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

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Message-ID: <Y1ZQSEMLkybFCadS@kroah.com>
Date:   Mon, 24 Oct 2022 10:43:52 +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 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
> + * 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.

thanks,

greg k-h