[mc1322x] Non-aligned structure copies

[Jon Smirl]

On Tue, Nov 3, 2009 at 11:15 PM, Ian Martin <martini at alum.mit.edu> wrote:
> Hi Jon,
>
> I had a similar issue in either Contiki or uIP and inevitably had to
> redefine the uip_ipaddr_copy macro with a call to the ANSI memcpy
> function. It was something along the lines of:
>
> #define uip_ipaddr_copy(dest, src) memcpy((void*)(dest), (void*)(src),
> sizeof(uip_ipaddr_t))
>
> ...which shouldn't have any trouble with unaligned addressing.  I see
> that you have created the equivalent using inline code.  I think uIP
> even defines the macro differently for IPv4 vs IPv6 (besides for the
> obvious reason).  IIRC, the only other occasion where I ran into
> trouble with unaligned access was in the DHCP client code.

Now I'm afraid of how many other places this problem may be hidden.

Maybe Mar can get Qemu fired up and convince it to generate exceptions
on unaligned access. that would help locate the problems.
My familiarity with Qemu is zero.

>
> I don't know of a compiler switch offhand.

I don't think it is possible to build a switch. I'm sprinkling
_attribute__ ((__packed__)) everywhere and looking for more struct
copies.

Some ARM7 core have hardware that does machine checks on unaligned
access. mc13224 manual doesn't mention supporting it.


>
> -Ian
>
> On Tue, Nov 3, 2009 at 10:48 PM, Jon Smirl <jonsmirl at gmail.com> wrote:
>> My TTL field was getting stomped in the packets so I added some debug
>>
>> uip_nd6_io_rs_output(void) {
>>  UIP_IP_BUF->vtc = 0x60;
>>  UIP_IP_BUF->tcflow = 0;
>>  UIP_IP_BUF->flow = 0;
>>  UIP_IP_BUF->proto = UIP_PROTO_ICMP6;
>>  UIP_IP_BUF->ttl = UIP_ND6_HOP_LIMIT;
>> printf("ttl 1 %d\n", UIP_IP_BUF->ttl);
>>  uip_create_linklocal_allrouters_mcast(&UIP_IP_BUF->destipaddr);
>> printf("ttl 2 %d %p %p\n", UIP_IP_BUF->ttl, &UIP_IP_BUF->ttl,
>> &UIP_IP_BUF->srcipaddr);
>>  uip_netif_select_src(&UIP_IP_BUF->srcipaddr, &UIP_IP_BUF->destipaddr);
>> printf("ttl 3 %d\n", UIP_IP_BUF->ttl);
>>
>> The select_src(... was stomping TTL
>>
>> More debug and I track it down to ipaddr_copy...
>>
>> printf("uip_netif_select_src %d %d\n", best, index);
>>
>>  uip_ipaddr_copy(src, &(uip_netif_physical_if.addresses[index].ipaddr));
>>  return;
>>
>> --- It's doing a struct copy. I'm suspicious about non-aligned access on ARM
>>
>> #ifndef uip_ipaddr_copy
>> #define uip_ipaddr_copy(dest, src) (*(dest) = *(src))
>> #endif
>>
>> I change it to:
>>
>> #ifndef uip_ipaddr_copy
>> //#define uip_ipaddr_copy(dest, src) (*(dest) = *(src))
>> #define uip_ipaddr_copy(dest, src) \
>> { \
>> int i; \
>> for (i = 0; i < sizeof *(dest); i++) { \
>>        *(((char *)(dest)) + i) = *(( (char *)(src)) + i); \
>> } \
>> }
>> #endif
>>
>> --- TTL doesn't get stomped any more
>>
>> Sending NS to ff00:0000:0000:0000:0000:0001:ffff:ee00 from
>> 0000:0000:0000:0000:0000:0000:0000:0000 with target address
>> fe80:0000:0000:0000:dfcc:bbaa:aa99:ffee
>> output;
>> Sending RS 0
>> ttl 1 255
>> ttl 2 255 40e59a 40e59b
>> uip_netif_select_src
>> uip_netif_select_src 0 0
>> ttl 3 255
>> ttl 4 255
>>
>> The code still doesn't work because there are more of these floating around.
>>
>> Is there some compiler switch to tell it to do the right thing with an
>> unaligned struct copy?
>>
>> --
>> Jon Smirl
>> jonsmirl at gmail.com
>>
>> _______________________________________________
>> mc1322x mailing list
>> mc1322x at devl.org
>> http://devl.org/cgi-bin/mailman/listinfo/mc1322x
>>
>



-- 
Jon Smirl
jonsmirl at gmail.com