[kbuild-all] 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 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&gt; 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&gt; 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&gt; > > > > > > 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