On Tue, Jan 11, 2022 at 4:28 AM Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx> wrote: > > On Tue, Jan 11, 2022 at 12:47:17PM +0200, Sakari Ailus wrote: > > Hi Nick, > > > > 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@xxxxxxxxxxxxxxx> 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; 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. > > > > Wouldn't that be the case only if the __packed attribute resulted in a > > different memory layout than not having that attribute? > > > > All these fields are aligned by 4 so I don't see how this could be an > > actual problem. > > packed means two things and developers often forgot about the second one: > - the gaps between members in the data structures are removed > - the instance of the data object may be on unaligned address Well put; the second is something that surprised me yesterday. I'd like to say I'd forgotten, but I'm not sure I ever really knew that in the first place...marking a struct as being packed seems like shorthand for marking all of the members as having alignment of 1, which makes sense since natural alignment requirements are what prevent structure packing in the first place. `I think this case should demonstrate the second point: https://godbolt.org/z/77P484e4o -- Thanks, ~Nick Desaulniers
