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

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Date:   Mon, 10 Dec 2018 16:10:31 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Dave Jiang <dave.jiang@...el.com>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        "Zhang, Yu C" <yu.c.zhang@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>, zwisler@...nel.org,
        Jan Kara <jack@...e.cz>, "Zhang, Yi Z" <yi.z.zhang@...el.com>
Subject: Re: [PATCH V2 1/1] device-dax: check for vma range while dax_mmap.

On Tue, Aug 21, 2018 at 12:38 AM Yi Zhang <yi.z.zhang@...ux.intel.com> 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@...ux.intel.com>
> > >> ---
> > >>  drivers/dax/device.c | 29 +++++++++++++++++++++++++++++
> > >>  1 file changed, 29 insertions(+)
> > >>
> > >
> > > Looks good to me:
> > > Reviewed-by: Vishal Verma <vishal.l.verma@...el.com>
> >
> > 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.

Could you be clearer about why this is a problem? The fault sounds
like the correct result.