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    SubjectRe: [PATCH] x_tables: fix alignment on [at least] ppc32
    On Thu, Jan 19, 2006 at 04:56:35PM -0800, David S. Miller wrote:
    > > [NETFILTER] x_tables: Fix XT_ALIGN() macro on [at least] ppc32
    > > [...]
    > > The fix is an ugly kludge, but it has been tested to solve the problem. Yet
    > > another reason to move away from the current {ip,ip6,arp,eb}tables like
    > > data structures.
    > >
    > > Signed-off-by: Harald Welte <>
    > Harald, I'm going to modify this to just use u_int64_t as that
    > should be totally sufficient.
    > It is the largest type, and will produce the desired results without
    > the silly structure.

    Sorry dave, as I just learned, it isn't. As reported by Jiri Slaby
    <>, Linus' tree now breaks on i386 :(

    Interestingly, on i386:

    __alignof__(struct _xt_align) 4
    __alignof__(u_int64_t) 8
    __alignof__(void *) 4

    whereas on ppc:

    __alignof__(struct _xt_align) 8
    __alignof__(u_int64_t) 8
    __alignof__(void *) 4

    So your assumption that __alignof__(u_int64_t) == __alignof__(struct xt_align)
    doesn't hold true for all archs.

    I would therefore recommend applying my unmodified patch, and hope that
    it then works on all archs simultaneously.

    > Some malloc() implementations use "long double" to figure out the
    > largest type size and alignment requirements any C type might have
    > on the machine. But there is no reason to use that here.

    Our main problem is that we have to stay compatible with old userspace
    programs that had a different definition for what has now become
    XT_ALIGN(). So independent what might be the best solution from an
    alignment point of view, we must match what old userspace thinks.

    Yes, this all sucks. And yes, we will see a new interface this year.


    - Harald Welte <>
    "Fragmentation is like classful addressing -- an interesting early
    architectural error that shows how much experimentation was going
    on while IP was being designed." -- Paul Vixie
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