CoAP utils (coap_utils.h) cause stack overflow

RE: CoAP utils (coap_utils.h) cause stack overflow

Øyvind — Wed, 10 Jun 2020 09:19:35 GMT

I see that R&D has already given an answer to your PR (https://github.com/nrfconnect/sdk-nrf/pull/2451)

Thanks for spotting this, we've only tested the library with Zephyr native networking stack so far, so the issue was not spotted with bsdlib.

Kind regards,
Øyvind

RE: CoAP utils (coap_utils.h) cause stack overflow

Øyvind — Wed, 10 Jun 2020 08:49:12 GMT

Hi Pontus,

Happy to hear that you found the solution. Regarding if this is a bug I don't know at the moment. I have forwarded your question to our R&D department and will get back to you within today.

Kind regards,
Øyvind

RE: CoAP utils (coap_utils.h) cause stack overflow

Pwntus — Tue, 09 Jun 2020 20:53:50 GMT

All the clues point me to this #define: https://github.com/nrfconnect/sdk-nrf/blob/master/subsys/net/lib/coap_utils/coap_utils.c#L20

I manually doubled the stack size (500 > 1000) and voilà, it works!

I don't know why 500 is the configured size, maybe this is a bug?

RE: CoAP utils (coap_utils.h) cause stack overflow

Pwntus — Tue, 09 Jun 2020 20:44:21 GMT

I've traced the faulting address 29376 in build/zephyr/zephyr.lst:

static inline int nrf91_socket_offload_poll(struct pollfd *fds, int nfds,
					    int timeout)
{
   29374:	b580      	push	{r7, lr}
   29376:	b098      	sub	sp, #96	; 0x60 <------ here
   29378:	af00      	add	r7, sp, #0
   2937a:	60f8      	str	r0, [r7, #12]
   2937c:	60b9      	str	r1, [r7, #8]
   2937e:	607a      	str	r2, [r7, #4]
	int retval = 0;
	
	...

...which points me to the nRF implementation of BSD sockets: https://github.com/nrfconnect/sdk-nrf/blob/master/lib/bsdlib/nrf91_sockets.c#L862

I do however suspect the issue is with the stack size of the current thread used by the socket, which in this case appears to be listening/polling.

Thankful for any input.