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

[Amir Goldstein]

On Wed, Nov 13, 2019 at 6:02 PM Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
>
> On Thu, Nov 7, 2019 at 11:50 AM Colin King <colin.king@xxxxxxxxxxxxx> wrote:
> >
> > From: Colin Ian King <colin.king@xxxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxx>
> > Link: https://lore.kernel.org/lkml/20191106234301.283006-1-colin.king@xxxxxxxxxxxxx/
> > Fixes: 9df085f3c9a2 ("ovl: relax requirement for non null uuid ...")
> > Cc: stable@xxxxxxxxxxxxxxx # v4.20+
> > Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
> > Signed-off-by: Colin Ian King <colin.king@xxxxxxxxxxxxx>
> > ---
> >  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.

Thanks,
Amir.