On Mon, 20 Aug 2018 11:05:08 -0700
Ori Bernstein <o...@eigenstate.org> wrote:
> On Mon, 20 Aug 2018 23:38:28 +0900 (JST), YASUOKA Masahiko
> <yasu...@openbsd.org> wrote:
>> As far as I know, switch(4) assumes it receives packets located at 64
>> bit aligned memory. So if the code like below
>>
>> *(uint64_t *)oxm->oxm_value = htobe64(val);
>>
>> cause alignment faults, the assumption may be broken. If so, the
>> place we should fix may not be here.
>
> The reason we are suspicious here is because oxm_value comes from struct
> ofp_ox_match, which is defined like this:
>
> struct ofp_ox_match {
> uint16_t oxm_class;
> uint8_t oxm_fh;
> uint8_t oxm_length;
> uint8_t oxm_value[0];
> } __packed;
>
> While the packet may be aligned to 8 bytes, my reading is that the oxm_value
> itself is not at the start of the packet, and is not aligned within the
> ofp_ox_match. I'm not familiar enough with the open flowz to know if
> ofp_ox_match is guaranteed to start at a 4-byte alignment (which would
> push oxm_value out to an 8 byte alignment), but I suspect that's not the
> case.
You are right. I'm sorry. My understanding was wrong.
Overall flow match structures is designed to align to 64-bits, but as
for each fields, there is no such care. So the diff seems ok.
--yasuoka
