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    Subject[PATCH 4.14 18/82] kbuild: mkcompile_h: consider timestamp if KBUILD_BUILD_TIMESTAMP is set
    Date
    From: Matthias Maennich <maennich@google.com>

    [ 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@google.com>
    Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    scripts/mkcompile_h | 14 +++++++++++---
    1 file changed, 11 insertions(+), 3 deletions(-)

    diff --git a/scripts/mkcompile_h b/scripts/mkcompile_h
    index 959199c3147e..49f92fffa098 100755
    --- a/scripts/mkcompile_h
    +++ b/scripts/mkcompile_h
    @@ -83,15 +83,23 @@ 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
    + 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
    --
    2.30.2


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