linux-kernel - Re: [PATCH 04/24] x86/entry: Expose the address of .Lgs

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Message-ID: <1a33582b-63ef-581e-07da-3e3b970df4ab@linux.alibaba.com>
Date:   Thu, 2 Sep 2021 17:20:26 +0800
From:   Lai Jiangshan <laijs@...ux.alibaba.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 04/24] x86/entry: Expose the address of .Lgs_change to
 traps.c



On 2021/9/2 17:14, Peter Zijlstra wrote:
> On Wed, Sep 01, 2021 at 01:50:05AM +0800, Lai Jiangshan wrote:
> 
>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>> index e38a4cf795d9..9164e85b36b8 100644
>> --- a/arch/x86/entry/entry_64.S
>> +++ b/arch/x86/entry/entry_64.S
>> @@ -729,6 +729,7 @@ _ASM_NOKPROBE(common_interrupt_return)
>>   SYM_FUNC_START(asm_load_gs_index)
>>   	FRAME_BEGIN
>>   	swapgs
>> +SYM_INNER_LABEL(asm_load_gs_index_gs_change, SYM_L_GLOBAL)
>>   .Lgs_change:
> 
> After all the patches, there's only a single .Lgs_change reference left.
> Can't we convert the _ASM_EXTABLE() entry to use the new label and
> totally remove it?

The label .Lgs_change is still needed in ASM code for extable.

I tried the way as you suggested, and the result is:

warning: objtool: __ex_table+0x0: don't know how to handle non-section reloc symbol asm_load_gs_index_gs_change

But never mind, I have already converted load_gs_index into C code.
I will send it later.

> 
>>   	movl	%edi, %gs
>>   2:	ALTERNATIVE "", "mfence", X86_BUG_SWAPGS_FENCE
>> @@ -1011,7 +1012,7 @@ SYM_CODE_START_LOCAL(error_entry)
>>   	movl	%ecx, %eax			/* zero extend */
>>   	cmpq	%rax, RIP+8(%rsp)
>>   	je	.Lbstep_iret
>> -	cmpq	$.Lgs_change, RIP+8(%rsp)
>> +	cmpq	$asm_load_gs_index_gs_change, RIP+8(%rsp)
>>   	jne	.Lerror_entry_done_lfence