RE: [PATCH] flow-dissector: Fix alignment issue in __skb_flow_get_ports

From: David Miller

From: Alexander Duyck <redacted>
Date: Fri, 10 Oct 2014 09:50:17 -0700

quoted

If I just use get_unaligned that is pretty easy in terms of cleanup
for the ports and IPv4 addresses, the IPv6 will still be a significant
hurdle to overcome though.

Actually, it's not that simple.

When the compiler sees things like "th->doff" it will load the 32-bit
word that 4-bit field contains and extract the value using shifts and
masking.

So we might need to sprinkle a "attribute((packed))" here and there
to make it work.

Marking a structure 'packed' forces the compiler to generate byte accesses.
It is enough to mark the 32bit members with __attribute__((aligned(2)))
(or use a typedef for u32 that includes that attribute).
Then the compiler will use 16bit accesses for that field.

	David