linux-kernel - Re: [RFC v4 07/18] objtool: Introduce INSN

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Message-ID: <3c4e3227-eeb3-371a-d015-a0e0e60e5332@gmail.com>
Date:   Thu, 22 Aug 2019 21:45:00 +0100
From:   Julien <julien.thierry.kdev@...il.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Raphael Gault <raphael.gault@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        peterz@...radead.org, catalin.marinas@....com, will.deacon@....com,
        raph.gault+kdev@...il.com
Subject: Re: [RFC v4 07/18] objtool: Introduce INSN_UNKNOWN type

Hi Josh,

On 22/08/19 21:04, Josh Poimboeuf wrote:
> On Fri, Aug 16, 2019 at 01:23:52PM +0100, Raphael Gault wrote:
>> On arm64 some object files contain data stored in the .text section.
>> This data is interpreted by objtool as instruction but can't be
>> identified as a valid one. In order to keep analysing those files we
>> introduce INSN_UNKNOWN type. The "unknown instruction" warning will thus
>> only be raised if such instructions are uncountered while validating an
>> execution branch.
>>
>> This change doesn't impact the x86 decoding logic since 0 is still used
>> as a way to specify an unknown type, raising the "unknown instruction"
>> warning during the decoding phase still.
>>
>> Signed-off-by: Raphael Gault <raphael.gault@....com>
> 
> Is there a reason such data can't be moved to .rodata?  That would seem
> like the proper fix.
> 

Raphaël can confirm, if I remember correctly, that issue was encountered 
on assembly files implementing crypto algorithms were some 
words/double-words of data were in the middle of the .text. I think it 
is done this way to make sure the data can be loaded in a single 
instruction. So moving it to another section could impact the crypto 
performance depending on the relocations.

That was my understanding at least.

Cheers,

-- 
Julien Thierry