* a question about protection_map[]
@ 2016-07-11 10:12 Xishi Qiu
2016-07-11 13:30 ` Kirill A. Shutemov
0 siblings, 1 reply; 4+ messages in thread
From: Xishi Qiu @ 2016-07-11 10:12 UTC (permalink / raw)
To: alan; +Cc: Linux MM, LKML
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?
pgprot_t protection_map[16] = {
__P000, __P001, __P010, __P011, __P100, __P101, __P110, __P111,
__S000, __S001, __S010, __S011, __S100, __S101, __S110, __S111
};
#define __P001 PAGE_READONLY
#define __P010 PAGE_COPY
#define PAGE_READONLY __pgprot(_PAGE_PRESENT | _PAGE_USER | \
_PAGE_ACCESSED | _PAGE_NX)
#define PAGE_COPY_NOEXEC __pgprot(_PAGE_PRESENT | _PAGE_USER | \
_PAGE_ACCESSED | _PAGE_NX)
#define PAGE_COPY PAGE_COPY_NOEXEC
Thanks,
Xishi Qiu
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^ permalink raw reply [flat|nested] 4+ messages in thread* Re: a question about protection_map[]
2016-07-11 10:12 a question about protection_map[] Xishi Qiu
@ 2016-07-11 13:30 ` Kirill A. Shutemov
2016-07-12 1:31 ` Xishi Qiu
0 siblings, 1 reply; 4+ messages in thread
From: Kirill A. Shutemov @ 2016-07-11 13:30 UTC (permalink / raw)
To: Xishi Qiu; +Cc: alan, Linux MM, LKML
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.
--
Kirill A. Shutemov
--
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^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: a question about protection_map[]
2016-07-11 13:30 ` Kirill A. Shutemov
@ 2016-07-12 1:31 ` Xishi Qiu
2016-07-12 1:46 ` Kirill A. Shutemov
0 siblings, 1 reply; 4+ messages in thread
From: Xishi Qiu @ 2016-07-12 1:31 UTC (permalink / raw)
To: Kirill A. Shutemov; +Cc: alan, Linux MM, LKML
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
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^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: a question about protection_map[]
2016-07-12 1:31 ` Xishi Qiu
@ 2016-07-12 1:46 ` Kirill A. Shutemov
0 siblings, 0 replies; 4+ messages in thread
From: Kirill A. Shutemov @ 2016-07-12 1:46 UTC (permalink / raw)
To: Xishi Qiu; +Cc: alan, Linux MM, LKML
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
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^ permalink raw reply [flat|nested] 4+ messages in thread
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2016-07-11 10:12 a question about protection_map[] Xishi Qiu
2016-07-11 13:30 ` Kirill A. Shutemov
2016-07-12 1:31 ` Xishi Qiu
2016-07-12 1:46 ` Kirill A. Shutemov
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