linux-kernel - Re: [PATCH 05/15] kbuild: build init/built-in.a just once

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Date:   Tue, 13 Sep 2022 01:32:20 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org
Subject: Re: [PATCH 05/15] kbuild: build init/built-in.a just once

Hi Masahiro,

On Sun, Aug 28, 2022 at 11:39:53AM +0900, 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@...nel.org>

<snip>

> diff --git a/init/build-version b/init/build-version
> new file mode 100755
> index 000000000000..39225104f14d
> --- /dev/null
> +++ b/init/build-version
> @@ -0,0 +1,10 @@
> +#!/bin/sh
> +# SPDX-License-Identifier: GPL-2.0-only
> +
> +VERSION=$(cat .version) 2>/dev/null &&
> +VERSION=$(expr $VERSION + 1) 2>/dev/null ||
> +VERSION=1
> +
> +echo ${VERSION} > .version
> +
> +echo ${VERSION}

I am seeing

  cat: .version: No such file or directory

at some point in nearly all of my builds in -next. Does the 2>/dev/null
want to be moved into the subshell?

Cheers,
Nathan