On 02/19/2012 04:51 PM, Linus Torvalds wrote:

On Sun, Feb 19, 2012 at 4:07 PM, H. Peter Anvin [off-list ref] wrote:

quoted

Use explicit sizes (__u64) instead of implicit sizes (unsigned long)
in the definition for sigcontext.h; this will allow this structure to
be shared between the x86-64 native ABI and the x32 ABI.

Btw, since we had this issue just with autofs: what are the x32 ABI
alignment issues for __u64? Are they like x86-64 ("natural alignment")
or x86-32 ("4-byte alignment")?

I assume they are natural alignment, and as pointed out by Davem, we
do have the versions of u64 that make this explicit: "compat_u64" is
the 4-byte-aligned one, while "__aligned_u64" is the natively aligned
one.

Just plain "__u64" doesn't tell which it is, which is sad and wrong,
but we're likely stuck with it forever. Unless some shining knight
comes and says "__u64 is native alignment, and if you want anything
else, you need to use __compat_u64", and actually fixes the cases
where x86-32 depends on the 4-byte aligned one.

Which would be nice, but sounds unlikely. Shining knights tend to be
rare. But this *could* possibly be automated, so it's not entirely out
of the question.

We are using __u64 as x86-32 compatible since we are sharing most of the
really complex path (like ioctl) with i386 much more so than x86-64.  So
it is defined in userspace as:

typedef unsigned long long __u64 __attribute__((aligned(4)));

__aligned_u64 obviously is naturally aligned, which matches uint64_t is
userspace.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.