RE: [PATCH v2] netlink: align attributes on 64-bits

quoted

Consider the structure:
	struct bar {
		__u32 foo1;
		__u64 foo2;
	}
On i386 it will have size 12 and foo2 will be at offset 4.
On sparc32 (and most 64bit) it will have size 16 with foo2
at offset 8 (and 4 bytes of pad after foo1).

This is a known problem and I can't think of anything
that can be done about it except for memcpy()ing the
data before accessing it.

You can't use memcpy() to copy a pointer to a misaligned
structure into an aligned buffer. The compiler assumes
the pointer is aligned and will use instructions that
depend on the alignment.

If you have ideas, I'm more that willing to listen :)
... Netlink has and will be host bound. It also
uses host byte order for that reason.

I think:
1) Alignment is only needed on systems that have 'strict alignment'
   requirements (maybe disable for testing?)
2) Alignment is only needed for parameters whose size is a
   multiple of the alignment (a structure containing a
   field that needs 8 byte alignment will always be a multiple
   of 8 bytes long).
3) You need to add NA_HDR_LEN to the write pointer before
   determining the size of the pad.

So a structure of three uint32_t will never need aligning.

	David