RE: [PATCH v2 3/4] secure_seq: use siphash24 instead of md5_transform

From: Jason A. Donenfeld

Sent: 14 December 2016 13:44
To: Hannes Frederic Sowa

quoted

__packed not only removes all padding of the struct but also changes the
alignment assumptions for the whole struct itself. The rule, the struct
is aligned by its maximum alignment of a member is no longer true. That
said, the code accessing this struct will change (not on archs that can
deal efficiently with unaligned access, but on others).

That's interesting. There currently aren't any alignment requirements
in siphash because we use the unaligned helper functions, but as David
pointed out in another thread, maybe that too should change. In that
case, we'd have an aligned-only version of the function that requires
8-byte aligned input. Perhaps the best way to go about that would be
to just mark the struct as __packed __aligned(8). Or, I guess, since
64-bit accesses gets split into two on 32-bit, that'd be best descried
as __packed __aligned(sizeof(long)). Would that be an acceptable
solution?

Just remove the __packed and ensure that the structure is 'nice'.
This includes ensuring there is no 'tail padding'.
In some cases you'll need to put the port number into a 32bit field.

I'd also require that the key be aligned.
It probably ought to be a named structure type with two 64bit members
(or with an array member that has two elements).

	David