Re: HEADSUP: GEOM label may be broken [Was Re: svn commit: r361838 - in head/sys/geom: . label]

On Sat, Jun 6, 2020 at 2:17 AM Xin Li <delphij_at_delphij.net> wrote:

> I just spent quite some time to revive my laptop.  TL;DR: if you are
> using /dev/diskid or /dev/gptid labels and GELI, please wait until
> things settled.
>
> ===
>
> On 6/5/20 9:12 AM, Conrad Meyer wrote:
> > Author: cem
> > Date: Fri Jun  5 16:12:21 2020
> > New Revision: 361838
> > URL: https://svnweb.freebsd.org/changeset/base/361838
> >
> > Log:
> >   geom_label: Use provider aliasing to alias upstream geoms
> >
> >   For synthetic aliases (just pseudonyms inferred from metadata like GPT
> or
> >   UFS labels, GPT UUIDs, etc), use the GEOM provider aliasing system to
> create
> >   a symlink to the real device instead of creating an independent device.
> >   This makes it more clear which labels and devices correspond, and we
> can
> >   safely have multiple labels to a single device accessed at once.
> >
> >   The confusingly named geom_label on-disk construct continues to behave
> >   identically to how it did before.
> >
> >   This requires teaching GEOM's provider aliasing about the possibility
> >   that aliases might be added later in time, and GEOM's devfs interaction
> >   layer not to worry about existing aliases during retaste.
> >
> >   Discussed with:     imp
> >   Relnotes:   sure, if we don't end up reverting it
>
> This would break (and the effect can be quite persistent and hard to
> repair, see explanation below) existing configuration as some GEOM
> classes are not converted to support the new way of device
> representation, so I'd like to request this change be reverted for now
> until these are fixed.
>
> Consider the following configuration, where one have a hard drive
> partitioned with GPT, like:
>
> =>        40  1953525088  ada1  GPT  (932G)
>           40      262144     1  efi  (128M)
>       262184     8388608     2  freebsd-zfs  (4.0G)
>      8650792    67108864     3  freebsd-swap  (32G)
>     75759656  1877765472     4  freebsd-zfs  (895G)
>
> Now, the first ZFS pool is created as root pool.  ZFS gets an exclusive
> hold of 'ada1p2' despite the pool is carefully created to use
> /dev/diskid/<DISKSERIAL>p2 instead of ada1p2.
>
> ZFS writes the new device path to vdev label.  For ZFS, this doesn't
> matter much as it always checks the label.
>
> However, this will prevent GEOM from properly creating
> /dev/diskid/<DISKSERIAL>.
>
> In order to prevent accidentally writing data to wrong disk, for "raw"
> disk partitions it's usually a good idea to reference them with labels,
> either by /dev/diskid or /dev/gptid.  With /dev/ada1p2 exclusively
> accessed by ZFS, the /dev/diskid/<DISKSERIAL> representing the disk is
> now gone.
>
> And to make the situation even worse, simply changing the partition
> reference to the corresponding /dev/gptid/<uuid> doesn't really work
> either, when one is encrypting partitions individually.  In the example
> above, adap4 is an encrypted partition and with the alias change, one
> can no longer "geli attach" them via /dev/gptid/<uuid>.  They can still
> be attached by the "canonical" path (/dev/ada1p4), but for the swap
> partition that would completely defeat the purpose of using label (to
> prevent accidentally writing to the wrong disk).
>
> For my case, reverting to an older kernel is not sufficient to fix the
> configuration, because ZFS is recording the "canonical" device path
> (/dev/ada1p2) and /dev/diskid label for this disk disappeared somewhat
> permanently (I would have to find a USB drive somewhere to fix the root
> pool to use the "right" device path).
>
> >   Differential Revision:      https://reviews.freebsd.org/D24968
> [...]
> > Modified: head/sys/geom/label/g_label.c
> >
> ==============================================================================
> > --- head/sys/geom/label/g_label.c     Fri Jun  5 16:05:09 2020
> (r361837)
> > +++ head/sys/geom/label/g_label.c     Fri Jun  5 16:12:21 2020
> (r361838)
> > _at__at_ -344,18 +345,16 _at__at_ g_label_taste(struct g_class *mp, struct
> g_provider *p
> >  {
> >       struct g_label_metadata md;
> >       struct g_consumer *cp;
> > +     struct g_class *clsp;
> >       struct g_geom *gp;
> >       int i;
> > +     bool changed;
> >
> >       g_trace(G_T_TOPOLOGY, "%s(%s, %s)", __func__, mp->name, pp->name);
> >       g_topology_assert();
> >
> >       G_LABEL_DEBUG(2, "Tasting %s.", pp->name);
> >
> > -     /* Skip providers that are already open for writing. */
> > -     if (pp->acw > 0)
> > -             return (NULL);
> > -
> >       if (strcmp(pp->geom->class->name, mp->name) == 0)
> >               return (NULL);
> >
> > _at__at_ -391,9 +390,16 _at__at_ g_label_taste(struct g_class *mp, struct g_provider
> *p
> >               if (md.md_provsize != pp->mediasize)
> >                       break;
> >
> > +             /* Skip providers that are already open for writing. */
> > +             if (pp->acw > 0) {
> > +                     g_access(cp, -1, 0, 0);
> > +                     goto end;
> > +             }
> > +
>
> (Is this still necessary when the eventual provider would be the real one?)
>
> I think symlink aliasing is a the right direction but we need to be
> really careful to not break existing and legitimate usage.  Since this
> also breaks GELI when using with labels, I'd like to request that this
> change be backed out for now until the consumers are correctly fixed.
>

These were the sorts of concerns I had when talking to cem_at_, but in his
defense I wasn't able to come up with a concrete example so we convinced
ourselves that my worry wasn't real. In light of this new data, I agree we
should revert. This is complicated enough that we should have a couple of
'gotta work' use cases and make sure those work before we redo. There's a
big reason I only did it for the 'nvd compat for nda' use case: I had what
I thought was an irrational fear of sorting out this mess... Guess it turns
out to be rational. :(

Thanks Xin for the excellent explanation and example.

Warner