linux-kernel - Re: [PATCH 1/2] modules: Ensure natural alignment for .altinstructions and __bug

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Message-ID: <26020ab7-3700-89cd-b58a-0d529282dce3@gmx.de>
Date:   Fri, 1 Jul 2022 20:22:34 +0200
From:   Helge Deller <deller@....de>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     jeyu@...nel.org, linux-modules@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-parisc@...r.kernel.org,
        live-patching@...r.kernel.org
Subject: Re: [PATCH 1/2] modules: Ensure natural alignment for
 .altinstructions and __bug_table sections

On 7/1/22 20:02, Luis Chamberlain wrote:
> On Mon, Jun 27, 2022 at 09:05:50PM +0200, Helge Deller wrote:
>> In the kernel image vmlinux.lds.S linker scripts the .altinstructions
>> and __bug_table sections are 32- or 64-bit aligned because they hold 32-
>> and/or 64-bit values.
>>
>> But for modules the module.lds.S linker script doesn't define a default
>> alignment yet, so the linker chooses the default byte-alignment, which
>> then leads to unnecessary unaligned memory accesses at runtime.
>>
>> This patch adds the missing alignments.
>>
>> Signed-off-by: Helge Deller <deller@....de>
>
> Good catch but does this fix a real world issue? When are
> altinstructions used for modules? When are alternatives used
> for modules?
>
> How did you notice this issue?

You are right - usually this is unnoticed, because either the
hardware (on x86 cpus) or by exception handlers (e.g. on hppa
or sparc) fix up such unaligned accesses at runtime.

I noticed that, because on hppa the 32-bit unalignment handler
was broken due a bad commit, and as such wrong values were
returned on unaligned accesses, which then led to other bugs.
For hppa I fixed it with this commit:
https://patchwork.kernel.org/project/linux-parisc/patch/20220626233911.1023515-1-deller@gmx.de/

This happened when loading the mptbase kernel module:
https://lore.kernel.org/all/07d91863-dacc-a503-aa2b-05c3b92a1e39@bell.net/T/#mab602dfa32be5e229d5e192ab012af196d04d75d
The summary why it went wrong is at the end of that thread.

On hppa we replace "sync" and some other instructions in kernel
and modules - depending on the CPU type (detected at runtime), or
on the environment (e.g. when running in qemu).

IMHO this really should be fixed, esp. since the fix is trivial.

> This information should go into the commit log.

Sure, I'll resend with updated info.

Thanks!
Helge

>>  scripts/module.lds.S | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/scripts/module.lds.S b/scripts/module.lds.S
>> index 1d0e1e4dc3d2..3a3aa2354ed8 100644
>> --- a/scripts/module.lds.S
>> +++ b/scripts/module.lds.S
>> @@ -27,6 +27,8 @@ SECTIONS {
>>  	.ctors			0 : ALIGN(8) { *(SORT(.ctors.*)) *(.ctors) }
>>  	.init_array		0 : ALIGN(8) { *(SORT(.init_array.*)) *(.init_array) }
>>
>> +	.altinstructions	0 : ALIGN(8) { KEEP(*(.altinstructions)) }
>> +	__bug_table		0 : ALIGN(8) { KEEP(*(__bug_table)) }
>>  	__jump_table		0 : ALIGN(8) { KEEP(*(__jump_table)) }
>>
>>  	__patchable_function_entries : { *(__patchable_function_entries) }
>> --
>> 2.35.3
>>