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

On 12/19/12 at 09:17am, David Laight wrote:

quoted

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.

I am not sure I understand this correctly. Are you saying
that the following does not work on i386?

struct foo {
  uint32_t a;
  uint64_t b;
};

struct foo buf;

memcpy(&buf, nla_data(attr), nla_len(attr));
printf([...], buf.b);

That will be fine on all systems.
But if, instead, you have:
	struct foo buf, *bufp;
	bufp = nla_data(attr);
	memcpy(&buf, bufp, sizeof buf);
The compiler is allowed to assume that 'bufp' is aligned,
so the copy will be done using 64bit accesses.

(Basically because all you are allowed to do with 'void *'
is cast a point to 'void *', then back to its original type.
So when you cast back from 'void *' the pointer can be assumed
to be aligned.)

This will fault on systems that require strict alignment
of 64bit items.

	David