linux-kernel - Re: [RFC 5/8] x86: refcount: prevent gcc distortions

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Date:   Thu, 17 May 2018 01:18:21 -0600
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "Nadav Amit" <namit@...are.com>
Cc:     <keescook@...omium.org>,
        "the arch/x86 maintainers" <x86@...nel.org>, <tglx@...utronix.de>,
        "Josh Poimboeuf" <jpoimboe@...hat.com>, <mingo@...hat.com>,
        <linux-kernel@...r.kernel.org>, <hpa@...or.com>
Subject: Re: [RFC 5/8] x86: refcount: prevent gcc distortions

>>> On 16.05.18 at 18:44, <namit@...are.com> wrote:
> Jan Beulich <JBeulich@...e.com> wrote:
>>>>> On 15.05.18 at 16:11, <namit@...are.com> wrote:
>>> --- a/arch/x86/include/asm/refcount.h
>>> +++ b/arch/x86/include/asm/refcount.h
>>> @@ -14,34 +14,43 @@
>>>  * central refcount exception. The fixup address for the exception points
>>>  * back to the regular execution flow in .text.
>>>  */
>>> -#define _REFCOUNT_EXCEPTION				\
>>> -	".pushsection .text..refcount\n"		\
>>> -	"111:\tlea %[counter], %%" _ASM_CX "\n"		\
>>> -	"112:\t" ASM_UD2 "\n"				\
>>> -	ASM_UNREACHABLE					\
>>> -	".popsection\n"					\
>>> -	"113:\n"					\
>>> +
>>> +asm ("\n"
>>> +	".macro __REFCOUNT_EXCEPTION counter:vararg\n\t"
>>> +	".pushsection .text..refcount\n"
>>> +	"111:\tlea \\counter, %" _ASM_CX "\n"
>>> +	"112:\t" ASM_UD2 "\n\t"
>>> +	ASM_UNREACHABLE
>>> +	".popsection\n\t"
>>> +	"113:\n"
>>> 	_ASM_EXTABLE_REFCOUNT(112b, 113b)
>>> +	".endm");
>> 
>> A few comments on assembly code formatting - while gas at present is
>> relatively lax in this regard, I wouldn't exclude that there might be a
>> more strict mode in the future, and that such a mode might eventually
>> become the default. Furthermore these formatting aspects affect
>> readability of the assembly produced, should anyone ever find a need
>> to look at it (perhaps because of some breakage) - I certainly do every
>> once in a while.
>> 
>> Labels should be placed without any indentation (but of course there
>> may be more than one on a line, in which case subsequent ones may
>> of course be arbitrarily indented). Instructions and directives, otoh,
>> should be placed with at least a single tab or space of indentation
>> (unless preceded by a label, in which case the extra white space still
>> helps readability).
> 
> Writing these patches, I looked at the generated assembly, and there did not
> appear to be a standard. IIRC, .pushsection directives were not always
> inlined. I will fix it according to your comments.

Right, I should have made explicit that there's no consistency at all in
pre-existing code. I merely think the issue shouldn't be made worse.

Jan