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kbuild: mkcompile_h: consider timestamp if KBUILD_BUILD_TIMESTAMP is set

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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@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
This commit is contained in:
Matthias Maennich 2021-06-12 15:18:38 +01:00 committed by Masahiro Yamada
parent 4a6795933a
commit a979522a1a
1 changed files with 11 additions and 3 deletions
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14
scripts/mkcompile_h
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@ -68,15 +68,23 @@ UTS_VERSION="$(echo $UTS_VERSION $CONFIG_FLAGS $TIMESTAMP | cut -b -$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
IGNORE_PATTERN="UTS_VERSION"
else
IGNORE_PATTERN="NOT_A_PATTERN_TO_BE_MATCHED"
fi
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
else