Re: [PATCH][V2] ovl: fix lookup failure on multi lower squashfs

[Miklos Szeredi <miklos@szeredi.hu>]

On Wed, Nov 13, 2019 at 5:34 PM Amir Goldstein <amir73il@gmail.com> wrote:
>
> On Wed, Nov 13, 2019 at 6:02 PM Miklos Szeredi <miklos@szeredi.hu> wrote:
> >
> > On Thu, Nov 7, 2019 at 11:50 AM Colin King <colin.king@canonical.com> wrote:
> > >
> > > From: Colin Ian King <colin.king@canonical.com>
> > >
> > > In the past, overlayfs required that lower fs have non null uuid in
> > > order to support nfs export and decode copy up origin file handles.
> > >
> > > Commit 9df085f3c9a2 ("ovl: relax requirement for non null uuid of
> > > lower fs") relaxed this requirement for nfs export support, as long
> > > as uuid (even if null) is unique among all lower fs.
> > >
> > > However, said commit unintentionally also relaxed the non null uuid
> > > requirement for decoding copy up origin file handles, regardless of
> > > the unique uuid requirement.
> > >
> > > Amend this mistake by disabling decoding of copy up origin file handle
> > > from lower fs with a conflicting uuid.
> > >
> > > We still encode copy up origin file handles from those fs, because
> > > file handles like those already exist in the wild and because they
> > > might provide useful information in the future.
> > >
> > > Reported-by: Colin Ian King <colin.king@canonical.com>
> > > Link: https://lore.kernel.org/lkml/20191106234301.283006-1-colin.king@canonical.com/
> > > Fixes: 9df085f3c9a2 ("ovl: relax requirement for non null uuid ...")
> > > Cc: stable@vger.kernel.org # v4.20+
> > > Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> > > Signed-off-by: Colin Ian King <colin.king@canonical.com>
> > > ---
> > >  fs/overlayfs/namei.c     |  8 ++++++++
> > >  fs/overlayfs/ovl_entry.h |  2 ++
> > >  fs/overlayfs/super.c     | 16 ++++++++++------
> > >  3 files changed, 20 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
> > > index e9717c2f7d45..f47c591402d7 100644
> > > --- a/fs/overlayfs/namei.c
> > > +++ b/fs/overlayfs/namei.c
> > > @@ -325,6 +325,14 @@ int ovl_check_origin_fh(struct ovl_fs *ofs, struct ovl_fh *fh, bool connected,
> > >         int i;
> > >
> > >         for (i = 0; i < ofs->numlower; i++) {
> > > +               /*
> > > +                * If lower fs uuid is not unique among lower fs we cannot match
> > > +                * fh->uuid to layer.
> > > +                */
> > > +               if (ofs->lower_layers[i].fsid &&
> > > +                   ofs->lower_layers[i].fs->bad_uuid)
> > > +                       continue;
> > > +
> > >                 origin = ovl_decode_real_fh(fh, ofs->lower_layers[i].mnt,
> > >                                             connected);
> > >                 if (origin)
> > > diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
> > > index a8279280e88d..28348c44ea5b 100644
> > > --- a/fs/overlayfs/ovl_entry.h
> > > +++ b/fs/overlayfs/ovl_entry.h
> > > @@ -22,6 +22,8 @@ struct ovl_config {
> > >  struct ovl_sb {
> > >         struct super_block *sb;
> > >         dev_t pseudo_dev;
> > > +       /* Unusable (conflicting) uuid */
> > > +       bool bad_uuid;
> > >  };
> > >
> > >  struct ovl_layer {
> > > diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> > > index afbcb116a7f1..5d4faab57ba0 100644
> > > --- a/fs/overlayfs/super.c
> > > +++ b/fs/overlayfs/super.c
> > > @@ -1255,17 +1255,18 @@ static bool ovl_lower_uuid_ok(struct ovl_fs *ofs, const uuid_t *uuid)
> > >  {
> > >         unsigned int i;
> > >
> > > -       if (!ofs->config.nfs_export && !(ofs->config.index && ofs->upper_mnt))
> > > -               return true;
> > > -
>
> Colin, I mislead you, this should be (I think):
>
>        if (!ofs->config.nfs_export && !ofs->upper_mnt)
>                return true;
>
> > >         for (i = 0; i < ofs->numlowerfs; i++) {
> > >                 /*
> > >                  * We use uuid to associate an overlay lower file handle with a
> > >                  * lower layer, so we can accept lower fs with null uuid as long
> > >                  * as all lower layers with null uuid are on the same fs.
> > > +                * if we detect multiple lower fs with the same uuid, we
> > > +                * disable lower file handle decoding on all of them.
> > >                  */
> > > -               if (uuid_equal(&ofs->lower_fs[i].sb->s_uuid, uuid))
> > > +               if (uuid_equal(&ofs->lower_fs[i].sb->s_uuid, uuid)) {
> > > +                       ofs->lower_fs[i].bad_uuid = true;
> > >                         return false;
> > > +               }
> > >         }
> > >         return true;
> > >  }
> > > @@ -1277,6 +1278,7 @@ static int ovl_get_fsid(struct ovl_fs *ofs, const struct path *path)
> > >         unsigned int i;
> > >         dev_t dev;
> > >         int err;
> > > +       bool bad_uuid = false;
> > >
> > >         /* fsid 0 is reserved for upper fs even with non upper overlay */
> > >         if (ofs->upper_mnt && ofs->upper_mnt->mnt_sb == sb)
> > > @@ -1287,10 +1289,11 @@ static int ovl_get_fsid(struct ovl_fs *ofs, const struct path *path)
> > >                         return i + 1;
> > >         }
> > >
> > > -       if (!ovl_lower_uuid_ok(ofs, &sb->s_uuid)) {
> > > +       if (ofs->upper_mnt && !ovl_lower_uuid_ok(ofs, &sb->s_uuid)) {
> >
> > This seems bogus: why only check conflicting lower layers if there's
> > an upper layer?
>
> It is bogus - it was my (wrong) suggestion.
> The thinking was that we only decode origin fh if we have an upper layer
> and index only valid with upper layer.
> I forgot the case of nfs_export and lower-only setup.
> Suggested fix above.
>
> >
> > > +               bad_uuid = true;
> > >                 ofs->config.index = false;
> > >                 ofs->config.nfs_export = false;
> > > -               pr_warn("overlayfs: %s uuid detected in lower fs '%pd2', falling back to index=off,nfs_export=off.\n",
> > > +               pr_warn("overlayfs: %s uuid detected in lower fs '%pd2', enforcing index=off,nfs_export=off.\n",
> >
> > And this while this makes sense, it doesn't really fit into this patch
> > (no change of behavior regarding how index and nfs_export are
> > handled).
> >
>
> Again, this was my (not wrong?) suggestion.
> What this patch changes is that ovl_lower_uuid_ok() can now return false
> and we get to this print although user did not ask for index nor nfs_export.
> So the "falling back" language no longer makes sense.

But does "enforcing" makes sense in this light?  That's not what the
detected bad_uuid condition is about, it's about failing to utilize
origin markings to make inode numbers persistent for filesystems that
have null uuid.   Is that correct?   Can we do a message that makes
that somewhat more clearer?

Thanks,
Miklos