linux-kernel - Re: [PATCH v2 1/4] container_of: add container_of_const() that preserves const-ness of the pointer

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Message-ID: <Y49cGRDBVP3bHJuT@casper.infradead.org>
Date:   Tue, 6 Dec 2022 15:13:29 +0000
From:   Matthew Wilcox <willy@...radead.org>
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>,
        Jason Gunthorpe <jgg@...dia.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Subject: Re: [PATCH v2 1/4] container_of: add container_of_const() that
 preserves const-ness of the pointer

On Mon, Dec 05, 2022 at 01:12:03PM +0100, Greg Kroah-Hartman wrote:
> +/**
> + * container_of_const - cast a member of a structure out to the containing
> + *			structure and preserve the const-ness of the pointer
> + * @ptr:		the pointer to the member
> + * @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)				\
> +	_Generic(ptr,							\
> +		const typeof(*(ptr)) *: ((const type *)container_of(ptr, type, member)),\
> +		default: ((type *)container_of(ptr, type, member))	\
> +	)
> +

Reviewed-by: Matthew Wilcox (Oracle) <willy@...radead.org>

I tried doing this:

+++ b/include/linux/container_of.h
@@ -15,11 +15,17 @@
  *
  * WARNING: any const qualifier of @ptr is lost.
  */
-#define container_of(ptr, type, member) ({                             \
+#define _c_of(ptr, type, member) ({                                    \
        void *__mptr = (void *)(ptr);                                   \
        static_assert(__same_type(*(ptr), ((type *)0)->member) ||       \
                      __same_type(*(ptr), void),                        \
                      "pointer type mismatch in container_of()");       \
        ((type *)(__mptr - offsetof(type, member))); })

+#define container_of(ptr, type, m)                                     \
+       _Generic(ptr,                                                   \
+               const typeof(*(ptr)) *: (const type *)_c_of(ptr, type, m),\
+               default: ((type *)_c_of(ptr, type, m))                  \
+       )
+
 #endif /* _LINUX_CONTAINER_OF_H */

(whitespace damaged, yes the kernel-doc is now in the wrong place, etc)

It found a few problems; just building the mlx5 driver (I happened to be
doing some work on it in that tree).  We're definitely not ready to do
that yet, but I'll send a few patches to prepare for it.