linux-kernel - Re: [PATCH] x86: Stop relying on magic jmp behavior for early_idt

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Message-ID: <555E7D3B.3070804@zytor.com>
Date:	Thu, 21 May 2015 17:50:03 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"H.J. Lu" <hjl.tools@...il.com>, Borislav Petkov <bp@...en8.de>,
	Jan Beulich <JBeulich@...e.com>,
	Binutils <binutils@...rceware.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: Stop relying on magic jmp behavior for early_idt_handlers

On 05/21/2015 05:38 PM, Andy Lutomirski wrote:
> On Thu, May 21, 2015 at 5:32 PM, H. Peter Anvin <hpa@...or.com> wrote:
>> On 05/21/2015 05:17 PM, Andy Lutomirski wrote:
>>>
>>> diff --git a/arch/x86/include/asm/segment.h b/arch/x86/include/asm/segment.h
>>> index 5a9856eb12ba..4bbe0eb24d7e 100644
>>> --- a/arch/x86/include/asm/segment.h
>>> +++ b/arch/x86/include/asm/segment.h
>>> @@ -231,12 +231,21 @@
>>>  #define TLS_SIZE                     (GDT_ENTRY_TLS_ENTRIES* 8)
>>>
>>>  #ifdef __KERNEL__
>>> -#ifndef __ASSEMBLY__
>>>
>>> -extern const char early_idt_handlers[NUM_EXCEPTION_VECTORS][2+2+5];
>>> +/*
>>> + * early_idt_handlers is an array of entry points.  For simplicity, it's
>>> + * a real array.  We allocate nine bytes for each entry: two one-byte
>>> + * push instructions and a five-byte jump in the worst case.
>>> + */
>>> +#define EARLY_IDT_HANDLER_STRIDE 9
>>> +#ifndef __ASSEMBLY__
>>> +extern const char early_idt_handlers[NUM_EXCEPTION_VECTORS][EARLY_IDT_HANDLER_STRIDE];
>>>  #ifdef CONFIG_TRACING
>>>  # define trace_early_idt_handlers early_idt_handlers
>>>  #endif
>>> +#endif
>>> +
>>> +#ifndef __ASSEMBLY__
>>>
>>
>> Please get rid of the unnecessary #ifndef/#endif pair.
> 
> I did it to preserve logical nesting.  I didn't want the pile of
> early_idt_handlers declarations and comments to have the side affect
> of starting an #ifdef block.
> 
> If you still think I should change it, I'll change it.
> 

Please change it.  We generally don't do that kind of logical nesting,
it is just clutter.

	-hpa

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