[PATCH] rtc: class: Make the time32 hctosys limit configurable

[Karl Mehltretter <kmehltretter@gmail.com>]

On 32-bit systems with RTC_HCTOSYS enabled, rtc_hctosys() rejects
RTC dates whose seconds value exceeds INT_MAX, even when userspace
uses a 64-bit time_t. This leaves system time uninitialized by the RTC.

Commit b3a5ac42ab18 ("rtc: hctosys: Ensure system time doesn't overflow
time_t") introduced this limit to protect time32 userspace from future
RTC dates that can break boot. The same limit prevents systems with
time64 userspace from initializing the clock from valid post-2038 dates.

Add RTC_HCTOSYS_TIME32_LIMIT for 32-bit kernels, defaulting to y to
preserve the existing safeguard. Allow integrators to disable it once
their entire userspace, including init, supports post-2038 dates.
Warn on rejection with the RTC date and option name so users can
identify the cause and find the setting.

Keep the option independent of COMPAT_32BIT_TIME. Userspace with a
64-bit time_t may still need legacy syscalls for operations that do
not represent post-2038 dates.

Fixes: b3a5ac42ab18 ("rtc: hctosys: Ensure system time doesn't overflow time_t")
Link: https://lore.kernel.org/all/20220908115337.1604277-1-arnd@kernel.org/
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
Tested this exact patch on SAM9X75 Curiosity (32-bit ARM, rtc0),
using three full kernel builds and six powered warm boots:

  TIME32_LIMIT  COMPAT_32BIT_TIME  RTC in 2026    RTC in 2040
  y             y                 hctosys=1      hctosys=0, epoch + 2 s
  n             y                 hctosys=1      hctosys=1, correct time
  n             n                 hctosys=1      hctosys=1, correct time

The default-limit 2040 boot logged the rejected date and
CONFIG_RTC_HCTOSYS_TIME32_LIMIT=y. The other boots did not warn.
Legacy clock_gettime and futex returned -ENOSYS with COMPAT_32BIT_TIME=n.
Tests used a minimal time64 initramfs without NTP. Current clocks and
all original boot files were restored after testing.

Host tests checked Kconfig defaults and dependencies, the INT_MAX
boundary and error paths.

 drivers/rtc/Kconfig | 23 +++++++++++++++++++++++
 drivers/rtc/class.c |  8 +++++---
 2 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/drivers/rtc/Kconfig b/drivers/rtc/Kconfig
index 05b9233b9418..a594c041bc35 100644
--- a/drivers/rtc/Kconfig
+++ b/drivers/rtc/Kconfig
@@ -30,6 +30,29 @@ config RTC_HCTOSYS
 	  the value read from a specified RTC device. This is useful to avoid
 	  unnecessary fsck runs at boot time, and to network better.
 
+config RTC_HCTOSYS_TIME32_LIMIT
+	bool "Reject RTC dates after the 2038 cutoff"
+	depends on RTC_HCTOSYS && !64BIT
+	default y
+	help
+	  Reject RTC dates after 03:14:07 UTC on 19 January 2038 when
+	  initializing the system clock. This preserves the existing
+	  safeguard for userspace using a signed 32-bit time_t.
+
+	  A rejected date leaves system time unchanged, normally near the
+	  Unix epoch, and the RTC's hctosys sysfs attribute reads 0. Userspace
+	  must supply the time if no other source has initialized it.
+
+	  Disable this only if the entire userspace, including the init
+	  system, supports dates beyond 2038. Otherwise, an invalid future
+	  date in the RTC may prevent booting.
+
+	  Disabling this check does not require disabling COMPAT_32BIT_TIME:
+	  userspace with a 64-bit time_t may still use legacy system calls
+	  for operations that do not represent dates beyond 2038.
+
+	  If unsure, say Y.
+
 config RTC_HCTOSYS_DEVICE
 	string "RTC used to set the system time"
 	depends on RTC_HCTOSYS
diff --git a/drivers/rtc/class.c b/drivers/rtc/class.c
index 01ba04028f1f..18ea141ab8c2 100644
--- a/drivers/rtc/class.c
+++ b/drivers/rtc/class.c
@@ -72,12 +72,14 @@ static void rtc_hctosys(struct rtc_device *rtc)
 
 	tv64.tv_sec = rtc_tm_to_time64(&tm);
 
-#if BITS_PER_LONG == 32
-	if (tv64.tv_sec > INT_MAX) {
+	if (IS_ENABLED(CONFIG_RTC_HCTOSYS_TIME32_LIMIT) &&
+	    tv64.tv_sec > INT_MAX) {
+		dev_warn(rtc->dev.parent,
+			 "hctosys: rejecting %ptR UTC due to CONFIG_RTC_HCTOSYS_TIME32_LIMIT=y\n",
+			 &tm);
 		err = -ERANGE;
 		goto err_read;
 	}
-#endif
 
 	err = do_settimeofday64(&tv64);
 

base-commit: 587858367581b9c55c3690f4e63382ad622719d4
-- 
2.39.5 (Apple Git-154)