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

From: Ian Abbott
Date: Wed May 09 2012 - 07:01:30 EST

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

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

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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