Re: [PATCH -V3] Generic name to handle and open by handle syscalls

["Aneesh Kumar K. V" <aneesh.kumar@linux.vnet.ibm.com>]

On Thu, 22 Apr 2010 17:49:58 -0500, "Serge E. Hallyn" <serue@us.ibm.com> wrote:
> Quoting Aneesh Kumar K.V (aneesh.kumar@linux.vnet.ibm.com):
> > Hi,
> > 
> > The below set of patches implement open by handle support using exportfs
> > operations. This allows user space application to map a file name to file 
> > handle and later open the file using handle. This should be usable
> > for userspace NFS [1] and 9P server [2]. XFS already support this with the ioctls
> > XFS_IOC_PATH_TO_HANDLE and XFS_IOC_OPEN_BY_HANDLE.
> > 
> > [1] http://nfs-ganesha.sourceforge.net/
> > [2] http://lists.gnu.org/archive/html/qemu-devel/2010-03/msg01087.html
> > 
> > TODO:
> > I guess we would need to optimize how we get the vfsmount for the filesystem
> > uuid specified. Searching the file system list and task name space may be a big
> > overhead for each open by handle call.
> > 
> > Chages from V2:
> > a) Support system wide unique handle.
> > 
> > Changes from v1:
> > a) handle size is now specified in bytes
> > b) returns -EOVERFLOW if the handle size is small
> > c) dropped open_handle syscall and added open_by_handle_at syscall
> >    open_by_handle_at takes mount_fd as the directory fd of the mount point
> >    containing the file
> > e) handle will only be unique in a given file system. So for an NFS server
> >    exporting multiple file system, NFS server will have to internally track the
> >    mount point to which a file handle belongs to. We should be able to do it much
> >    easily than expecting kernel to give a system wide unique file handle. System
> >    wide unique file handle would need much larger changes to the exportfs or VFS
> >    interface and I was not sure whether we really need to do that in the kernel or
> >    in the user space
> > f) open_handle_at now only check for DAC_OVERRIDE capability
> 
> Hi Aneesh,
> 
> it still scares me a bit as I expect to see some privileged userspace
> daemon accept (forged) handles from unprivileged users and pass back
> open fds, but adding some random token to prevent that would require
> storing it, so I think you're doing what you reasonably can by
> requiring DAC_OVERRIDE.

We still do access permission check using the file permission
modes. What we don't do is to check the execute (x) bit on the
directory. But the idea of adding DAC_OVERRIDE is to make sure
that we allow only privileged user to use the open by handle interface.


> 
> nsproxy bit looks fine, and i saw no problems in the rest of the set.
> As Andreas was saying I don't see why you're calling it _at, but
> meanwhile
> 
> Acked-by: Serge Hallyn <serue@us.ibm.com>


I missed adding Acked-by:  in V4 iteration. But will add in V5
iteration. Can i add the same to all the patches ? Or do you want me to
add only to the nsproxy bits.

> 
> to the set.
> 
> -serge

-aneesh