linux-kernel - Re: [PATCH] jump_label: align jump

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Message-ID: <23989c10-7b47-3fda-f790-25b539704bec@akamai.com>
Date:   Tue, 28 Feb 2017 17:41:33 -0500
From:   Jason Baron <jbaron@...mai.com>
To:     David Daney <ddaney@...iumnetworks.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 03:15 PM, David Daney wrote:
> On 02/28/2017 11:34 AM, Jason Baron wrote:
>>
>>
>> On 02/28/2017 02:22 PM, David Daney wrote:
>>> 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.
>>>
>>>
>>
>> Ok, I can try adding it to the linger script.
>>
>> FWIW, here is my before and after with the .balign thing for the nfsd.ko
>> module on powperc (using a cross-compiler):
>>
>> before:
>>
>>   [31] __jump_table      PROGBITS        0000000000000000 03ee3e 0000f0
>> 00  WA  0   0  1
>>
>> after:
>>
>>  [31] __jump_table      PROGBITS        0000000000000000 03ee40 0000f0
>> 00  WA  0   0  4
>>
>
> Try the (lightly tested) attached.
>
> If it works and Steven likes it, perhaps someone can merge it.
>
> David.
>
>
>

So before your module.lds script:

# powerpc64-linux-readelf -eW fs/nfsd/nfsd.o | grep jump 

   [31] __jump_table      PROGBITS        0000000000000000 03edfe 0000f0 
00  WA  0   0  1
 

# powerpc64-linux-readelf -eW fs/nfsd/nfsd.ko | grep jump 

   [32] __jump_table      PROGBITS        0000000000000000 044046 0000f0 
00  WA  0   0  1

With your patch:
 
 
 

# powerpc64-linux-readelf -eW fs/nfsd/nfsd.o | grep jump 

   [31] __jump_table      PROGBITS        0000000000000000 03edfe 0000f0 
00  WA  0   0  1
 

# powerpc64-linux-readelf -eW fs/nfsd/nfsd.ko | grep jump 

   [18] __jump_table      PROGBITS        0000000000000000 03e358 0000f0 
00  WA  0   0  8


I also checked all the other .ko files and they were properly aligned. 
So I think this should hopefully work, and I like that its not a 
per-arch fix.

Sachin, sorry to bother you again, but I'm hoping you can try David's 
latest patch to scripts/module-common.lds, just to test in your setup.

Thanks,

-Jason