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References: <20231122221814.139916-1-deller@kernel.org> <20231122221814.139916-4-deller@kernel.org>
In-Reply-To: <20231122221814.139916-4-deller@kernel.org>
From: Masahiro Yamada <masahiroy@kernel.org>
Date: Thu, 21 Dec 2023 22:07:54 +0900
Message-ID: <CAK7LNARgQ0t=4dfkJXDhSzdFGbxDuN2kPGxTgDR7siCYTtGU5w@mail.gmail.com>
Subject: Re: [PATCH 3/4] vmlinux.lds.h: Fix alignment for __ksymtab*,
 __kcrctab_* and .pci_fixup sections
To: deller@kernel.org
Cc: linux-kernel@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>, 
	linux-modules@vger.kernel.org, linux-arch@vger.kernel.org, 
	Luis Chamberlain <mcgrof@kernel.org>
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On Thu, Nov 23, 2023 at 7:18=E2=80=AFAM <deller@kernel.org> wrote:
>
> From: Helge Deller <deller@gmx.de>
>
> On 64-bit architectures without CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
> (e.g. ppc64, ppc64le, parisc, s390x,...) the __KSYM_REF() macro stores
> 64-bit pointers into the __ksymtab* sections.
> Make sure that the start of those sections is 64-bit aligned in the vmlin=
ux
> executable, otherwise unaligned memory accesses may happen at runtime.


Are you solving a real problem?


1/4 already ensures the proper alignment of __ksymtab*, doesn't it?


I applied the following hack to attempt to
break the alignment intentionally.


diff --git a/include/asm-generic/vmlinux.lds.h
b/include/asm-generic/vmlinux.lds.h
index bae0fe4d499b..e2b5c9acee97 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -482,7 +482,7 @@
        TRACEDATA                                                       \
                                                                        \
        PRINTK_INDEX                                                    \
-                                                                       \
+       . =3D . + 1;                                                      \
        /* Kernel symbol table: Normal symbols */                       \
        __ksymtab         : AT(ADDR(__ksymtab) - LOAD_OFFSET) {         \
                __start___ksymtab =3D .;                                  \


The __ksymtab section and __start___ksymtab symbol
are still properly aligned due to the '.balign'
in <linux/export-internal.h>


So, my understanding is this patch is unneeded.


Or, does the behaviour depend on toolchains?


> The __kcrctab* sections store 32-bit entities, so make those sections
> 32-bit aligned.
>
> The pci fixup routines want to be 64-bit aligned on 64-bit platforms
> which don't define CONFIG_HAVE_ARCH_PREL32_RELOCATIONS. An alignment
> of 8 bytes is sufficient to guarantee aligned accesses at runtime.
>
> Signed-off-by: Helge Deller <deller@gmx.de>
> Cc: <stable@vger.kernel.org> # v6.0+


> ---
>  include/asm-generic/vmlinux.lds.h | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmli=
nux.lds.h
> index bae0fe4d499b..fa4335346e7d 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -467,6 +467,7 @@
>         }                                                               \
>                                                                         \
>         /* PCI quirks */                                                \
> +       . =3D ALIGN(8);                                                  =
 \
>         .pci_fixup        : AT(ADDR(.pci_fixup) - LOAD_OFFSET) {        \
>                 BOUNDED_SECTION_PRE_LABEL(.pci_fixup_early,  _pci_fixups_=
early,  __start, __end) \
>                 BOUNDED_SECTION_PRE_LABEL(.pci_fixup_header, _pci_fixups_=
header, __start, __end) \
> @@ -484,6 +485,7 @@
>         PRINTK_INDEX                                                    \
>                                                                         \
>         /* Kernel symbol table: Normal symbols */                       \
> +       . =3D ALIGN(8);                                                  =
 \
>         __ksymtab         : AT(ADDR(__ksymtab) - LOAD_OFFSET) {         \
>                 __start___ksymtab =3D .;                                 =
 \
>                 KEEP(*(SORT(___ksymtab+*)))                             \
> @@ -491,6 +493,7 @@
>         }                                                               \
>                                                                         \
>         /* Kernel symbol table: GPL-only symbols */                     \
> +       . =3D ALIGN(8);                                                  =
 \
>         __ksymtab_gpl     : AT(ADDR(__ksymtab_gpl) - LOAD_OFFSET) {     \
>                 __start___ksymtab_gpl =3D .;                             =
 \
>                 KEEP(*(SORT(___ksymtab_gpl+*)))                         \
> @@ -498,6 +501,7 @@
>         }                                                               \
>                                                                         \
>         /* Kernel symbol table: Normal symbols */                       \
> +       . =3D ALIGN(4);                                                  =
 \
>         __kcrctab         : AT(ADDR(__kcrctab) - LOAD_OFFSET) {         \
>                 __start___kcrctab =3D .;                                 =
 \
>                 KEEP(*(SORT(___kcrctab+*)))                             \
> @@ -505,6 +509,7 @@
>         }                                                               \
>                                                                         \
>         /* Kernel symbol table: GPL-only symbols */                     \
> +       . =3D ALIGN(4);                                                  =
 \
>         __kcrctab_gpl     : AT(ADDR(__kcrctab_gpl) - LOAD_OFFSET) {     \
>                 __start___kcrctab_gpl =3D .;                             =
 \
>                 KEEP(*(SORT(___kcrctab_gpl+*)))                         \
> --
> 2.41.0
>


--=20
Best Regards
Masahiro Yamada