linux-kernel - Re: [PATCH v3] device property: Get rid of union aliasing

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Date:   Wed, 16 May 2018 12:41:33 +0300
From:   Mika Westerberg <mika.westerberg@...ux.intel.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        linux-efi@...r.kernel.org,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Lukas Wunner <lukas@...ner.de>
Subject: Re: [PATCH v3] device property: Get rid of union aliasing

On Tue, May 15, 2018 at 08:32:02PM +0300, Andy Shevchenko wrote:
> The commit
> 
>   318a19718261 ("device property: refactor built-in properties support")
> 
> went way too far and brought a union aliasing. Partially revert it here
> to get rid of union aliasing.
> 
> Note, all Apple properties are considered as u8 arrays. To get a value
> of any of them the caller must use device_property_read_u8_array().
> 
> What union aliasing is?
> ~~~~~~~~~~~~~~~~~~~~~~~
> The C99 standard in section 6.2.5 paragraph 20 defines union type as
> "an overlapping nonempty set of member objects". It also states in
> section 6.7.2.1 paragraph 14 that "the value of at most one of the
> members can be stored in a union object at any time'.
> 
> Union aliasing is a type punning mechanism using union members to store
> as one type and read back as another.
> 
> Why it's not good?
> ~~~~~~~~~~~~~~~~~~
> Section 6.2.6.1 paragraph 6 says that a union object may not be a trap
> representation, although its member objects may be.
> 
> Meanwhile annex J.1 says that "the value of a union member other than
> the last one stored into" is unspecified [removed in C11].
> 
> In TC3, a footnote is added which specifies that accessing a member of a
> union other than the last one stored causes "the object representation"
> to be re-interpreted in the new type and specifically refers to this as
> "type punning". This conflicts to some degree with Annex J.1.
> 
> While it's working in Linux with GCC, the use of union members to do
> type punning is not clear area in the C standard and might lead to
> unspecified behaviour.
> 
> More information is available in this [1] blog post.
> 
> [1]: https://davmac.wordpress.com/2010/02/26/c99-revisited/
> 
> Acked-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>

Reviewed-by: Mika Westerberg <mika.westerberg@...ux.intel.com>