build system: Make it possible to build with 64bit time_t

On most 32bit architectures time_t (and a few other time related types)
are a signed 32bit wide integer type.
As a consequence they can only represent dates between

	Fri Dec 13 08:45:52 PM UTC 1901

(-0x80000000 seconds before Jan 1 1970) and

	Tue Jan 19 03:14:07 AM UTC 2038

(0x7fffffff seconds after Jan 1 1970). Given that some machines that are
built today have an expected lifetime of >15 years, this needs to be
extended. To to that, define the cpp symbol _TIME_BITS to 64 which
results in some magic in glibc to make time_t (and the few other time
related types) 64 bit wide.

This new switch CONFIG_TIME64 is in the spirit of CONFIG_LFS and only
expected to have the expected effect with glibc. On musl for examples
time_t already defaults to 64bit wide types.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

2 files changed, 11 insertions, 0 deletions

diff --git a/Config.in b/Config.in
index a98a8b15b..214eba546 100644
--- a/Config.in
+++ b/Config.in
@@ -108,6 +108,16 @@ config LFS
         programs that can benefit from large file support include dd, gzip,
         cp, mount, tar.
 
+config TIME64
+        bool "Support 64bit wide time types"
+        default y
+        depends on LFS
+        help
+        Make times later than 2038 representable for several libc syscalls
+        (stat, clk_gettime etc.). Note this switch is specific to glibc and has
+        no effect on platforms that already use 64bit wide time types (i.e. all
+        64bit archs and some selected 32bit archs (currently riscv and x32)).
+
 config PAM
         bool "Support PAM (Pluggable Authentication Modules)"
         default n
diff --git a/Makefile.flags b/Makefile.flags
index 1cec5ba20..e4cd658fd 100644
--- a/Makefile.flags
+++ b/Makefile.flags
@@ -15,6 +15,7 @@ CPPFLAGS += \
         -include include/autoconf.h \
         -D_GNU_SOURCE -DNDEBUG \
         $(if $(CONFIG_LFS),-D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64) \
+        $(if $(CONFIG_TIME64),-D_TIME_BITS=64) \
         -DBB_VER=$(squote)$(quote)$(BB_VER)$(quote)$(squote)
 
 CFLAGS += $(call cc-option,-Wall,)