[Virtio-fs] [PATCH] virtiofsd: handle NULL dir in lo_do_lookup

[Vivek Goyal]

On Thu, May 23, 2019 at 11:22:54AM +0800, Eryu Guan wrote:
> On Thu, May 23, 2019 at 10:06:34AM +0800, Liu Bo wrote:
> > Reported by fstests/generic/467.
> > 
> > open_by_handle_at() called from fuse inside guest can carry fuse mount
> > point to daemon but lo_do_lookup() doesn't know its inode info because
> > it's out of fuse's scope, thus lo_inode(req, parent) ends up with
> > returning a NULL dir and breaks virtiofsd immediately.
> > 
> > Note that it'd break applications that uses open_by_handle_at.
> > 
> > It seems to me that nothing could be done to support open_by_handle_at in
> > this case.
> 
> Then perhaps we should consider disabling exportfs support in virtio-fs
> case? I noticed that currently fuse set
> 
> 	sb->s_export_op = &fuse_export_operations
> 
> unconditionally, maybe we could implement a feature mask in fuse and let
> filesystem implementation registers what features it supports, e.g.
> exportfs in this case.
> 
> Or we could try adding real exportfs support to virtiofs? Though I'm not
> sure if it's possible or worth it at the moment.

If we don't support export operations, we should disable it. Looks like
we have enabled it in virtiofsd.

lo_init() {
        if(conn->capable & FUSE_CAP_EXPORT_SUPPORT)
                conn->want |= FUSE_CAP_EXPORT_SUPPORT;
}

So if we were to disable it, I guess we will have to remove it.

Not sure if it is possible to support export ops in virtio-fs case. Need
to read more about it.

Vivek