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Date: Fri, 22 Dec 2023 10:02:20 +0100
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Subject: Re: [PATCH 3/4] vmlinux.lds.h: Fix alignment for __ksymtab*,
 __kcrctab_* and .pci_fixup sections
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To: Masahiro Yamada <masahiroy@kernel.org>, 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>
References: <20231122221814.139916-1-deller@kernel.org>
 <20231122221814.139916-4-deller@kernel.org>
 <CAK7LNARgQ0t=4dfkJXDhSzdFGbxDuN2kPGxTgDR7siCYTtGU5w@mail.gmail.com>
From: Helge Deller <deller@gmx.de>
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On 12/21/23 14:07, Masahiro Yamada wrote:
> 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 vml=
inux
>> executable, otherwise unaligned memory accesses may happen at runtime.
>
>
> Are you solving a real problem?

Not any longer.
I faced a problem on parisc when neither #1 and #3 were applied
because of a buggy unalignment exception handler. But this is
not something which I would count a "real generic problem".

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

Yes, it does.

>...
> So, my understanding is this patch is unneeded.

Yes, it's not required and I'm fine if we drop it.

But regarding __kcrctab:

>> @@ -498,6 +501,7 @@
>>          }                                                             =
  \
>>                                                                        =
  \
>>          /* Kernel symbol table: Normal symbols */                     =
  \
>> +       . =3D ALIGN(4);                                                =
   \
>>          __kcrctab         : AT(ADDR(__kcrctab) - LOAD_OFFSET) {       =
  \
>>                  __start___kcrctab =3D .;                              =
    \
>>                  KEEP(*(SORT(___kcrctab+*)))                           =
  \

I think this patch would be beneficial to get proper alignment:

diff --git a/include/linux/export-internal.h b/include/linux/export-intern=
al.h
index cd253eb51d6c..d445705ac13c 100644
=2D-- a/include/linux/export-internal.h
+++ b/include/linux/export-internal.h
@@ -64,6 +64,7 @@

  #define SYMBOL_CRC(sym, crc, sec)   \
         asm(".section \"___kcrctab" sec "+" #sym "\",\"a\""     "\n" \
+           ".balign 4"                                         "\n" \
             "__crc_" #sym ":"                                   "\n" \
             ".long " #crc                                       "\n" \
             ".previous"                                         "\n")


Helge