[PATCH 5.4 36/71] kbuild: mkcompile_h: consider timestamp if KBUILD_BUILD_TIMESTAMP is set

From: Greg Kroah-Hartman
Date: Thu Jul 22 2021 - 12:33:56 EST

From: Matthias Maennich <maennich@xxxxxxxxxx>

[ Upstream commit a979522a1a88556e42a22ce61bccc58e304cb361 ]

To avoid unnecessary recompilations, mkcompile_h does not regenerate
compile.h if just the timestamp changed.
Though, if KBUILD_BUILD_TIMESTAMP is set, an explicit timestamp for the
build was requested, in which case we should not ignore it.

If a user follows the documentation for reproducible builds [1] and
defines KBUILD_BUILD_TIMESTAMP as the git commit timestamp, a clean
build will have the correct timestamp. A subsequent cherry-pick (or
amend) changes the commit timestamp and if an incremental build is done
with a different KBUILD_BUILD_TIMESTAMP now, that new value is not taken
into consideration. But it should for reproducibility.

Hence, whenever KBUILD_BUILD_TIMESTAMP is explicitly set, do not ignore
UTS_VERSION when making a decision about whether the regenerated version
of compile.h should be moved into place.

[1] https://www.kernel.org/doc/html/latest/kbuild/reproducible-builds.html

Signed-off-by: Matthias Maennich <maennich@xxxxxxxxxx>
Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
scripts/mkcompile_h | 14 +++++++++++---
1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/scripts/mkcompile_h b/scripts/mkcompile_h
index d1d757c6edf4..06c1e9e3bc38 100755
--- a/scripts/mkcompile_h
+++ b/scripts/mkcompile_h
@@ -80,15 +80,23 @@ UTS_TRUNCATE="cut -b -$UTS_LEN"
# Only replace the real compile.h if the new one is different,
# in order to preserve the timestamp and avoid unnecessary
# recompilations.
-# We don't consider the file changed if only the date/time changed.
+# We don't consider the file changed if only the date/time changed,
+# unless KBUILD_BUILD_TIMESTAMP was explicitly set (e.g. for
+# reproducible builds with that value referring to a commit timestamp).
# A kernel config change will increase the generation number, thus
# causing compile.h to be updated (including date/time) due to the
# changed comment in the
# first line.

+if [ -z "$KBUILD_BUILD_TIMESTAMP" ]; then
if [ -r $TARGET ] && \
- grep -v 'UTS_VERSION' $TARGET > .tmpver.1 && \
- grep -v 'UTS_VERSION' .tmpcompile > .tmpver.2 && \
+ grep -v $IGNORE_PATTERN $TARGET > .tmpver.1 && \
+ grep -v $IGNORE_PATTERN .tmpcompile > .tmpver.2 && \
cmp -s .tmpver.1 .tmpver.2; then
rm -f .tmpcompile