From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH -V4 6/6] x86: Add new syscalls for x86_64
Date: Mon, 26 Apr 2010 05:53:11 -0400
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Cc: Andreas Dilger <adilger@sun.com>, hch@infradead.org,
	viro@zeniv.linux.org.uk, corbet@lwn.net, serue@us.ibm.com,
	linux-fsdevel@vger.kernel.org, sfrench@us.ibm.com
To: "Aneesh Kumar K. V" <aneesh.kumar@linux.vnet.ibm.com>
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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.