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Date:   Thu, 17 Nov 2022 10:00:46 +0000
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Subject: Re: [PATCH 05/15] kbuild: build init/built-in.a just once
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To:     Masahiro Yamada <masahiroy@kernel.org>,
        linux-kbuild@vger.kernel.org
Cc:     linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        "linux-tegra@vger.kernel.org" <linux-tegra@vger.kernel.org>
References: <20220828024003.28873-1-masahiroy@kernel.org>
 <20220828024003.28873-6-masahiroy@kernel.org>
From:   Jon Hunter <jonathanh@nvidia.com>
In-Reply-To: <20220828024003.28873-6-masahiroy@kernel.org>
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Hi Masahiro

On 28/08/2022 03:39, Masahiro Yamada wrote:
> Kbuild builds init/built-in.a twice; first during the ordinary
> directory descending, second from scripts/link-vmlinux.sh.
> 
> We do this because UTS_VERSION contains the build version and the
> timestamp. We cannot update it during the normal directory traversal
> since we do not yet know if we need to update vmlinux. UTS_VERSION is
> temporarily calculated, but omitted from the update check. Otherwise,
> vmlinux would be rebuilt every time.
> 
> When Kbuild results in running link-vmlinux.sh, it increments the
> version number in the .version file and takes the timestamp at that
> time to really fix UTS_VERSION.
> 
> However, updating the same file twice is a footgun. To avoid nasty
> timestamp issues, all build artifacts that depend on init/built-in.a
> must be atomically generated in link-vmlinux.sh, where some of them
> do not need rebuilding.
> 
> To fix this issue, this commit changes as follows:
> 
> [1] Split UTS_VERSION out to include/generated/utsversion.h from
>      include/generated/compile.h
> 
>      include/generated/utsversion.h is generated just before the
>      vmlinux link. It is generated under include/generated/ because
>      some decompressors (s390, x86) use UTS_VERSION.
> 
> [2] Split init_uts_ns and linux_banner out to init/version-timestamp.c
>      from init/version.c
> 
>      init_uts_ns and linux_banner contain UTS_VERSION. During the ordinary
>      directory descending, they are compiled with __weak and used to
>      determine if vmlinux needs relinking. Just before the vmlinux link,
>      they are compiled without __weak to embed the real version and
>      timestamp.
> 
> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>


Since this change I have noticed that the kernel image (at least on ARM64) now contains two version strings ...

$ strings arch/arm64/boot/Image | grep "Linux version"
Linux version 6.0.0-rc7-00011-g2df8220cc511 (jonathanh@moonraker) (aarch64-linux-gnu-gcc (Linaro GCC 6.4-2017.08) 6.4.1 20170707, GNU ld (Linaro_Binutils-2017.08) 2.27.0.20161019) # SMP PREEMPT
Linux version 6.0.0-rc7-00011-g2df8220cc511 (jonathanh@moonraker) (aarch64-linux-gnu-gcc (Linaro GCC 6.4-2017.08) 6.4.1 20170707, GNU ld (Linaro_Binutils-2017.08) 2.27.0.20161019) #20 SMP PREEMPT Thu Nov 17 09:49:18 GMT 2022

Is this expected?

Thanks!
Jon

-- 
nvpublic