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    SubjectRe: [-next] FATAL: drivers/gpu/drm/udl/udl: sizeof(struct usb_device_id)=24 is not a modulo of the size of section __mod_usb_device_table=44.
    On Sat, Jun 16, 2012 at 03:23:31PM +0200, Bjørn Mork wrote:
    > Greg Kroah-Hartman <gregkh@linuxfoundation.org> writes:
    > > On Fri, Jun 15, 2012 at 11:02:55PM +0200, Geert Uytterhoeven wrote:
    > >
    > >> As "kernel_ulong_t  driver_info" is no longer naturally aligned, the
    > >> compiler will
    > >> add implicit padding. But the padding depends on the architecture.
    > >
    > > Ah, so we were "lucky" before, nice.
    >
    > I don't really believe in luck :-) I think someone has been really
    > smart here. Maybe too smart...

    No, I think the previous structure was just "lucky" in that it just
    happened to be the right alignment. I say this as I think I was the one
    who created that structure years ago. Or maybe not, this was back in
    the 2.3 kernel days, I can't remember what patches I wrote last week...

    > >> It can be fixed by adding explicit padding. Probably it should be padded by
    > >> 7 bytes (not 3), as kernel_ulong_t may require 8-byte alignment on some 64-bit
    > >> platforms. Or by an explicit alignment attribute.
    > >>
    > >> See also
    > >> * commit 8175fe2dda1c93a9c596921c8ed4a0b4baccdefe ("HID: fix
    > >> hid_device_id for cross compiling")
    > >> * commit 7492d4a416d68ab4bd254b36ffcc4e0138daa8ff ("sdio: fix module
    > >> device table definition for m68k")
    > >> * commit 9e2d3cd34a159948dc753a14573e16bffc04dba8 ("[PATCH]
    > >> mod_devicetable.h fixes")
    > >
    > > So would the patch below fix this? It should force alignment of the
    > > driver_data field, which is all you want here, right?
    > >
    > >> Still, there's a bug in file2alias (which is compiled by the host
    > >> compiler), in that
    > >> it may use different padding than the target platform when cross-compiling.
    > >
    > > That's not good, but outside of this specific issue, right? Have we
    > > just been fortunate it hasn't really hit us yet?
    > >
    > > thanks,
    > >
    > > greg k-h
    > >
    > > diff --git a/include/linux/mod_devicetable.h b/include/linux/mod_devicetable.h
    > > index 7771d45..6955045 100644
    > > --- a/include/linux/mod_devicetable.h
    > > +++ b/include/linux/mod_devicetable.h
    > > @@ -122,7 +122,8 @@ struct usb_device_id {
    > > __u8 bInterfaceNumber;
    > >
    > > /* not matched against */
    > > - kernel_ulong_t driver_info;
    > > + kernel_ulong_t driver_info
    > > + __attribute__((aligned(sizeof(kernel_ulong_t))));
    > > };
    >
    >
    > This feels a lot like papering over the real problem. It will solve
    > this instance, but the list of such previous "paper work" that Geert
    > provided should be enough evidence that this will happen again the next
    > time someone modifies a device id struct for some subsystem.

    Hopefully not, if you add another field here, the alignment force will
    keep things lined up properly, from what I can tell. Is that not true?

    > And adding forced aligment here feels wrong since there is no good
    > reason why the (target) compiler shouldn't know the proper alignment for
    > this structure, is there? OK, "feels wrong" is not a good argument. But
    > it would be better to solve this problem once and for all.

    C doesn't require the structure to be aligned. Actually the spec says
    it doesn't guarantee anything about this, we just "know" that gcc is
    going to be semi-sane and try to do the best it can. Hopefully clang is
    also semi-sane as well.

    So because of that, we have to give it some guidance, hence the patch.

    thanks,

    greg k-h
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