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kheaders: make headers archive reproducible

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In commit 43d8ce9d65 ("Provide in-kernel headers to make
extending kernel easier") a new mechanism was introduced, for kernels
>=5.2, which embeds the kernel headers in the kernel image or a module
and exposes them in procfs for use by userland tools.

The archive containing the header files has nondeterminism caused by
header files metadata. This patch normalizes the metadata and utilizes
KBUILD_BUILD_TIMESTAMP if provided and otherwise falls back to the
default behaviour.

In commit f7b101d330 ("kheaders: Move from proc to sysfs") it was
modified to use sysfs and the script for generation of the archive was
renamed to what is being patched.

Signed-off-by: Dmitry Goldin <dgoldin+lkml@protonmail.ch>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
This commit is contained in:
Dmitry Goldin 2019-10-04 10:40:07 +00:00 committed by Masahiro Yamada
parent d188b8c901
commit 86cdd2fdc4
2 changed files with 13 additions and 5 deletions
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13
Documentation/kbuild/reproducible-builds.rst
View File

@ -16,16 +16,21 @@ the kernel may be unreproducible, and how to avoid them.
Timestamps
----------
The kernel embeds a timestamp in two places:
The kernel embeds timestamps in three places:
* The version string exposed by ``uname()`` and included in
``/proc/version``
* File timestamps in the embedded initramfs
By default the timestamp is the current time. This must be overridden
using the `KBUILD_BUILD_TIMESTAMP`_ variable. If you are building
from a git commit, you could use its commit date.
* If enabled via ``CONFIG_IKHEADERS``, file timestamps of kernel
headers embedded in the kernel or respective module,
exposed via ``/sys/kernel/kheaders.tar.xz``
By default the timestamp is the current time and in the case of
``kheaders`` the various files' modification times. This must
be overridden using the `KBUILD_BUILD_TIMESTAMP`_ variable.
If you are building from a git commit, you could use its commit date.
The kernel does *not* use the ``__DATE__`` and ``__TIME__`` macros,
and enables warnings if they are used. If you incorporate external

5
kernel/gen_kheaders.sh
View File

@ -71,7 +71,10 @@ done | cpio --quiet -pd $cpio_dir >/dev/null 2>&1
find $cpio_dir -type f -print0 |
xargs -0 -P8 -n1 perl -pi -e 'BEGIN {undef $/;}; s/\/\*((?!SPDX).)*?\*\///smg;'
tar -Jcf $tarfile -C $cpio_dir/ . > /dev/null
# Create archive and try to normalize metadata for reproducibility
tar "${KBUILD_BUILD_TIMESTAMP:+--mtime=$KBUILD_BUILD_TIMESTAMP}" \
--owner=0 --group=0 --sort=name --numeric-owner \
-Jcf $tarfile -C $cpio_dir/ . > /dev/null
echo "$src_files_md5" > kernel/kheaders.md5
echo "$obj_files_md5" >> kernel/kheaders.md5