Re: a question about protection_map[]

From: Xishi Qiu
Date: Mon Jul 11 2016 - 21:31:44 EST

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?

Xishi Qiu