linux-kernel - Re: [PATCH 2/2] x86-64/Xen: eliminate W+X mappings

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Date:   Tue, 12 Dec 2017 03:48:56 -0700
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "Ingo Molnar" <mingo@...nel.org>
Cc:     <mingo@...e.hu>, <tglx@...utronix.de>,
        "xen-devel" <xen-devel@...ts.xenproject.org>,
        "Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
        "Juergen Gross" <jgross@...e.com>, <linux-kernel@...r.kernel.org>,
        <hpa@...or.com>
Subject: Re: [PATCH 2/2] x86-64/Xen: eliminate W+X mappings

>>> On 12.12.17 at 11:38, <mingo@...nel.org> wrote:
> * Jan Beulich <JBeulich@...e.com> wrote:
>> --- 4.15-rc3/arch/x86/xen/mmu_pv.c
>> +++ 4.15-rc3-x86_64-Xen-avoid-W+X/arch/x86/xen/mmu_pv.c
>> @@ -1902,6 +1902,16 @@ void __init xen_setup_kernel_pagetable(p
>>  	/* Graft it onto L4[511][510] */
>>  	copy_page(level2_kernel_pgt, l2);
>>  
>> +	/* Zap execute permission from the ident map. Due to the sharing of
>> +	 * L1 entries we need to do this in the L2. */
> 
> please use the customary (multi-line) comment style:
> 
>   /*
>    * Comment .....
>    * ...... goes here.
>    */
> 
> specified in Documentation/CodingStyle.

I would have but didn't because all other comments in this function
use this (wrong) style. I've concluded that consistency is better
here than matching the style doc. If the Xen maintainers tell me
otherwise, I'll happily adjust the patch.

>> +	if (__supported_pte_mask & _PAGE_NX)
>> +		for (i = 0; i < PTRS_PER_PMD; ++i) {
>> +			if (pmd_none(level2_ident_pgt[i]))
>> +				continue;
>> +			level2_ident_pgt[i] =
>> +				pmd_set_flags(level2_ident_pgt[i], _PAGE_NX);
> 
> So the line break here is quite distracting, especially considering how similar it 
> is to the alignment of the 'continue' statement. I.e. visually it looks like 
> control flow alignment.
> 
> Would be much better to just leave it a single page and ignore checkpatch 
> here.

Again I'll wait to see what the Xen maintainers think. I too dislike
line splits like this one, but the line ended up quite a bit too long,
not just a character or two. I also wasn't sure whether splitting
between the function arguments would be okay, leaving the first
line just slightly too long.

Jan