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

[Nicolas Schier <n.schier@avm.de>]

On Tue, Aug 22, 2023 at 09:13:18AM +1200, Paulo Miguel Almeida wrote:
> 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.::

This is still not true for IKHEADERS; there we still need to set TZ=UTC:

diff --git a/kernel/gen_kheaders.sh b/kernel/gen_kheaders.sh
--- a/kernel/gen_kheaders.sh
+++ b/kernel/gen_kheaders.sh
@@ -85,3 +85,3 @@ find $cpio_dir -type f -print0 |
 # Create archive and try to normalize metadata for reproducibility.
-tar "${KBUILD_BUILD_TIMESTAMP:+--mtime=$KBUILD_BUILD_TIMESTAMP}" \
+TZ=UTC tar "${KBUILD_BUILD_TIMESTAMP:+--mtime=$KBUILD_BUILD_TIMESTAMP}" \
     --owner=0 --group=0 --sort=name --numeric-owner \

but I can send such a patch later.

Reviewed-by: Nicolas Schier <n.schier@avm.de>