linux-kernel - Re: [PATCH 1/1] linux/container_of.h: Warn about loss of constness

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Date:   Mon, 24 Oct 2022 10:45:25 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        David Laight <David.Laight@...lab.com>
Subject: Re: [PATCH 1/1] linux/container_of.h: Warn about loss of constness

On Mon, Oct 24, 2022 at 10:43:52AM +0200, Greg Kroah-Hartman wrote:
> On Mon, Oct 24, 2022 at 11:26:10AM +0300, Sakari Ailus wrote:
> > container_of() casts the original type to another which leads to the loss
> > of the const qualifier if it is not specified in the caller-provided type.
> > This easily leads to container_of() returning a non-const pointer to a
> > const struct which the C compiler does not warn about.
> > 
> > Signed-off-by: Sakari Ailus <sakari.ailus@...ux.intel.com>
> > ---
> >  include/linux/container_of.h | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> > 
> > diff --git a/include/linux/container_of.h b/include/linux/container_of.h
> > index 2f4944b791b81..c7c21d0f41a87 100644
> > --- a/include/linux/container_of.h
> > +++ b/include/linux/container_of.h
> > @@ -13,6 +13,10 @@
> >   * @type:	the type of the container struct this is embedded in.
> >   * @member:	the name of the member within the struct.
> >   *
> > + * WARNING: as container_of() casts the given struct to another, also the
> 
> No need for "also" here (sorry for the grammar nit.)
> 
> > + * possible const qualifier of @ptr is lost unless it is also specified in
> > + * @type. This is not a problem if the containing object is not const. Use with
> > + * care.
> 
> I do not think these last two sentences you added here are needed
> either.
> 
> 
> >   */
> >  #define container_of(ptr, type, member) ({				\
> >  	void *__mptr = (void *)(ptr);					\
> > @@ -27,6 +31,11 @@
> >   * @type:	the type of the container struct this is embedded in.
> >   * @member:	the name of the member within the struct.
> >   *
> > + * WARNING: as container_of() casts the given struct to another, also the

Wrong function name here.

> > + * possible const qualifier of @ptr is lost unless it is also specified in
> > + * @type. This is not a problem if the containing object is not const. Use with
> > + * care.
> 
> Same comments here.

Wait, no one uses this macro, so why not just remove it entirely?

thanks,

greg k-h