linux-kernel - Re: a question about protection

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Date:	Tue, 12 Jul 2016 04:46:50 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Xishi Qiu <qiuxishi@...wei.com>
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 Tue, Jul 12, 2016 at 09:31:30AM +0800, Xishi Qiu wrote:
> 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?

Yes. See wp_page_reuse().

-- 
 Kirill A. Shutemov