linux-kernel - Re: [v8,02/12] objtool: Allow alternatives to be ignored

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Message-ID: <0a35672a-5bec-d4b2-732b-805b09d7b39b@roeck-us.net>
Date:   Sun, 28 Jan 2018 17:26:05 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Andi Kleen <ak@...ux.intel.com>, Paul Turner <pjt@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>, tglx@...utronix.de,
        Kees Cook <keescook@...gle.com>,
        Rik van Riel <riel@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>, gnomes@...rguk.ukuu.org.uk,
        x86@...nel.org, thomas.lendacky@....com
Subject: Re: [v8,02/12] objtool: Allow alternatives to be ignored

On 01/28/2018 01:06 PM, Josh Poimboeuf wrote:
> On Mon, Jan 22, 2018 at 12:25:22PM -0800, Guenter Roeck wrote:
>> Hi David,
>>
>> On Mon, Jan 22, 2018 at 07:34:04PM +0000, David Woodhouse wrote:
>>> On Thu, 2018-01-18 at 11:41 -0800, Guenter Roeck wrote:
>>>>
>>>>> Not sure, does your gcc have retpolines?  Give me your .o file and I can
>>>>> diagnose it.
>>>>>   
>>>> Yes, it does, only it is the gcc from the Google toolchain which may
>>>> generate different code than the upstream version.
>>>>
>>>> I attached an affected object file. Please let me know if there is anything else
>>>> I can do to help.
>>> Disassembly of section .text.__x86.indirect_thunk:
>>>
>>> 0000000000000000 <__x86.indirect_thunk>:
>>>     0:	e8 04 00 00 00       	callq  9 <__x86.indirect_thunk+0x9>
>>>     5:	f3 90                	pause
>>>     7:	eb fc                	jmp    5 <__x86.indirect_thunk+0x5>
>>>     9:	48 8d 64 24 08       	lea    0x8(%rsp),%rsp
>>>     e:	c3                   	retq
>>>
>>> That has the old-style CET-incompatible retpoline in a COMDAT section
>>> in the .o file. What compiler options are being used for that? The
>>> kernel should only use retpoline if GCC supports both of
>>> -mindirect-branch=thunk-extern and -mindirect-branch-register, and this
>>> compiler is doing *neither* of those.
>>
>> It uses "-mindirect-branch=thunk -mindirect-branch-loop=pause
>> -fno-jump-tables", though I don't know if that even exists in
>> upstream gcc (it is the gcc use for Chrome OS builds). I'll pass
>> your feedback to our compiler team.
>>
>> Either case, I think it is less than optimal that objtool crashes
>> with _any_ object code.
> 
> I've got a pending fix for this, so that objtool doesn't seg fault, and
> instead prints out a warning:
> 
>    quirks.o: warning: objtool: efi_delete_dummy_variable()+0x99: unsupported intra-function call
>    quirks.o: warning: objtool: If this is a retpoline, please patch it in with alternatives and annotate it with ANNOTATE_NOSPEC_ALTERNATIVE.
> 
> The code is here, along with a few more fixes:
> 
>    https://git.kernel.org/pub/scm/linux/kernel/git/jpoimboe/linux.git/log/?h=TODO-objtool-seg-fault
> 

Excellent. I'll give it a try tomorrow.

Thanks for looking into this!

Guenter