[multiple] Y2038 32-bit signed time_t mitigations

Most OS platforms have already provided solutions to
Y2038 32-bit signed time_t 5 - 10 years ago (or more!)
Notable exceptions are Linux i686 and FreeBSD i386.

Since 32-bit systems tend to be embedded systems,
and since many distros take years to pick up new software,
this commit aims to provide Y2038 mitigations for lighttpd
running on 32-bit systems with Y2038-unsafe 32-bit signed time_t

* Y2038: lighttpd 1.4.60 and later report Y2038 safety
  $ lighttpd -V
    + Y2038 support                                    # Y2038-SAFE
  $ lighttpd -V
    - Y2038 support (unsafe 32-bit signed time_t)      # Y2038-UNSAFE

* Y2038: general platform info
  * Y2038-SAFE: lighttpd 64-bit builds on platforms using 64-bit time_t
      - all major 64-bit platforms (known to this author) use 64-bit time_t
  * Y2038-SAFE: lighttpd 32-bit builds on platforms using 64-bit time_t
      - Linux x32 ABI (different from i686)
      - FreeBSD all 32-bit and 64-bit architectures *except* 32-bit i386
      - NetBSD 6.0 (released Oct 2012) all 32-bit and 64-bit architectures
      - OpenBSD 5.5 (released May 2014) all 32-bit and 64-bit architectures
      - Microsoft Windows XP and Visual Studio 2005 (? unsure ?)
        Another reference suggests Visual Studio 2015 defaults to 64-bit time_t
      - MacOS 10.15 Catalina (released 2019) drops support for 32-bit apps
  * Y2038-SAFE: lighttpd 32-bit builds on platforms using 32-bit unsigned time_t
      - e.g. OpenVMS (unknown if lighttpd builds on this platform)
  * Y2038-UNSAFE: lighttpd 32-bit builds on platforms using 32-bit signed time_t
      - Linux 32-bit (including i686)
          - glibc 32-bit library support not yet available for 64-bit time_t
              - https://sourceware.org/glibc/wiki/Y2038ProofnessDesign
              - Linux kernel 5.6 on 32-bit platforms does support 64-bit time_t
                https://itsubuntu.com/linux-kernel-5-6-to-fix-the-year-2038-issue-unix-y2k/
              - https://www.gnu.org/software/libc/manual/html_node/64_002dbit-time-symbol-handling.html
                "Note: at this point, 64-bit time support in dual-time
                 configurations is work-in-progress, so for these
                 configurations, the public API only makes the 32-bit time
                 support available. In a later change, the public API will
                 allow user code to choose the time size for a given
                 compilation unit."
              - compiling with -D_TIME_BITS=64 currently has no effect
          - glibc recent (Jul 2021) mailing list discussion
              - https://public-inbox.org/bug-gnulib/878s2ozq70.fsf@oldenburg.str.redhat.com/T/
      - FreeBSD i386
      - DragonFlyBSD 32-bit

* Y2038 mitigations attempted on Y2038-UNSAFE platforms (32-bit signed time_t)
  * lighttpd prefers system monotonic clock instead of realtime clock
    in places where realtime clock is not required
  * lighttpd treats negative time_t values as after 19 Jan 2038 03:14:07 GMT
  * (lighttpd presumes that lighttpd will not encounter dates before 1970
    during normal operation.)
  * lighttpd casts struct stat st.st_mtime (and st.st_*time) through uint64_t
    to convert negative timestamps for comparisions with 64-bit timestamps
    (treating negative timestamp values as after 19 Jan 2038 03:14:07 GMT)
  * lighttpd provides unix_time64_t (int64_t) and
  * lighttpd provides struct unix_timespec64 (unix_timespec64_t)
    (struct timespec equivalent using unix_time64_t tv_sec member)
  * lighttpd provides gmtime64_r() and localtime64_r() wrappers
    for platforms 32-bit platforms using 32-bit time_t and
    lighttpd temporarily shifts the year in order to use
    gmtime_r() and localtime_r() (or gmtime() and localtime())
    from standard libraries, before readjusting year and passing
    struct tm to formatting functions such as strftime()
  * lighttpd provides TIME64_CAST() macro to cast signed 32-bit time_t to
    unsigned 32-bit and then to unix_time64_t

* Note: while lighttpd tries handle times past 19 Jan 2038 03:14:07 GMT
  on 32-bit platforms using 32-bit signed time_t, underlying libraries and
  underlying filesystems might not behave properly after 32-bit signed time_t
  overflows (19 Jan 2038 03:14:08 GMT).  If a given 32-bit OS does not work
  properly using negative time_t values, then lighttpd likely will not work
  properly on that system.

* Other references and blogs
  - https://en.wikipedia.org/wiki/Year_2038_problem
  - https://en.wikipedia.org/wiki/Time_formatting_and_storage_bugs
  - http://www.lieberbiber.de/2017/03/14/a-look-at-the-year-20362038-problems-and-time-proofness-in-various-systems/

1 files changed, 5 insertions, 5 deletions

diff --git a/src/stat_cache.c b/src/stat_cache.c
index 707e0d2d..ccb47262 100644
--- a/src/stat_cache.c
+++ b/src/stat_cache.c
@@ -215,7 +215,7 @@ typedef struct fam_dir_entry {
 	buffer name;
 	int refcnt;
 	FAMRequest req;
-	time_t stat_ts;
+	unix_time64_t stat_ts;
 	dev_t st_dev;
 	ino_t st_ino;
 	struct fam_dir_entry *fam_parent;
@@ -679,7 +679,7 @@ static fam_dir_entry * fam_dir_monitor(stat_cache_fam *scf, char *fn, uint32_t d
         /* directory already registered */
     }
 
-    const time_t cur_ts = log_monotonic_secs;
+    const unix_time64_t cur_ts = log_monotonic_secs;
     struct stat lst;
     int ck_dir = fn_is_dir;
     if (!fn_is_dir && (NULL==fam_dir || cur_ts - fam_dir->stat_ts >= 16)) {
@@ -1268,7 +1268,7 @@ stat_cache_entry * stat_cache_get_entry(const buffer * const name) {
 	 * check if the directory for this file has changed
 	 */
 
-	const time_t cur_ts = log_monotonic_secs;
+	const unix_time64_t cur_ts = log_monotonic_secs;
 
 	const int file_ndx = splaytree_djbhash(name->ptr, len);
 	splay_tree *sptree = sc.files = splaytree_splay(sc.files, file_ndx);
@@ -1469,7 +1469,7 @@ int stat_cache_open_rdonly_fstat (const buffer *name, struct stat *st, int symli
  * and remove them in a second loop
  */
 
-static void stat_cache_tag_old_entries(splay_tree * const t, int * const keys, int * const ndx, const time_t max_age, const time_t cur_ts) {
+static void stat_cache_tag_old_entries(splay_tree * const t, int * const keys, int * const ndx, const time_t max_age, const unix_time64_t cur_ts) {
     if (*ndx == 8192) return; /*(must match num array entries in keys[])*/
     if (t->left)
         stat_cache_tag_old_entries(t->left, keys, ndx, max_age, cur_ts);
@@ -1482,7 +1482,7 @@ static void stat_cache_tag_old_entries(splay_tree * const t, int * const keys, i
         keys[(*ndx)++] = t->key;
 }
 
-static void stat_cache_periodic_cleanup(const time_t max_age, const time_t cur_ts) {
+static void stat_cache_periodic_cleanup(const time_t max_age, const unix_time64_t cur_ts) {
     splay_tree *sptree = sc.files;
     int max_ndx, i;
     int keys[8192]; /* 32k size on stack */