Re: [PATCH V2 1/1] device-dax: check for vma range while dax_mmap.

[Yi Zhang]

On 2018-12-10 at 16:10:31 -0800, Dan Williams wrote:
> On Tue, Aug 21, 2018 at 12:38 AM Yi Zhang <yi.z.zhang@xxxxxxxxxxxxxxx> wrote:
> >
> > On 2018-08-20 at 12:50:31 -0700, Dave Jiang wrote:
> > >
> > >
> > > On 08/20/2018 10:53 AM, Verma, Vishal L wrote:
> > > >
> > > > On Mon, 2018-08-13 at 20:02 +0800, Zhang Yi wrote:
> > > >> This patch prevents a user mapping an illegal vma range that is larger
> > > >> than a dax device physical resource.
> > > >>
> > > >> When qemu maps the dax device for virtual nvdimm's backend device, the
> > > >> v-nvdimm label area is defined at the end of mapped range. By using an
> > > >> illegal size that exceeds the range of the device dax, it will trigger a
> > > >> fault with qemu.
> > > >>
> > > >> Signed-off-by: Zhang Yi <yi.z.zhang@xxxxxxxxxxxxxxx>
> > > >> ---
> > > >>  drivers/dax/device.c | 29 +++++++++++++++++++++++++++++
> > > >>  1 file changed, 29 insertions(+)
> > > >>
> > > >
> > > > Looks good to me:
> > > > Reviewed-by: Vishal Verma <vishal.l.verma@xxxxxxxxx>
> > >
> > > Applied.
> > Thanks Dava and Vishal's kindly review. Thank you.
> 
> So, it turns out this patch did not get merged for 4.20. I fumbled it
> when returning from vacation. However, I'm not sure it is needed. As
> long as attempts to access the out-of-range capacity results in SIGBUS
> then the implementation is correct. This is similar to the case where
> a file is truncated after the vma is established. That size is
> validated at fault time.
The problem is that we didn't get the fault at we initial the mapping
until attempt to access it, then qemu will failed unexpect without any
output, I think is is better to mention user that we are starting at a 
illegal size, but not faulting at an uncertained time.
> 
> Could you be clearer about why this is a problem? The fault sounds
> like the correct result.
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