On Mon, Jan 10, 2022 at 12:50 PM Andy Shevchenko
<andy.shevchenko(a)gmail.com> wrote:
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.
Regardless of your unaligned access patch
(
https://lore.kernel.org/lkml/20220110205049.11696-1-andriy.shevchenko@lin...),
taking the address of a packed member will still have a warning:
https://godbolt.org/z/8r1rcocPY
So it looks like packing a struct sets the alignment of members to 1,
ie. under-aligning them. You need to restore the alignment of the
members you plan to take the address of.
ie.
struct w {
char x, y, z;
} __attribute__((packed));
is equivalent in layout and alignment to:
struct w {
char __attribute__((aligned(1))) x;
char __attribute__((aligned(1))) y;
char __attribute__((aligned(1))) z;
} __attribute__((aligned(1)));
For `struct v4l2_pix_format_mplane` we probably want the trailing
char's to have alignment 1, rather than every member. We can either be
explicit with __aligned(1) on them, or __aligned(4) on the uint
members and retain __packed.
> 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
--
Thanks,
~Nick Desaulniers