[Virtio-fs] [PATCH 2/4] fuse: Fix infinite loop in sget_fc()

[Greg Kurz]

On Thu, 27 May 2021 12:08:36 +0200
Max Reitz <mreitz at redhat.com> wrote:

> On 25.05.21 17:02, Greg Kurz wrote:
> > We don't set the SB_BORN flag on submounts. This is wrong as these
> > superblocks are then considered as partially constructed or dying
> > in the rest of the code and can break some assumptions.
> > 
> > One such case is when you have a virtiofs filesystem with submounts
> > and you try to mount it again : virtio_fs_get_tree() tries to obtain
> > a superblock with sget_fc(). The logic in sget_fc() is to loop until
> > it has either found an existing matching superblock with SB_BORN set
> > or to create a brand new one. It is assumed that a superblock without
> > SB_BORN is transient and should go away. Forgetting to set SB_BORN on
> > submounts hence causes sget_fc() to retry forever.
> > 
> > Setting SB_BORN requires special care, i.e. a write barrier for
> > super_cache_count() which can check SB_BORN without taking any lock.
> > We should call vfs_get_tree() to deal with that but this requires
> > to have a proper ->get_tree() implementation for submounts, which
> > is a bigger piece of work. Go for a simple bug fix in the meatime.
> > 
> > Fixes: bf109c64040f ("fuse: implement crossmounts")
> > Cc: mreitz at redhat.com
> > Cc: stable at vger.kernel.org # v5.10+
> > Signed-off-by: Greg Kurz <groug at kaod.org>
> > ---
> >   fs/fuse/dir.c | 10 ++++++++++
> >   1 file changed, 10 insertions(+)
> > 
> > diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
> > index 01559061cbfb..3b0482738741 100644
> > --- a/fs/fuse/dir.c
> > +++ b/fs/fuse/dir.c
> > @@ -346,6 +346,16 @@ static struct vfsmount *fuse_dentry_automount(struct path *path)
> >   		goto out_put_sb;
> >   	}
> >   
> > +	/*
> > +	 * FIXME: setting SB_BORN requires a write barrier for
> > +	 *        super_cache_count(). We should actually come
> > +	 *        up with a proper ->get_tree() implementation
> > +	 *        for submounts and call vfs_get_tree() to take
> > +	 *        care of the write barrier.
> > +	 */
> > +	smp_wmb();
> > +	sb->s_flags |= SB_BORN;
> > +
> 
> I’m not sure whether we should have the order be exactly the same as in 
> vfs_get_tree(), i.e. whether this should be put after fsc->root has been 
> set.  Or maybe even after fm has been added to fc->mounts, because that 
> too was part of the fuse_get_tree_submount() function of your “fuse: 
> Call vfs_get_tree() for submounts” patch.
> 

Good catch !

>  From a quick look at SB_BORN users, it doesn’t seem to make a 
> difference to me, though, so:
> 

Same here but I'm fine with posting a new version that
preserves the order.

> Reviewed-by: Max Reitz <mreitz at redhat.com>
> 

Thanks !

--
Greg

> >   	sb->s_flags |= SB_ACTIVE;
> >   	fsc->root = dget(sb->s_root);
> >   	/* We are done configuring the superblock, so unlock it */
> > 
>