Re: [PATCH] staging: comedi: remove __user annotation inside of struct's

[Ian Abbott]

On 2012-05-09 11:31, Dan Carpenter wrote:
> On Wed, May 09, 2012 at 11:20:07AM +0100, Ian Abbott wrote:
> > On 2012-05-09 00:55, H Hartley Sweeten wrote:
> > > On Tuesday, May 08, 2012 4:41 PM, H Hartley Sweeten wrote:
> > > > The structs' comedi_insn, coomedi_insnlist, comedi_cmd,
> > > > comedi_chaninfo, and comedi_rangeinfo are all passed to
> > > > the kernel from user space using ioctl commands. They
> > > > are then copied to kernel space using copy_from_user()
> > > > before the data is passed to the drivers.
> > > >
> > > > The __user annotation should not be used with variables
> > > > inside the struct. This produces a lot of sparse warnings
> > > > like:
> > > >
> > > > warning: dereference of noderef expression
> > > >
> > > > Note: This patch exposes some new warnings about different
> > > > address space. These will be addressed.
> > >
> > > Please ignore this patch.
> > >
> > > It appears the annotations in the struct definitions are correct.
> >
> > Personally, I think you were on the mark with the patch.  It's
> > better to avoid using __user in comedi.h so it can be used as-is in
> > user-space.
>
> Sparse is useful so we shouldn't break it.  I always run sparse over
> my patches before submission and look at the warnings.  Except if
> they scroll off the page.  In that case, I just figure that the
> author deserves the bugs.
>
> We could just do some ifdeferry to fix it for userspace.

That doesn't help in cases such as 'struct comedi_insn' where the 'data' 
pointer is a user-space pointer in the user-space copy of the object and 
a kernel-space pointer in the kernel-space copy of the object.  The only 
fix for that is to have separate "k" versions of the struct or to do a 
load of casting, which is slightly error-prone and makes the code less 
readable.

Are there any handy macros for casting pointers to __user pointers, 
something like

#define _user(p) ((typeof(*(p)) __user *)(p))

but preferably without the repeated expansion of 'p' in case of 
side-effects?

--
-=( Ian Abbott @ MEV Ltd.    E-mail: <abbotti@xxxxxxxxx>        )=-
-=( Tel: +44 (0)161 477 1898   FAX: +44 (0)161 718 3587         )=-
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