From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mika Westerberg Subject: Re: [PATCH v3] device property: Get rid of union aliasing Date: Wed, 16 May 2018 12:41:33 +0300 Message-ID: <20180516094133.GN32438@lahna.fi.intel.com> References: <20180515173202.84296-1-andriy.shevchenko@linux.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20180515173202.84296-1-andriy.shevchenko@linux.intel.com> Sender: linux-kernel-owner@vger.kernel.org To: Andy Shevchenko Cc: Greg Kroah-Hartman , linux-kernel@vger.kernel.org, Ard Biesheuvel , linux-efi@vger.kernel.org, Sakari Ailus , "Rafael J . Wysocki" , ACPI Devel Maling List , Lukas Wunner List-Id: linux-acpi@vger.kernel.org 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 > Signed-off-by: Andy Shevchenko Reviewed-by: Mika Westerberg