7:49 a.m.
On Mon, Jan 10, 2022 at 11:44:12AM -0800, Nick Desaulniers wrote:
On Sun, Jan 9, 2022 at 6:26 AM Andy Shevchenko
<andy.shevchenko(a)gmail.com> wrote:
>
> +Nick Desaulniers
>
> Any ideas on how to fix this?
Not obviously.
Thanks for looking into this!
This code probably will trigger a splat when run with
UBSan though.
I'm curious if the structure needs to be packed for interfacing with
hardware or some ABI, or whether we could add an explicit alignment to
the member if that would be ok (which may add back some padding)?
Looking into it I think removing packed attribute brings a regression
immediately on 64-bit platforms since u32 member followed by u8.
Otherwise, I suspect to actually access this properly we may have
macros for performing underaligned loads and stores? I suspect you'd
read potentially unaligned data into an aligned copy, then do
operations on that, at which point printing the address of the copy is
legal, but perhaps useless.
Current code does this:
val = *fourcc & ~BIT(31);
...
for (i = 0; i < sizeof(*fourcc); i++) {
...
strcpy(p, *fourcc & BIT(31) ? " big-endian" : " little-endian");
...
p = special_hex_number(p, output + sizeof(output) - 2, *fourcc, sizeof(u32));
If unaligned access is not good it will crash on some architectures AFAIU.
So, it should use get_unaligned() in the first place.
Perhaps:
- __u32 pixelformat;
+ __u32 pixelformat __aligned(4);
This looks weird, however I can't immediately see any side effects of it.
What if the address of the entire structure is unaligned, would we have a
gap here? In any case I wouldn't go this way.
__u32 field;
__u32 colorspace;
Perhaps we could tighten up this warning in clang; we don't have any
holes before this member, so I _wouldn't_ have assumed
__attribute__((packed)) would have caused the address of pixelformat
member of an instance of struct v4l2_pix_format_mplane to ever be
underaligned.
> On Sun, Jan 9, 2022 at 3:53 PM kernel test robot <lkp(a)intel.com> wrote:
> >
> > Hi Sakari,
> >
> > FYI, the error/warning still remains.
> >
> > tree: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
master
> > head: d1587f7bfe9a0f97a75d42ac1489aeda551106bc
> > commit: e927e1e0f0dd3e353d5556503a71484008692c82 v4l: ioctl: Use %p4cc printk
modifier to print FourCC codes
> > date: 11 months ago
> > config: mips-buildonly-randconfig-r002-20220107
(https://download.01.org/0day-ci/archive/20220108/202201081852.uTfBqS4b-lk...)
> > compiler: clang version 14.0.0 (https://github.com/llvm/llvm-project
32167bfe64a4c5dd4eb3f7a58e24f4cba76f5ac2)
> > reproduce (this is a W=1 build):
> > wget
https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O
~/bin/make.cross
> > chmod +x ~/bin/make.cross
> > # install mips cross compiling tool for clang build
> > # apt-get install binutils-mips-linux-gnu
> > #
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit...
> > git remote add linus
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
> > git fetch --no-tags linus master
> > git checkout e927e1e0f0dd3e353d5556503a71484008692c82
> > # save the config file to linux build tree
> > mkdir build_dir
> > COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1
O=build_dir ARCH=mips SHELL=/bin/bash drivers/hid/ drivers/media/v4l2-core/ fs/
> >
> > If you fix the issue, kindly add following tag as appropriate
> > Reported-by: kernel test robot <lkp(a)intel.com>
> >
> > All warnings (new ones prefixed by >>):
> >
> > >> 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
[-Waddress-of-packed-member]
> > mp->width, mp->height,
&mp->pixelformat,
> > ^~~~~~~~~~~~~~~
> > include/linux/printk.h:385:26: note: expanded from macro 'pr_cont'
> > printk(KERN_CONT fmt, ##__VA_ARGS__)
> > ^~~~~~~~~~~
> > >> 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]
> > pr_cont(", pixelformat=%p4cc\n",
&sdr->pixelformat);
> > ^~~~~~~~~~~~~~~~
> > include/linux/printk.h:385:26: note: expanded from macro 'pr_cont'
> > printk(KERN_CONT fmt, ##__VA_ARGS__)
> > ^~~~~~~~~~~
> > >> drivers/media/v4l2-core/v4l2-ioctl.c:353:5: warning: taking address of
packed member 'dataformat' of class or structure 'v4l2_meta_format' may
result in an unaligned pointer value [-Waddress-of-packed-member]
> > &meta->dataformat, meta->buffersize);
> > ^~~~~~~~~~~~~~~~
> > include/linux/printk.h:385:26: note: expanded from macro 'pr_cont'
> > printk(KERN_CONT fmt, ##__VA_ARGS__)
--
With Best Regards,
Andy Shevchenko