Re: [Xen-devel] [PATCH v4 8/9] xen/gntdev.c: Convert to use vm_map_pages()

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Tue, Jul 30, 2019 at 08:22:02PM +0530, Souptick Joarder wrote:
> On Tue, Jul 30, 2019 at 7:52 PM Marek Marczykowski-Górecki
> <marmarek@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Tue, Jul 30, 2019 at 10:05:42AM -0400, Boris Ostrovsky wrote:
> > > On 7/30/19 2:03 AM, Souptick Joarder wrote:
> > > > On Mon, Jul 29, 2019 at 7:06 PM Marek Marczykowski-Górecki
> > > > <marmarek@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> > > >> On Mon, Jul 29, 2019 at 02:02:54PM +0530, Souptick Joarder wrote:
> > > >>> On Mon, Jul 29, 2019 at 1:35 PM Souptick Joarder 
> > > >>> <jrdr.linux@xxxxxxxxx> wrote:
> > > >>>> On Sun, Jul 28, 2019 at 11:36 PM Marek Marczykowski-Górecki
> > > >>>> <marmarek@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> > > >>>>> On Fri, Feb 15, 2019 at 08:18:31AM +0530, Souptick Joarder wrote:
> > > >>>>>> Convert to use vm_map_pages() to map range of kernel
> > > >>>>>> memory to user vma.
> > > >>>>>>
> > > >>>>>> map->count is passed to vm_map_pages() and internal API
> > > >>>>>> verify map->count against count ( count = vma_pages(vma))
> > > >>>>>> for page array boundary overrun condition.
> > > >>>>> This commit breaks gntdev driver. If vma->vm_pgoff > 0, vm_map_pages
> > > >>>>> will:
> > > >>>>>  - use map->pages starting at vma->vm_pgoff instead of 0
> > > >>>> The actual code ignores vma->vm_pgoff > 0 scenario and mapped
> > > >>>> the entire map->pages[i]. Why the entire map->pages[i] needs to be 
> > > >>>> mapped
> > > >>>> if vma->vm_pgoff > 0 (in original code) ?
> > > >> vma->vm_pgoff is used as index passed to gntdev_find_map_index. It's
> > > >> basically (ab)using this parameter for "which grant reference to map".
> > > >>
> > > >>>> are you referring to set vma->vm_pgoff = 0 irrespective of value 
> > > >>>> passed
> > > >>>> from user space ? If yes, using vm_map_pages_zero() is an alternate
> > > >>>> option.
> > > >> Yes, that should work.
> > > > I prefer to use vm_map_pages_zero() to resolve both the issues. 
> > > > Alternatively
> > > > the patch can be reverted as you suggested. Let me know you opinion and 
> > > > wait
> > > > for feedback from others.
> > > >
> > > > Boris, would you like to give any feedback ?
> > >
> > > vm_map_pages_zero() looks good to me. Marek, does it work for you?
> >
> > Yes, replacing vm_map_pages() with vm_map_pages_zero() fixes the
> > problem for me.
> 
> Marek, I can send a patch for the same if you are ok.
> We need to cc stable as this changes are available in 5.2.4.

Sounds good, thanks!

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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