RE: [PATCH] ovl: do not set overlay.opaque for new directories

["Yurkov, Vyacheslav" <Vyacheslav.Yurkov@bruker.com>]

Hi Amir,
Thanks for the follow-up.

Yes, this is existing problem I solved with a proposed patch on our devices. Of course having a proper solution would be better.
The only option I see enabled in my config is CONFIG_OVERLAY_FS_REDIRECT_ALWAYS_FOLLOW.

Took me some time to backport proposed changes to my kernel, but they worked fine. Will send a v2 shortly

The


- confidential -

> -----Original Message-----
> From: Amir Goldstein <amir73il@gmail.com>
> Sent: Tuesday, May 25, 2021 10:32
> To: Yurkov, Vyacheslav <Vyacheslav.Yurkov@bruker.com>
> Cc: Miklos Szeredi <miklos@szeredi.hu>; overlayfs <linux-
> unionfs@vger.kernel.org>
> Subject: Re: [PATCH] ovl: do not set overlay.opaque for new directories
> 
> **EXTERNAL EMAIL**
> 
> On Mon, May 3, 2021 at 9:43 PM Vyacheslav Yurkov
> <Vyacheslav.Yurkov@bruker.com> wrote:
> >
> > This optimization breaks existing use case when a lower layer directory
> > appears after directory was created on a merged layer. If overlay.opaque
> > is applied, new files on lower layer are not visible.
> >
> > Consider the following scenario:
> > - /lower and /upper are mounted to /merged
> > - directory /merged/new-dir is created with a file test1
> > - overlay is unmounted
> > - directory /lower/new-dir is created with a file test2
> > - overlay is mounted again
> >
> > If opaque is applied by default, file test2 is not going to be visible
> > without explicitly clearing the overlay.opaque attribute
> >
> > Signed-off-by: Vyacheslav Yurkov <Vyacheslav.Yurkov@bruker.com>
> 
> Hi Vyacheslav,
> 
> Sorry for the late reply.
> Is the described regression really happening in real deployments?
> I would like to avoid removing the optimization if possible.
> 
> In any case, if we have to support existing deployments that use this
> practice,
> the optimization should be removed only for the case where the user did not
> opt-in to any of the new features, quoting overlayfs.rst:
> '
>   Offline changes to the lower tree are only allowed if the
>   "metadata only copy up", "inode index", "xino" and "redirect_dir" features
>   have not been used. If the lower tree is modified and any of these
>   features has been used, the behavior of the overlay is undefined,
>   though it will not result in a crash or deadlock.
> '
> 
> This means putting this check from ovl_lower_uuid_ok() into a helper:
> 
> static inline bool ovl_allow_offline_changes(struct ovl_fs *ofs)
> {
>         /*
>          * To avoid regressions in existing setups with overlay lower offline
>          * changes, we allow lower changes only if none of the new features
>          * are used.
>          */
>         return (!ofs->config.index && !ofs->config.metacopy &&
>                     !ofs>config.redirect_dir && ofs->config.xino !=
> OVL_XINO_ON);
> }
> 
> Note that ovl_lower_uuid_ok() does not currently check the redirect_dir
> feature, but it would be better to use the same helper in that case as well.
> 
> Thanks,
> Amir.
> 
> > ---
> >  fs/overlayfs/dir.c | 5 -----
> >  1 file changed, 5 deletions(-)
> >
> > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > index 93efe7048a77..f66f96dd9f0c 100644
> > --- a/fs/overlayfs/dir.c
> > +++ b/fs/overlayfs/dir.c
> > @@ -338,11 +338,6 @@ static int ovl_create_upper(struct dentry *dentry,
> struct inode *inode,
> >         if (IS_ERR(newdentry))
> >                 goto out_unlock;
> >
> > -       if (ovl_type_merge(dentry->d_parent) && d_is_dir(newdentry)) {
> > -               /* Setting opaque here is just an optimization, allow to fail */
> > -               ovl_set_opaque(dentry, newdentry);
> > -       }
> > -
> >         err = ovl_instantiate(dentry, inode, newdentry, !!attr->hardlink);
> >         if (err)
> >                 goto out_cleanup;
> > --
> > 2.25.1
> >