linux-kernel - Re: [PATCH] [next] initramfs: Parse KBUILD_BUILD

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Date:   Wed, 16 Aug 2023 08:38:46 +0200
From:   Nicolas Schier <n.schier@....de>
To:     Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@...il.com>
Cc:     masahiroy@...nel.org, bgray@...ux.ibm.com, ajd@...ux.ibm.com,
        linux-kernel@...r.kernel.org, keescook@...omium.org
Subject: Re: [PATCH] [next] initramfs: Parse KBUILD_BUILD_TIMESTAMP as UTC
 date

On Sat, Jul 29, 2023 at 05:02:09PM +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@...il.com>
> ---

Thanks for the patch!

Looking at 1970-01-01 concretely, you could circumvent your reported
issue by using KBUILD_BUILD_TIMESTAMP="1970-01-01 UTC".  But I think you
found an interesting point w/ regard to reproducibility, as time zone is
neither mentioned in Documentation/kbuild/reproducibility.rst
nor in Documentation/kbuild/kbuild.rst but influences the output of
'date -d' (w/o '-u') (or comparable) calls in usr/gen_initramfs.sh as
well as in kernel/gen_kheaders.sh ('TZ=UTC tar'?) and possibly in
scripts/kernel-doc.

I think some might be suprised that giving a YYYY-MM-DD date to
KBUILD_BUILD_TIMESTAMP leads to an effectively different timestamp in
initramfs_data.cpio, but I think your change is still reasonable.

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


>  usr/gen_initramfs.sh | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/usr/gen_initramfs.sh b/usr/gen_initramfs.sh
> index 14b5782f961a..a90316d9a080 100755
> --- a/usr/gen_initramfs.sh
> +++ b/usr/gen_initramfs.sh
> @@ -221,7 +221,7 @@ while [ $# -gt 0 ]; do
>  			shift
>  			;;
>  		"-d")	# date for file mtimes
> -			timestamp="$(date -d"$1" +%s || :)"
> +			timestamp="$(date -d"$1" -u +%s || :)"
>  			if test -n "$timestamp"; then
>  				timestamp="-t $timestamp"
>  			fi
> -- 
> 2.40.1
> 

-- 
Nicolas Schier