Re: [PATCH v10 12/13] libnvdimm/pfn: Fix fsdax-mode namespace info-block zero-fields

[Dan Williams <dan.j.williams@intel.com>]

On Wed, Jun 19, 2019 at 9:30 AM Aneesh Kumar K.V
<aneesh.kumar@linux.ibm.com> wrote:
>
> Dan Williams <dan.j.williams@intel.com> writes:
>
> > At namespace creation time there is the potential for the "expected to
> > be zero" fields of a 'pfn' info-block to be filled with indeterminate
> > data. While the kernel buffer is zeroed on allocation it is immediately
> > overwritten by nd_pfn_validate() filling it with the current contents of
> > the on-media info-block location. For fields like, 'flags' and the
> > 'padding' it potentially means that future implementations can not rely
> > on those fields being zero.
> >
> > In preparation to stop using the 'start_pad' and 'end_trunc' fields for
> > section alignment, arrange for fields that are not explicitly
> > initialized to be guaranteed zero. Bump the minor version to indicate it
> > is safe to assume the 'padding' and 'flags' are zero. Otherwise, this
> > corruption is expected to benign since all other critical fields are
> > explicitly initialized.
> >
> > Note The cc: stable is about spreading this new policy to as many
> > kernels as possible not fixing an issue in those kernels. It is not
> > until the change titled "libnvdimm/pfn: Stop padding pmem namespaces to
> > section alignment" where this improper initialization becomes a problem.
> > So if someone decides to backport "libnvdimm/pfn: Stop padding pmem
> > namespaces to section alignment" (which is not tagged for stable), make
> > sure this pre-requisite is flagged.
>
> Don't we need a change like below in this patch?
>
> modified   drivers/nvdimm/pfn_devs.c
> @@ -452,10 +452,11 @@ int nd_pfn_validate(struct nd_pfn *nd_pfn, const char *sig)
>         if (memcmp(pfn_sb->parent_uuid, parent_uuid, 16) != 0)
>                 return -ENODEV;
>
> -       if (__le16_to_cpu(pfn_sb->version_minor) < 1) {
> -               pfn_sb->start_pad = 0;
> -               pfn_sb->end_trunc = 0;
> -       }
> +       if ((__le16_to_cpu(pfn_sb->version_minor) < 1) ||
> +           (__le16_to_cpu(pfn_sb->version_minor) >= 3)) {
> +                       pfn_sb->start_pad = 0;
> +                       pfn_sb->end_trunc = 0;
> +               }

No, this kills off start_pad and end_trunc permanently.

> IIUC we want to force the start_pad and end_truc to zero if the pfn_sb
> minor version number >= 3. So once we have this patch backported and
> older kernel finds a pfn_sb with minor version 3, it will ignore the
> start_pad read from the nvdimm and overwrite that with zero here.
> This patch doesn't enforce that right? After the next patch we can have
> values other than 0 in pfn_sb->start_pad?

The reason for the version bump is for the kernel to safely assume
that uninitialized fields default to zero, but it's otherwise a nop
when the implementation is explicitly initializing every field by
default.