[En-Nut-Discussion] Possible race condition in DHCP and DHCPKick

[Ole Reinhardt]

Hi Henrik,

I just looked into the code and for me it seems very likely that this
race condition can occur. The problem can occur, as the dhcp code often
first signals dhcpDone and waits then for a wake which might result in a
race condition.

But shouldn't we change the other places as well:

multiple times in NutDhcpClient()

... 
NutEventBroadcast(&dhcpDone);
NutEventWait(&dhcpWake, NUT_WAIT_INFINITE);
...

To

... mutiple time:
NutEventBroadcastAsync(&dhcpDone);
NutEventWait(&dhcpWake, NUT_WAIT_INFINITE);
...


And another more complex situation:


        else if (dhcpState == DHCPST_BOUND) {
            retries = 0;
            NutEventBroadcast(&dhcpDone);
            if (dhcpConfig->dyn_renewalTime <= NutGetSeconds() -
leaseTime) {
                dhcpState = DHCPST_RENEWING;
            } else {
                /* Calculate the remaining lease time and take a nap. */
                napTime = dhcpConfig->dyn_renewalTime - (NutGetSeconds()
- leaseTime);
                if (napTime > MAX_DHCP_NAPTIME) {
                    napTime = MAX_DHCP_NAPTIME;
                }
                NutEventWait(&dhcpWake, napTime * 1000UL);
            }
        }

to 

        else if (dhcpState == DHCPST_BOUND) {
	    uint32_t now = NutGetSeconds();
            retries = 0;
            NutEventBroadcastAsync(&dhcpDone);
            if (dhcpConfig->dyn_renewalTime <= now - leaseTime) {
                dhcpState = DHCPST_RENEWING;
            } else {
                /* Calculate the remaining lease time and take a nap. */
                napTime = dhcpConfig->dyn_renewalTime - (now -
leaseTime);
                if (napTime > MAX_DHCP_NAPTIME) {
                    napTime = MAX_DHCP_NAPTIME;
                }
                NutEventWait(&dhcpWake, napTime * 1000UL);
            }
        }

The above is more complex as NutGetSeconds also could result in a
context switch.

Regards,

Ole


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