fix: Update the Y2K38 guide

Since the Y2K38 issue is resolved in Scarthgap, update the guide to
reflect the current state.

Also, add explanation of how Yocto/OE ensures Y2K38 compliance,
and expand guidance on ensuring third-party code is Y2K38-friendly.

Signed-off-by: Telukula Jeevan Kumar Sahu <j-sahu@ti.com>

Author: jeevantelukula

source/linux/How_to_Guides/Target/How_to_fix_y2k38.rst

@@ -1,51 +1,157 @@
-######################
-How to Fix Y2K38 Issue
-######################
+##########################
+How to prevent Y2K38 issue
+##########################
 
 ************************
-What is the Y2K38 Issue?
+What is the Y2K38 issue?
 ************************
 
-The Y2K38 issue is a bug which prevents a date from exceeding 03:14:07 UTC on January 19, 2038, on 32-bit systems. This
-is because the structures used to store the date experience an integer overflow when the date is set beyond the mentioned
-one. This causes the system to crash.
+The Y2K38 issue is a time computing problem that prevents 32-bit systems from representing
+dates beyond 03:14:07 UTC on January 19, 2038. On 32-bit platforms, the UNIX time is stored
+as a signed 32-bit integer (``time_t``) counting seconds since the UNIX epoch
+(00:00:00 UTC on January 1, 1970). This integer overflows at 2,147,483,647 (0x7FFFFFFF),
+wrapping to a large negative value and causing the system to interpret the time as a date in 1901.
 
 For more information, see: https://en.wikipedia.org/wiki/Year_2038_problem
 
-There are new data structures that are used on 32-bit systems to store date/time in longer fields, thereby avoiding the
-overflow and the crash. However, since setting and displaying date/time uses multiple software components (such as the
-kernel, glibc etc), **all** code on a filesystem must be Y2K38-compliant (i.e. all code must be using the new data
-structures and functions) for the filesystem to be Y2K38-friendly. If any piece of software uses the older structures
-and functions, it makes the whole filesystem vulnerable.
+The fix requires updates at every layer of the software stack: kernel, C library (glibc),
+and all userspace applications. If any component lacks Y2K38 support, the entire
+system is vulnerable.
 
-To understand how the problem was fixed in Scarthgap filesystem, refer:
-https://static.sched.com/hosted_files/osseu2024/8b/EOSS%20Vienna%202024%20-%20Surviving%20Y2038.pdf
+********************
+Y2K38 issue resolved
+********************
 
-*******************************
-Building a Y2K38-friendly Image
-*******************************
+**The current SDK release uses the Scarthgap filesystem, which has the Y2K38 issue resolved.**
 
-In SDK 9.3, TI's 32-bit platforms are still based on Kirkstone filesystem, which is NOT Y2K38-friendly. The Y2K38 issue
-is fixed in OE-core Scarthgap, on which the next release will be based.
+TI's 32-bit platforms (AM335x, AM437x) now use OE-core Scarthgap, which includes all
+necessary fixes applied at every software layer. The filesystem images provided in this SDK
+are Y2K38-compliant by default.
 
-Until then, if a Y2K38-friendly filesystem is required, obtain a :file:`tisdk-thinlinux-image` from the latest
-`cicd.scarthgap.x`:
+How Yocto/OpenEmbedded ensures Y2K38 compliance
+===============================================
 
-.. ifconfig:: CONFIG_part_variant in ('AM335X')
+Starting with Yocto Nanbield 4.3 (eventually in Scarthgap 5.0), OE-core builds all packages with
+the following compile-time defines enabled globally:
 
-   https://software-dl.ti.com/cicd-report/linux/index.html?section=platform&platform=am335x
+.. code-block:: c
 
-.. ifconfig:: CONFIG_part_variant in ('AM437X')
+   #define _TIME_BITS 64
+   #define _FILE_OFFSET_BITS 64
 
-   https://software-dl.ti.com/cicd-report/linux/index.html?section=platform&platform=am437x
+``_TIME_BITS=64`` instructs glibc to use a 64-bit ``time_t`` on 32-bit platforms, making
+all time-related types and functions (``struct timespec``, ``struct timeval``, ``time()``,
+``clock_gettime()``, etc.) Y2K38-safe. ``_FILE_OFFSET_BITS=64`` enables large file support
+(files >2 GB). Yocto enables both flags together to ensure a consistent 64-bit ABI across
+the system.
 
-The above images are based on the 6.6 Kernel and Scarthgap filesystem, and thus has the Y2K38 issue fixed.
+Since all packages in the distribution are rebuilt with these defines, the entire filesystem
+is consistently Y2K38-safe.
 
-.. note::
-   While the above images are Y2K38-friendly, it must be ensured that even the code introduced to it from other sources
-   is Y2K38-friendly. Any code that is not Y2K38-friendly will break the Y2K38-compatibility of the filesystem. Thus it
-   is important to be intentional about writing Y2K38-friendly code, and to test this code to ensure that the final
-   image remains Y2K38-friendly.
+This approach requires:
 
-To build a Y2K38-friendly image on the 6.6 Kernel and Scarthgap filesystem baseline, follow the "Build Instructions"
-section present in the above link.
+- **glibc 2.34 or later**: introduces support for ``_TIME_BITS=64`` on 32-bit architectures
+- **Linux kernel 5.6 or later**: completes full 64-bit time syscall support
+  (``clock_gettime64``, ``clock_settime64``, etc.) required by glibc on 32-bit systems
+
+The Scarthgap filesystem satisfies both requirements (includes glibc 2.39 and kernel 6.6).
+
+Ensuring your code is Y2K38-friendly
+====================================
+
+While the SDK images are Y2K38-friendly, any code introduced from external sources must also be
+Y2K38-compliant. Code that is not Y2K38-compliant will break the Y2K38-compatibility of the filesystem.
+
+When building applications or libraries to run on the SDK filesystem, ensure the following compile-time
+defines are set:
+
+.. code-block:: c
+
+   #define _TIME_BITS 64
+   #define _FILE_OFFSET_BITS 64
+
+Or equivalently, pass these flags to the compiler:
+
+.. code-block:: console
+
+   $ CFLAGS="-D_TIME_BITS=64 -D_FILE_OFFSET_BITS=64" make
+
+With ``_TIME_BITS=64`` set, standard time functions (``time()``, ``clock_gettime()``,
+``localtime()``, etc.) automatically use 64-bit time internally (no API changes are
+required). However, audit your code for:
+
+- ``time_t`` casts or assignments that assume a 32-bit (4-byte) size
+- Hard-coded UNIX timestamps or future dates as raw integer constants
+- Use of ``sizeof(time_t)`` in serialization/protocol code
+
+******************************
+How to verify Y2K38 compliance
+******************************
+
+.. warning::
+
+   Set the date back to the correct value after testing. Running with an incorrect system clock
+   can disrupt services such as NTP, certificate validation, and cron jobs.
+
+Check ``sizeof(time_t)``
+========================
+
+The quickest way to verify a binary is Y2K38-safe is to check the size of ``time_t`` at
+compile time or runtime:
+
+.. code-block:: c
+
+   #include <stdio.h>
+   #include <time.h>
+   int main(void) {
+       printf("sizeof(time_t): %zu\n", sizeof(time_t));
+       return 0;
+   }
+
+- Compiled **without** ``_TIME_BITS=64``: ``sizeof(time_t)`` = 4 (32-bit, **not Y2K38-safe**)
+- Compiled **with** ``_TIME_BITS=64``: ``sizeof(time_t)`` = 8 (64-bit, **Y2K38-safe**)
+
+Test with a post-2038 date
+==========================
+
+You can also test whether your system handles dates beyond 2038 correctly by temporarily
+setting the system clock past January 19, 2038.
+
+**Test procedure:**
+
+#. Save the current date:
+
+   .. code-block:: console
+
+      $ date
+      Tue Jan 19 03:00:00 UTC 2038
+
+#. Set the clock past the Y2K38 boundary:
+
+   .. code-block:: console
+
+      $ date -s "2038-01-20 00:00:00"
+
+#. Verify the system handles the date correctly:
+
+   .. code-block:: console
+
+      $ date
+      Wed Jan 20 00:00:00 UTC 2038
+      $ date +%s
+      2147558400
+
+   On a Y2K38-safe system, ``date +%s`` returns a value greater than ``2147483647``
+   (0x7FFFFFFF) without overflow or sign error.
+
+#. Restore the correct date:
+
+   .. code-block:: console
+
+      $ date -s "<correct-date>"
+
+**On a non-Y2K38-safe system**, setting the date past the boundary may cause:
+
+- ``date`` to report a date in 1901 (signed overflow wraps to negative epoch)
+- System services to crash or behave unexpectedly
+- ``date +%s`` to return a small or negative value