8 @@ static int ftrtc010_rtc_set_time(struct device *dev, struct rtc_time *tm)
>  	hour = readl(rtc->rtc_base + FTRTC010_RTC_HOUR);
>  	day = readl(rtc->rtc_base + FTRTC010_RTC_DAYS);
>  
> -	offset = time - (day * 86400 + hour * 3600 + min * 60 + sec);
> +	offset = time - ((timeu64_t)day * 86400 + (timeu64_t)hour * 3600 +
> +			 (timeu64_t)min * 60 + (timeu64_t)sec);
>  
>  	writel(offset, rtc->rtc_base + FTRTC010_RTC_RECORD);
>  	writel(0x01, rtc->rtc_base + FTRTC010_RTC_CR);
> -- 
> 2.43.0
> 
>