Re: [PATCH] staging: comedi: cast to (unsigned int *)

From: Atul Gopinathan <atulgopinathan@gmail.com>
To: Greg KH <gregkh@linuxfoundation.org>
Cc: abbotti@mev.co.uk, hsweeten@visionengravers.com,
	devel@driverdev.osuosl.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] staging: comedi: cast to (unsigned int *)
Date: Wed, 17 Feb 2021 23:40:00 +0530	[thread overview]
Message-ID: <20210217181000.GB10124@atulu-ubuntu> (raw)
In-Reply-To: <YC1T06VCh0K2BBW5@kroah.com>

On Wed, Feb 17, 2021 at 06:35:15PM +0100, Greg KH wrote:
> On Wed, Feb 17, 2021 at 10:29:08PM +0530, Atul Gopinathan wrote:
> > Resolve the following warning generated by sparse:
> > 
> > drivers/staging//comedi/comedi_fops.c:2956:23: warning: incorrect type in assignment (different address spaces)
> > drivers/staging//comedi/comedi_fops.c:2956:23:    expected unsigned int *chanlist
> > drivers/staging//comedi/comedi_fops.c:2956:23:    got void [noderef] <asn:1> *
> > 
> > compat_ptr() has a return type of "void __user *"
> > as defined in "include/linux/compat.h"
> > 
> > cmd->chanlist is of type "unsigned int *" as defined
> > in drivers/staging/comedi/comedi.h" in struct
> > comedi_cmd.
> > 
> > Signed-off-by: Atul Gopinathan <atulgopinathan@gmail.com>
> > ---
> >  drivers/staging/comedi/comedi_fops.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/staging/comedi/comedi_fops.c b/drivers/staging/comedi/comedi_fops.c
> > index e85a99b68f31..fc4ec38012b4 100644
> > --- a/drivers/staging/comedi/comedi_fops.c
> > +++ b/drivers/staging/comedi/comedi_fops.c
> > @@ -2953,7 +2953,7 @@ static int get_compat_cmd(struct comedi_cmd *cmd,
> >  	cmd->scan_end_arg = v32.scan_end_arg;
> >  	cmd->stop_src = v32.stop_src;
> >  	cmd->stop_arg = v32.stop_arg;
> > -	cmd->chanlist = compat_ptr(v32.chanlist);
> > +	cmd->chanlist = (unsigned int __force *)compat_ptr(v32.chanlist);
> 
> __force?  That feels wrong, something is odd if that is ever needed.
> 
> Are you _sure_ this is correct?

The same file has instances of "(usigned int __force *)" cast being
used on the same "cmd->chanlist". For reference:

At line 1797 of comedi_fops.c:
1796                 /* restore chanlist pointer before copying back */
1797                 cmd->chanlist = (unsigned int __force *)user_chanlist;
1798                 cmd->data = NULL;

At line 1880:
1879         /* restore chanlist pointer before copying back */
1880         cmd->chanlist = (unsigned int __force *)user_chanlist;
1881         *copy = true;

Here "user_chanlist" is of type "unsigned int __user *".

Or perhaps, I shouldn't be relying on them?

Thanks!
Atul