Re: [PATCH 06/18] fsinfo: Add a uniquifier ID to struct mount [ver #21]

[Ian Kent]

On Tue, 2020-08-04 at 12:41 +0200, Miklos Szeredi wrote:
> On Mon, Aug 03, 2020 at 02:37:16PM +0100, David Howells wrote:
> > Add a uniquifier ID to struct mount that is effectively unique over
> > the
> > kernel lifetime to deal around mnt_id values being reused.  This
> > can then
> > be exported through fsinfo() to allow detection of replacement
> > mounts that
> > happen to end up with the same mount ID.
> > 
> > The normal mount handle is still used for referring to a particular
> > mount.
> > 
> > The mount notification is then changed to convey these unique mount
> > IDs
> > rather than the mount handle.
> > 
> > Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> > ---
> > 
> >  fs/mount.h        |    3 +++
> >  fs/mount_notify.c |    4 ++--
> >  fs/namespace.c    |    3 +++
> >  3 files changed, 8 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/mount.h b/fs/mount.h
> > index 85456a5f5a3a..1037781be055 100644
> > --- a/fs/mount.h
> > +++ b/fs/mount.h
> > @@ -79,6 +79,9 @@ struct mount {
> >  	int mnt_expiry_mark;		/* true if marked for
> > expiry */
> >  	struct hlist_head mnt_pins;
> >  	struct hlist_head mnt_stuck_children;
> > +#ifdef CONFIG_FSINFO
> > +	u64	mnt_unique_id;		/* ID unique over lifetime of
> > kernel */
> > +#endif
> 
> Not sure if it's worth making conditional.
> 
> >  #ifdef CONFIG_MOUNT_NOTIFICATIONS
> >  	struct watch_list *mnt_watchers; /* Watches on dentries within
> > this mount */
> >  #endif
> > diff --git a/fs/mount_notify.c b/fs/mount_notify.c
> > index 44f570e4cebe..d8ba66ed5f77 100644
> > --- a/fs/mount_notify.c
> > +++ b/fs/mount_notify.c
> > @@ -90,7 +90,7 @@ void notify_mount(struct mount *trigger,
> >  	n.watch.type	= WATCH_TYPE_MOUNT_NOTIFY;
> >  	n.watch.subtype	= subtype;
> >  	n.watch.info	= info_flags | watch_sizeof(n);
> > -	n.triggered_on	= trigger->mnt_id;
> > +	n.triggered_on	= trigger->mnt_unique_id;
> >  
> >  	switch (subtype) {
> >  	case NOTIFY_MOUNT_EXPIRY:
> > @@ -102,7 +102,7 @@ void notify_mount(struct mount *trigger,
> >  	case NOTIFY_MOUNT_UNMOUNT:
> >  	case NOTIFY_MOUNT_MOVE_FROM:
> >  	case NOTIFY_MOUNT_MOVE_TO:
> > -		n.auxiliary_mount	= aux->mnt_id;
> > +		n.auxiliary_mount = aux->mnt_unique_id;
> 
> Hmm, so we now have two ID's:
> 
>  - one can be used to look up the mount
>  - one is guaranteed to be unique
> 
> With this change the mount cannot be looked up with
> FSINFO_FLAGS_QUERY_MOUNT,
> right?
> 
> Should we be merging the two ID's into a single one which has both
> properties?

I'd been thinking we would probably need to change to 64 bit ids
for a while now and I thought that was what was going to happen.

We'll need to change libmount and current code but better early
on than later.

Ian

> 
> >  		break;
> >  
> >  	default:
> > diff --git a/fs/namespace.c b/fs/namespace.c
> > index b2b9920ffd3c..1db8a64cd76f 100644
> > --- a/fs/namespace.c
> > +++ b/fs/namespace.c
> > @@ -115,6 +115,9 @@ static int mnt_alloc_id(struct mount *mnt)
> >  	if (res < 0)
> >  		return res;
> >  	mnt->mnt_id = res;
> > +#ifdef CONFIG_FSINFO
> > +	mnt->mnt_unique_id = atomic64_inc_return(&vfs_unique_counter);
> > +#endif
> >  	return 0;
> >  }
> >  
> > 
> >