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

[Masahiro Yamada]

On Thu, Nov 17, 2022 at 7:00 PM Jon Hunter <jonathanh@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxx>
>
>
> 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?


Yes.

The first line is a weak (i.e. unused) symbol.
The second one is a real one.


2df8220cc511326508ec4da2f43ef69311bdd7b9 fixed up
arch/powerpc/boot/wrapper because it grep's vmlinux.


The weak symbols slightly increase the image size, but
I do not know how to do this better.






> Thanks!
> Jon
>
> --
> nvpublic



-- 
Best Regards
Masahiro Yamada