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

Hi David,

On Wed, Dec 14, 2016 at 10:51 AM, David Laight [off-list ref] wrote:

From: Jason A. Donenfeld

quoted

Sent: 14 December 2016 00:17
This gives a clear speed and security improvement. Rather than manually
filling MD5 buffers, we simply create a layout by a simple anonymous
struct, for which gcc generates rather efficient code.

...

quoted

+     const struct {
+             struct in6_addr saddr;
+             struct in6_addr daddr;
+             __be16 sport;
+             __be16 dport;
+     } __packed combined = {
+             .saddr = *(struct in6_addr *)saddr,
+             .daddr = *(struct in6_addr *)daddr,
+             .sport = sport,
+             .dport = dport
+     };

You need to look at the effect of marking this (and the other)
structures 'packed' on architectures like sparc64.

In all current uses of __packed in the code, I think the impact is
precisely zero, because all structures have members in descending
order of size, with each member being a perfect multiple of the one
below it. The __packed is therefore just there for safety, in case
somebody comes in and screws everything up by sticking a u8 in
between. In that case, it wouldn't be desirable to hash the structure
padding bits. In the worst case, I don't believe the impact would be
worse than a byte-by-byte memcpy, which is what the old code did. But
anyway, these structures are already naturally packed anyway, so the
present impact is nil.

Jason