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

Alexandre Belloni
2025-04-11T13:38:42Z

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