linux-kernel - Re: [PATCH 3/4] vmlinux.lds.h: Fix alignment for __ksymtab*, __kcrctab_* and .pci

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Date: Fri, 22 Dec 2023 10:02:20 +0100
From: Helge Deller <deller@....de>
To: Masahiro Yamada <masahiroy@...nel.org>, deller@...nel.org
Cc: linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
 linux-modules@...r.kernel.org, linux-arch@...r.kernel.org,
 Luis Chamberlain <mcgrof@...nel.org>
Subject: Re: [PATCH 3/4] vmlinux.lds.h: Fix alignment for __ksymtab*,
 __kcrctab_* and .pci_fixup sections

On 12/21/23 14:07, Masahiro Yamada wrote:
> On Thu, Nov 23, 2023 at 7:18 AM <deller@...nel.org> wrote:
>>
>> From: Helge Deller <deller@....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 vmlinux
>> 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 */                       \
>> +       . = ALIGN(4);                                                   \
>>          __kcrctab         : AT(ADDR(__kcrctab) - LOAD_OFFSET) {         \
>>                  __start___kcrctab = .;                                  \
>>                  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-internal.h
index cd253eb51d6c..d445705ac13c 100644
--- 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