build system: use SOURCE_DATE_EPOCH for timestamp if available

The SOURCE_DATE_EPOCH is an effort of the Reproducible Builds
organization to make timestamps/build dates in compiled tools
deterministic over several repetitive builds.

Busybox shows by default the build date timestamp which changes whenever
compiled. To have a reasonable accurate build date while staying
reproducible, it's possible to use the *date of last source
modification* rather than the current time and date.

Further information on SOURCE_DATE_EPOCH are available online [1].

This patch modifies `confdata.c` so that the content of the
SOURCE_DATE_EPOCH env variable is used as timestamp.

To be independent of different timezones between builds, whenever
SOURCE_DATE_EPOCH is defined the GMT time is used.

[1]: https://reproducible-builds.org/docs/source-date-epoch/

Signed-off-by: Paul Spooren <mail@aparcar.org>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 14 insertions, 3 deletions

diff --git a/scripts/kconfig/confdata.c b/scripts/kconfig/confdata.c
index 9976011a9..249a3195e 100644
--- a/scripts/kconfig/confdata.c
+++ b/scripts/kconfig/confdata.c
@@ -342,6 +342,8 @@ int conf_write(const char *name)
         time_t now;
         int use_timestamp = 1;
         char *env;
+        char *source_date_epoch;
+        struct tm *build_time;
 
         dirname[0] = 0;
         if (name && name[0]) {
@@ -378,7 +380,16 @@ int conf_write(const char *name)
         }
         sym = sym_lookup("KERNELVERSION", 0);
         sym_calc_value(sym);
-        time(&now);
+
+        source_date_epoch = getenv("SOURCE_DATE_EPOCH");
+        if (source_date_epoch && *source_date_epoch) {
+                now = strtoull(source_date_epoch, NULL, 10);
+                build_time = gmtime(&now);
+        } else {
+                time(&now);
+                build_time = localtime(&now);
+        }
+
         env = getenv("KCONFIG_NOTIMESTAMP");
         if (env && *env)
                 use_timestamp = 0;
@@ -398,14 +409,14 @@ int conf_write(const char *name)
                 if (use_timestamp) {
                         size_t ret = \
                                 strftime(buf, sizeof(buf), "#define AUTOCONF_TIMESTAMP "
-                                        "\"%Y-%m-%d %H:%M:%S %Z\"\n", localtime(&now));
+                                        "\"%Y-%m-%d %H:%M:%S %Z\"\n", build_time);
                         /* if user has Factory timezone or some other odd install, the
                          * %Z above will overflow the string leaving us with undefined
                          * results ... so let's try again without the timezone.
                          */
                         if (ret == 0)
                                 strftime(buf, sizeof(buf), "#define AUTOCONF_TIMESTAMP "
-                                        "\"%Y-%m-%d %H:%M:%S\"\n", localtime(&now));
+                                        "\"%Y-%m-%d %H:%M:%S\"\n", build_time);
                 } else { /* bbox */
                         strcpy(buf, "#define AUTOCONF_TIMESTAMP \"\"\n");
                 }