Messages in this thread | | | From | Nick Desaulniers <> | Date | Mon, 10 Jan 2022 13:22:43 -0800 | 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 |
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On Mon, Jan 10, 2022 at 12:50 PM Andy Shevchenko <andy.shevchenko@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@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@linux.intel.com/), 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@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-lkp@intel.com/config) > > > > 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/?id=e927e1e0f0dd3e353d5556503a71484008692c82 > > > > 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@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
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