Date:   Tue, 28 Feb 2017 11:22:18 -0800
From:   David Daney <ddaney@...iumnetworks.com>
To:     David Daney <ddaney@...iumnetworks.com>,
        Jason Baron <jbaron@...mai.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sachin Sant <sachinp@...ux.vnet.ibm.com>
Cc:     linux-mips@...ux-mips.org, Chris Metcalf <cmetcalf@...lanox.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Russell King <linux@...linux.org.uk>,
        Ralf Baechle <ralf@...ux-mips.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Rabin Vincent <rabin@....in>,
        Paul Mackerras <paulus@...ba.org>,
        Anton Blanchard <anton@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org, Ingo Molnar <mingo@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, Zhigang Lu <zlu@...hip.com>
Subject: Re: [PATCH] jump_label: align jump_entry table to at least 4-bytes

On 02/28/2017 11:05 AM, David Daney wrote:
> On 02/28/2017 10:39 AM, Jason Baron wrote:
>>
[...]
>>> I suspect that the alignment of the __jump_table section in the .ko
>>> files is not correct, and you are seeing some sort of problem due to
>>> that.
>>>
>>>
>>
>> Hi,
>>
>> Yes, if you look at the trace that Sachin sent the module being loaded
>> that does the WARN_ON() is nfsd.ko.
>>
>> That module from Sachin's trace has:
>>
>>   [31] __jump_table      PROGBITS        0000000000000000 03fd77 0000c0
>> 18 WAM  0   0  1
>
> The problem is then the section alignment (last column) for power.
>
> On mips with no patches applied, we get:
>
>   [17] __jump_table      PROGBITS        0000000000000000 00d2c0 000048
> 00  WA  0   0  8
>
> Look, proper alignment!
>
> The question I have is why do the power ".llong" and ".long" assembler
> directives not force section alignment?  Is there an alternative that
> could be used that would result in the proper alignment?  Would ".word"
> work?
>
> If not, then I would say patch only power with your balign thing. 8-byte
> alignment for 64-bit kernel, 4-byte alignment for 32-bit kernel
>

I think the proper fix is either:

A) Modify scripts/module-common.lds to force __jump_table alignment for 
all architectures.

B) Add arch/powerpc/kernel/module.lds to force __jump_table alignment 
for powerpc only.

David.



>
>>
>> So its not the size but rather the start offset '03fd77', that is the
>> problem here. That is what the WARN_ON triggers on, that the start of
>> the table is not 4-byte aligned.
>>
>> Using a ppc cross-compiler and the ENTSIZE patch that line does not
>> change, however if I use the initial patch posted in this thread, the
>> start does align to 4-bytes and thus the warning goes away, as Sachin
>> verified. In fact, without the patch I found several modules that don't
>> start at the proper alignment, however with the patch that started this
>> thread they were all properly aligned.
>>
>> In terms of the '.balign' causing holes, we originally added the
>> '_ASM_ALIGN' to x86 for precisely this reason. See commit:
>> ef64789 jump label: Add _ASM_ALIGN for x86 and x86_64 and discussion.
>>
>> In addition, we have a lot of runtime with the .balign in the tree and
>> I'm not aware of any holes in the table. I think the code would blow up
>> pretty badly if there were.
>>
>> A number of arches were already using the '.balign', and the patch I
>> proposed simply added it to remaining ones, now that we added a
>> WARN_ON() to catch this condition.
>>
>> Thanks,
>>
>> -Jason
>>
>>
>>
>>
>
>
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