linux-kernel - Re: a question about protection

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Date:	Tue, 12 Jul 2016 09:31:30 +0800
From:	Xishi Qiu <qiuxishi@...wei.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
CC:	<alan@...rguk.ukuu.org.uk>, Linux MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: a question about protection_map[]

On 2016/7/11 21:30, Kirill A. Shutemov wrote:

> On Mon, Jul 11, 2016 at 06:12:30PM +0800, Xishi Qiu wrote:
>> Hi,
>>
>> We can use mprotect to set read only or read/write.
>>
>> mprotect_fixup()
>> 	vma_set_page_prot()
>> 		vm_pgprot_modify()
>> 			vm_get_page_prot()
>> 				protection_map[vm_flags & (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)]
>>
>> The following code shows that prots from __P001(PROT_READ) and __P010(PROT_WRITE)
>> are the same, so how does it distinguish read only or read/write from mprotect?
> 
> It doesn't.
> 
> Write protection will be removed by fault handler on next write access to
> the page. Somewhat suboptiomal, but zero page implemenation relies on this
> to work properly.
> 

Hi Kirill,

I know, PAGE_READONLY and PAGE_COPY are both missed _PAGE_RW,
so it will cause page fault, then we will set new prot flag from
vma, right?

Thanks,
Xishi Qiu