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From:   Masahiro Yamada <masahiroy@kernel.org>
Date:   Tue, 13 Sep 2022 20:07:50 +0900
Message-ID: <CAK7LNAQTVZ7DNJj4tBNAgjkWAkHQaom01iOZAx3_C8+VDPVy4A@mail.gmail.com>
Subject: Re: [PATCH 05/15] kbuild: build init/built-in.a just once
To:     Nathan Chancellor <nathan@kernel.org>
Cc:     Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>
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On Tue, Sep 13, 2022 at 5:32 PM Nathan Chancellor <nathan@kernel.org> wrote:
>
> 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@kernel.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


Thanks, Nathan.


I did not notice it because /bin/sh is a symlink to dash
on my machine.
I see the warning by running it with bash.


$ rm -f .version ; dash init/build-version
1
$ rm -f .version ; bash init/build-version
cat: .version: No such file or directory
1


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


Agree.
I will fix it up locally.


Thanks.

>
> Cheers,
> Nathan


-- 
Best Regards
Masahiro Yamada