linux-kernel - Re: [PATCH v3 16/16] objtool,x86: Rewrite retpoline thunk calls

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Message-ID: <6caa3fa0-f71c-1a3f-b944-57b518645e74@kernel.org>
Date:   Wed, 9 Jun 2021 08:56:04 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>,
        Lukasz Majczak <lma@...ihalf.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Fāng-ruì Sòng <maskray@...gle.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Guenter Roeck <groeck@...gle.com>,
        Juergen Gross <jgross@...e.com>,
        Łukasz Bartosik <lb@...ihalf.com>,
        LKML <linux-kernel@...r.kernel.org>, mbenes@...e.com,
        Radosław Biernacki <rad@...ihalf.com>,
        upstream@...ihalf.com,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: Re: [PATCH v3 16/16] objtool,x86: Rewrite retpoline thunk calls

On 6/9/2021 8:08 AM, Peter Zijlstra wrote:
> On Wed, Jun 09, 2021 at 02:23:28PM +0200, Lukasz Majczak wrote:
>> śr., 9 cze 2021 o 09:20 Peter Zijlstra <peterz@...radead.org> napisał(a):
>>>
>>> On Wed, Jun 09, 2021 at 09:11:18AM +0200, Lukasz Majczak wrote:
>>>
>>>> I'm sorry I was on vacation last week - do you still need the requested debugs?
>>>
>>> If the patch here:
>>>
>>>    https://lkml.kernel.org/r/YL3q1qFO9QIRL/BA@hirez.programming.kicks-ass.net
>>>
>>> does not fix things for you (don't think it actually will), then yes,
>>> please send me the information requested.
>>
>> Ok, it didn't help. Peter, Josh I have sent you a private email with
>> requested information.
> 
> OK, I think I've found it. Check this one:
> 
>   5d5:   0f 85 00 00 00 00       jne    5db <cpuidle_reflect+0x22>       5d7: R_X86_64_PLT32     __x86_indirect_thunk_r11-0x4
> 
> 
> +Relocation section '.rela.altinstructions' at offset 0 contains 14 entries:
> +    Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
> 
> +0000000000000018  0000000200000002 R_X86_64_PC32          0000000000000000 .text + 5d5
> +000000000000001c  0000009200000002 R_X86_64_PC32          0000000000000000 __x86_indirect_alt_call_r11 + 0
> 
> Apparently we get conditional branches to retpoline thunks and objtool
> completely messes that up. I'm betting this also explains the problems
> Nathan is having.

Yes, the below patch gets my kernel back to booting so it seems the root 
cause is the same.

> *groan*,.. not sure what to do about this, except return to having
> objtool generate code, which everybody hated on. For now I'll make it
> skip the conditional branches.
> 
> I wonder if the compiler will also generate conditional tail calls, and
> what that does with static_call... now I have to check all that.
> 
> ---

Tested-by: Nathan Chancellor <nathan@...nel.org>

> diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
> index 24295d39713b..523aa4157f80 100644
> --- a/tools/objtool/arch/x86/decode.c
> +++ b/tools/objtool/arch/x86/decode.c
> @@ -747,6 +747,10 @@ int arch_rewrite_retpolines(struct objtool_file *file)
>   
>   	list_for_each_entry(insn, &file->retpoline_call_list, call_node) {
>   
> +		if (insn->type != INSN_JUMP_DYNAMIC &&
> +		    insn->type != INSN_CALL_DYNAMIC)
> +			continue;
> +
>   		if (!strcmp(insn->sec->name, ".text.__x86.indirect_thunk"))
>   			continue;
>   
>