Re: [PATCH 1/1] v4l: Avoid unaligned access warnings when printing 4cc modifiers

From: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Sakari Ailus <sakari.ailus@linux.intel.com>,
	linux-media@vger.kernel.org, hverkuil@xs4all.nl
Subject: Re: [PATCH 1/1] v4l: Avoid unaligned access warnings when printing 4cc modifiers
Date: Tue, 11 Jan 2022 12:07:57 +0200	[thread overview]
Message-ID: <Yd1W/cQ1G4/HpFMi@smile.fi.intel.com> (raw)
In-Reply-To: <CAKwvOdnfX1DzgkzCPY7N4LiJDJTGxsKNQbRMmmkt7o5z5O-W9w@mail.gmail.com>

On Mon, Jan 10, 2022 at 03:11:18PM -0800, Nick Desaulniers wrote:
> On Mon, Jan 10, 2022 at 2:48 PM Sakari Ailus
> <sakari.ailus@linux.intel.com> wrote:
> >
> > Pointers V4L2 pixelformat and dataformat fields in a few packed structs
> > are directly passed to printk family of functions.
> 
> I would rephrase the below statement...
> 
> > This could result in an
> > unaligned access albeit no such possibility appears to exist at the
> > moment i.e. this clang warning appears to be a false positive.
> 
> ...to:
> 
> warning: taking address of packed member 'pixelformat' of class or
> structure 'v4l2_pix_format_mplane' may result in an unaligned pointer
> value [-Waddress-of-packed-member]
> 
> The warning is correct;

Exactly. I'm going to repeat that in the comment to my patch.

> because `struct v4l2_pix_format_mplane` is
> __packed, it's members also have __aligned(1).  Taking the address of
> such members results in the use of underaligned pointers which is UB
> and may be caught by UBSAN or fault on architectures without unaligned
> loads should the struct instance happen to be allocated without any
> natural alignment.

-- 
With Best Regards,
Andy Shevchenko