Re: [PATCH -V4 6/6] x86: Add new syscalls for x86_64

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

On Mon, 26 Apr 2010 05:53:11 -0400, Christoph Hellwig <hch@infradead.org> wrote:
> On Sun, Apr 25, 2010 at 11:58:58PM +0530, Aneesh Kumar K. V wrote:
> > On Fri, 23 Apr 2010 16:09:07 -0600, Andreas Dilger <adilger@sun.com> wrote:
> > > On 2010-04-23, at 05:38, Aneesh Kumar K.V wrote:
> > > > Add necessary compat syscall support
> > > 
> > > Maybe I'm missing something, but why would we add a new syscall that immediately needs a compat interface?  I thought the kernel would simply return the packed file handle to userspace, instead of returning a pointer?
> > > 
> > > > +struct compat_file_handle {
> > > > +	int handle_size;
> > > > +	int handle_type;
> > > > +	/* File system identifier */
> > > > +	struct uuid fsid;
> > > > +	/* file identifier */
> > > > +	compat_uptr_t f_handle;
> > > > +};
> > > 
> > 
> > 
> > I want to make sure we put uuid part of the fs as a separate member. I
> > guess we could avoid compat interface if we define the syscall as
> > 
> > open_by_handle(struct file_handle *handle, int size, int flags);
> > 
> > But the handle in the above case will not be usable if the file handle
> > returned is too big to be usable by a cluster file system because of the
> > UUID part of handle. Having uuid as a seperate member allows such userspace
> > to generate a smaller unique identifier for the filesystem and still use
> > the void *f_handle part of the struct file_handle.
> > 
> > With struct file_handle since we have void * usage i guess we would need
> > a compat interface.
> 
> You can juse make the handle a
> 
>    char f_handle[0];
> 
> member and the structure variable size. 
> 

Even with the above change i guess we would need a
compat_sys_open_by_handle to make sure we don't pass O_LARGEFILE 
similar to what is done in compat_sys_open


-aneesh