linux-kernel - Re: drivers/media/v4l2-core/v4l2-ioctl.c:303:28: warning: taking address of packed member 'pixelformat' of class or structure 'v4l2_pix_format_mplane' may result in an unaligned pointer value

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Message-ID: <20210819081053.GW3@paasikivi.fi.intel.com>
Date:   Thu, 19 Aug 2021 11:10:53 +0300
From:   Sakari Ailus <sakari.ailus@...ux.intel.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     kernel test robot <lkp@...el.com>,
        Nick Desaulniers <nick.desaulniers@...il.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        kbuild-all@...ts.01.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Petr Mladek <pmladek@...e.com>
Subject: Re: drivers/media/v4l2-core/v4l2-ioctl.c:303:28: warning: taking
 address of packed member 'pixelformat' of class or structure
 'v4l2_pix_format_mplane' may result in an unaligned pointer value

On Fri, Jul 16, 2021 at 03:12:11PM +0300, Andy Shevchenko wrote:
> On Fri, Jul 16, 2021 at 02:41:05PM +0300, Sakari Ailus wrote:
> > On Wed, Jul 14, 2021 at 10:45:26PM +0300, Andy Shevchenko wrote:
> > > > >> drivers/media/v4l2-core/v4l2-ioctl.c:347:37: warning: taking address of packed member 'pixelformat' of class or structure 'v4l2_sdr_format' may result in an unaligned pointer value [-Waddress-of-packed-member]
> > > 
> > > Why is it packed in the first place? Is it used on unaligned addresses
> > > in other structures? But even so, why should it matter?
> > 
> > It's packed since we wanted to avoid having holes in the structs. There are
> > other ways to do that but it's ABI dependent and is prone to human errors,
> > too.
> 
> What holes can you think about in the above mention structure?

Probably not that one but it has happened in the past that the struct
memory layout has been unintentionally different in different ABIs and that
has not been the intention, but rather a bug. Packing has been added in
newer structs to avoid that.

-- 
Sakari Ailus