Re: [RFC][PATCH 4/4] fanotify: support reporting non-decodeable file handles

[Jan Kara <jack@suse.cz>]

On Thu 27-04-23 15:28:23, Amir Goldstein wrote:
> s_export_op
> 
> On Thu, Apr 27, 2023 at 2:48 PM Jan Kara <jack@suse.cz> wrote:
> >
> > On Tue 25-04-23 16:01:05, Amir Goldstein wrote:
> > > fanotify users do not always need to decode the file handles reported
> > > with FAN_REPORT_FID.
> > >
> > > Relax the restriction that filesystem needs to support NFS export and
> > > allow reporting file handles from filesystems that only support ecoding
> > > unique file handles.
> > >
> > > For such filesystems, users will have to use the AT_HANDLE_FID of
> > > name_to_handle_at(2) if they want to compare the object in path to the
> > > object fid reported in an event.
> > >
> > > Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> > ...
> > > diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
> > > index 8f430bfad487..a5af84cbb30d 100644
> > > --- a/fs/notify/fanotify/fanotify_user.c
> > > +++ b/fs/notify/fanotify/fanotify_user.c
> > > @@ -1586,11 +1586,9 @@ static int fanotify_test_fid(struct dentry *dentry)
> > >        * We need to make sure that the file system supports at least
> > >        * encoding a file handle so user can use name_to_handle_at() to
> > >        * compare fid returned with event to the file handle of watched
> > > -      * objects. However, name_to_handle_at() requires that the
> > > -      * filesystem also supports decoding file handles.
> > > +      * objects, but it does not need to support decoding file handles.
> > >        */
> > > -     if (!dentry->d_sb->s_export_op ||
> > > -         !dentry->d_sb->s_export_op->fh_to_dentry)
> > > +     if (!dentry->d_sb->s_export_op)
> > >               return -EOPNOTSUPP;
> >
> > So AFAICS the only thing you require is that s_export_op is set to
> > *something* as exportfs_encode_inode_fh() can deal with NULL ->encode_fh
> > just fine without any filesystem cooperation. What is the reasoning behind
> > the dentry->d_sb->s_export_op check? Is there an implicit expectation that
> > if s_export_op is set to something, the filesystem has sensible
> > i_generation? Or is it just a caution that you don't want the functionality
> > to be enabled for unexpected filesystems?
> 
> A little bit of both.
> Essentially, I do not want to use the generic encoding unless the filesystem
> opted-in to say "This is how objects should be identified".
> 
> The current fs that have s_export_op && !s_export_op->encode_fh
> practically make that statement because they support NFS export
> (i.e. they implement fh_to_dentry()).

Makes sense.

> I don't like the implicit fallback to generic encoding, especially when
> introducing this new functionality of encode_fid().
> 
> Before posting this patch set I had two earlier revisions.
> One that changed the encode_fh() to mandatory and converted
> all the INO32_GEN fs to explicitly set
> s_export_op.encode_fh = generic_encode_ino32_fh,
> And one that marked all the INO32_GEN fs with
> s_export_op.flags = EXPORT_OP_ENCODE_INO32_GEN
> in both cases there was no blind fallback to INO32_GEN.
> 
> But in the end, these added noise without actual value so
> I dropped them, because the d_sb->s_export_op check is anyway
> a pretty strong indication for opt-in to export fids.
> 
> CC exportfs maintainers in case they have an opinion one
> way or the other.

Yeah, I agree with your judgement. I just wanted to make sure I understand
everything properly.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR