[PATCH v2] [next] initramfs: Parse KBUILD_BUILD_TIMESTAMP as UTC date

[Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>]

When KBUILD_BUILD_TIMESTAMP is specified, the date command will parse
it to Unix Epoch time in UTC. However, the date command is
timezone-aware so it will convert from the local timezone to UTC first
which hits some of the sanity checks added on commit 5efb685bb3af1
("initramfs: Check negative timestamp to prevent broken cpio archive")

This creates an edge case for the UTC+<N> part of the world. For instance

 - In New Zealand (UTC+12:00):
     $ date -d"1970-01-01" +%s
     -43200

     $ make KBUILD_BUILD_TIMESTAMP=1970-01-01
     make[1]: Entering directory '<snip>/linux/'
       GEN     Makefile
       DESCEND objtool
       INSTALL libsubcmd_headers
       CALL    ../scripts/checksyscalls.sh
       GEN     usr/initramfs_data.cpio
     ERROR: Timestamp out of range for cpio format
     make[4]: *** [../usr/Makefile:76: usr/initramfs_data.cpio] Error 1

 - In Seattle, WA (UTC-07:00):
     $ date -d"1970-01-01" +%s
     32400

     $ make KBUILD_BUILD_TIMESTAMP=1970-01-01
     <builds fine>

Parse KBUILD_BUILD_TIMESTAMP date string as UTC so no localtime
conversion is done, which fixes the edge case aforementioned

Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>
---
Changelog:

- v2: Document behaviour and way to override it on
      Documentation/kbuild/kbuild.rst. (Req: Andrew Donnellan)

- v1: https://lore.kernel.org/lkml/ZMSdUS37BD5b%2Fdn7@mail.google.com/
---
 Documentation/kbuild/kbuild.rst | 9 ++++++++-
 usr/gen_initramfs.sh            | 2 +-
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/Documentation/kbuild/kbuild.rst b/Documentation/kbuild/kbuild.rst
index bd906407e307..8c204186c762 100644
--- a/Documentation/kbuild/kbuild.rst
+++ b/Documentation/kbuild/kbuild.rst
@@ -296,7 +296,14 @@ KBUILD_BUILD_TIMESTAMP
 Setting this to a date string overrides the timestamp used in the
 UTS_VERSION definition (uname -v in the running kernel). The value has to
 be a string that can be passed to date -d. The default value
-is the output of the date command at one point during build.
+is the output of the date command at one point during build. E.g.::
+
+    $ make KBUILD_BUILD_TIMESTAMP="1991-08-25"
+
+By default, the value is interpreted as UTC. To override this, append
+the desired timezone. E.g.::
+
+    $ make KBUILD_BUILD_TIMESTAMP="1991-08-25 UTC+03:00"
 
 KBUILD_BUILD_USER, KBUILD_BUILD_HOST
 ------------------------------------
diff --git a/usr/gen_initramfs.sh b/usr/gen_initramfs.sh
index 14b5782f961a..a90316d9a080 100755
--- a/usr/gen_initramfs.sh
+++ b/usr/gen_initramfs.sh
@@ -221,7 +221,7 @@ while [ $# -gt 0 ]; do
 			shift
 			;;
 		"-d")	# date for file mtimes
-			timestamp="$(date -d"$1" +%s || :)"
+			timestamp="$(date -d"$1" -u +%s || :)"
 			if test -n "$timestamp"; then
 				timestamp="-t $timestamp"
 			fi
-- 
2.41.0