linux-kernel - Re: [PATCH] container

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Date:   Wed, 8 Mar 2023 15:58:33 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, Jason Gunthorpe <jgg@...pe.ca>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "Rafael J . Wysocki" <rafael@...nel.org>
Subject: Re: [PATCH] container_of: add type safety

On Wed, Mar 08, 2023 at 06:57:13PM +0100, Greg Kroah-Hartman wrote:
> On Wed, Mar 08, 2023 at 12:35:03PM -0500, Michael S. Tsirkin wrote:
> > Using a wrong member in container_of will result in an error.
> > No so for container_of_const - it is just a cast so will
> > happily give you a wrong pointer.
> > 
> > Use logic from container_of to add safety.
> > 
> > Cc: Jason Gunthorpe <jgg@...pe.ca>
> > Cc: Sakari Ailus <sakari.ailus@...ux.intel.com>
> > Cc: Matthew Wilcox (Oracle) <willy@...radead.org>
> > Cc: Jason Gunthorpe <jgg@...dia.com>
> > Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> > Cc: Sakari Ailus <sakari.ailus@...ux.intel.com>
> > Cc: Rafael J. Wysocki <rafael@...nel.org>
> > Link: https://lore.kernel.org/r/20221205121206.166576-1-gregkh@linuxfoundation.org
> 
> That's the wrong link, that's not this patch, that was an old patch.
> 
> > Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> 
> I did not sign off on this.  NEVER do that, you just made a legal
> statement in my name, why?
> 
> Why did you not sign off on it?
> 
> totally confused...

Ooops. I started with people who signed off on the original one
and replaced S.O.B with a CC. Forgot to replace it for yours
and then the script checking patches saw a signoff and was happy
so I didn't notice I forgot to sign it myself ...

However ...


> > ---
> >  include/linux/container_of.h | 7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/linux/container_of.h b/include/linux/container_of.h
> > index 1d898f9158b4..5d87faf72e0a 100644
> > --- a/include/linux/container_of.h
> > +++ b/include/linux/container_of.h
> > @@ -29,10 +29,13 @@
> >   * @type:		the type of the container struct this is embedded in.
> >   * @member:		the name of the member within the struct.
> >   */
> > -#define container_of_const(ptr, type, member)				\
> > +#define container_of_const(ptr, type, member) ({			\
> > +	static_assert(__same_type(*(ptr), ((type *)0)->member) ||	\
> > +		      __same_type(*(ptr), void),			\
> > +		      "pointer type mismatch in container_of()");	\
> 
> Why is this needed because:
> 
> 
> >  	_Generic(ptr,							\
> >  		const typeof(*(ptr)) *: ((const type *)container_of(ptr, type, member)),\
> >  		default: ((type *)container_of(ptr, type, member))	\
> 
> container_of() is used here, so shouldn't the assert trigger there if
> you get things wrong?
> 
> So why is this change needed at all?
> 
> thanks,
> 
> greg k-h


Hmm. I think I was confused. Error did not seem to trigger but
I tried again and it does. Ignore this please.

Sorry about the noise.

-- 
MST