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Message-ID: <CAK7LNASdsWMP2jud4niOkrR5+a2jG-Vfo0XEa63bh3L3W6_t0Q@mail.gmail.com>
Date: Sat, 25 Mar 2023 15:05:30 +0900
From: Masahiro Yamada <masahiroy@...nel.org>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>,
linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
Nicolas Schier <nicolas@...sle.eu>,
linux-kernel@...r.kernel.org,
Martin KaFai Lau <martin.lau@...ux.dev>,
"open list:BPF JIT for MIPS (32-BIT AND 64-BIT)"
<bpf@...r.kernel.org>
Subject: Re: [PATCH] arm64: remove special treatment for the link order of head.o
On Fri, Mar 24, 2023 at 8:33 PM Ard Biesheuvel <ardb@...nel.org> wrote:
>
> (cc BTF list and maintainer)
>
> On Thu, 23 Mar 2023 at 22:12, Aurelien Jarno <aurelien@...el32.net> wrote:
> >
> > Hi,
> >
> > On 2023-03-22 15:51, Ard Biesheuvel wrote:
> > > On Tue, 21 Mar 2023 at 23:26, Aurelien Jarno <aurelien@...el32.net> wrote:
> > > >
> > > > Hi,
> > > >
> > > > On 2022-10-13 08:35, Masahiro Yamada wrote:
> > > > > In the previous discussion (see the Link tag), Ard pointed out that
> > > > > arm/arm64/kernel/head.o does not need any special treatment - the only
> > > > > piece that must appear right at the start of the binary image is the
> > > > > image header which is emitted into .head.text.
> > > > >
> > > > > The linker script does the right thing to do. The build system does
> > > > > not need to manipulate the link order of head.o.
> > > > >
> > > > > Link: https://lore.kernel.org/lkml/CAMj1kXH77Ja8bSsq2Qj8Ck9iSZKw=1F8Uy-uAWGVDm4-CG=EuA@mail.gmail.com/
> > > > > Suggested-by: Ard Biesheuvel <ardb@...nel.org>
> > > > > Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
> > > > > ---
> > > > >
> > > > > scripts/head-object-list.txt | 1 -
> > > > > 1 file changed, 1 deletion(-)
> > > > >
> > > > > diff --git a/scripts/head-object-list.txt b/scripts/head-object-list.txt
> > > > > index b16326a92c45..f226e45e3b7b 100644
> > > > > --- a/scripts/head-object-list.txt
> > > > > +++ b/scripts/head-object-list.txt
> > > > > @@ -15,7 +15,6 @@ arch/alpha/kernel/head.o
> > > > > arch/arc/kernel/head.o
> > > > > arch/arm/kernel/head-nommu.o
> > > > > arch/arm/kernel/head.o
> > > > > -arch/arm64/kernel/head.o
> > > > > arch/csky/kernel/head.o
> > > > > arch/hexagon/kernel/head.o
> > > > > arch/ia64/kernel/head.o
> > > >
> > > > This patch causes a significant increase of the arch/arm64/boot/Image
> > > > size. For instance the generic arm64 Debian kernel went from 31 to 39 MB
> > > > after this patch has been applied to the 6.1 stable tree.
> > > >
> > > > In turn this causes issues with some bootloaders, for instance U-Boot on
> > > > a Raspberry Pi limits the kernel size to 36 MB.
> > > >
> > >
> > > I cannot reproduce this with mainline
> > >
> > > With the patch
> > >
> > > $ size vmlinux
> > > text data bss dec hex filename
> > > 24567309 14752630 621680 39941619 26175f3 vmlinux
> > >
> > > With the patch reverted
> > >
> > > $ size vmlinux
> > > text data bss dec hex filename
> > > 24567309 14752694 621680 39941683 2617633 vmlinux
> >
> > I have tried with the current mainline, this is what I get, using GCC 12.2.0
> > and binutils 2.40:
> >
> > text data bss dec hex filename
> > 32531655 8192996 621968 41346619 276e63b vmlinux.orig
> > 25170610 8192996 621968 33985574 2069426 vmlinux.revert
> >
> > > It would help to compare the resulting vmlinux ELF images from both
> > > builds to see where the extra space is being allocated
> >
> > At a first glance, it seems the extra space is allocated in the BTF
> > section. I have uploaded the resulting files as well as the config file
> > I used there:
> > https://temp.aurel32.net/linux-arm64-size-head.o.tar.gz
> >
>
> Indeed. So we go from
>
> [15] .BTF PROGBITS ffff8000091d1ff4 011e1ff4
> 00000000005093d6 0000000000000000 A 0 0 1
>
> to
>
> [15] .BTF PROGBITS ffff8000091d1ff4 011e1ff4
> 0000000000c0e5eb 0000000000000000 A 0 0 1
>
> i.e, from 5 MiB to 12+ MiB of BTF metadata.
>
> To me, it is not clear at all how one would be related to the other,
> so it will leave it to the Kbuild and BTF experts to chew on this one.
Strange.
I used the .config file Aurelien provided, but
I still cannot reproduce this issue.
The vmlinux size is small
as-is in the current mainline.
[mainline]
masahiro@zoe:~/ref/linux(master)$ git log --oneline -1
65aca32efdcb (HEAD -> master, origin/master, origin/HEAD) Merge tag
'mm-hotfixes-stable-2023-03-24-17-09' of
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
masahiro@zoe:~/ref/linux(master)$ aarch64-linux-gnu-size vmlinux
text data bss dec hex filename
24561282 8186912 622032 33370226 1fd3072 vmlinux
masahiro@zoe:~/ref/linux(master)$ aarch64-linux-gnu-readelf -S
vmlinux | grep -A1 BTF
[15] .BTF PROGBITS ffff8000091c0708 011d0708
000000000048209c 0000000000000000 A 0 0 1
[16] .BTF_ids PROGBITS ffff8000096427a4 016527a4
0000000000000a1c 0000000000000000 A 0 0 1
[mainline + revert 994b7ac]
masahiro@zoe:~/ref/linux2(testing)$ git log --oneline -2
856c80dd789c (HEAD -> testing) Revert "arm64: remove special treatment
for the link order of head.o"
65aca32efdcb (origin/master, origin/HEAD, master) Merge tag
'mm-hotfixes-stable-2023-03-24-17-09' of
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
masahiro@zoe:~/ref/linux2(testing)$ aarch64-linux-gnu-size vmlinux
text data bss dec hex filename
24561329 8186912 622032 33370273 1fd30a1 vmlinux
masahiro@zoe:~/ref/linux2(testing)$ aarch64-linux-gnu-readelf -S
vmlinux | grep -A1 BTF
[15] .BTF PROGBITS ffff8000091c0708 011d0708
00000000004820cb 0000000000000000 A 0 0 1
[16] .BTF_ids PROGBITS ffff8000096427d4 016527d4
0000000000000a1c 0000000000000000 A 0 0 1
I still do not know what affects reproducibility.
(compiler version, pahole version, etc. ?)
Aurelien used GCC 12 + binutils 2.40, but
my toolchain is a bit older.
FWIW, I tested this on Ubuntu 22.04LTS.
masahiro@zoe:~/ref/linux(master)$ aarch64-linux-gnu-gcc --version
aarch64-linux-gnu-gcc (Ubuntu 11.3.0-1ubuntu1~22.04) 11.3.0
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
masahiro@zoe:~/ref/linux(master)$ pahole --version
v1.22
masahiro@zoe:~/ref/linux(master)$ aarch64-linux-gnu-as --version
GNU assembler (GNU Binutils for Ubuntu) 2.38
Copyright (C) 2022 Free Software Foundation, Inc.
This program is free software; you may redistribute it under the terms of
the GNU General Public License version 3 or later.
This program has absolutely no warranty.
This assembler was configured for a target of `aarch64-linux-gnu'.
--
Best Regards
Masahiro Yamada
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