Theoretical correctness fix:

Handle EAGAIN and EWOULDBLOCK more gracefully when dealing with safe_read().
All occurrences of safe_read() should only be invoked on fds that have signaled
ready-to-read state via the epoll() mechanism, so this change should not
result in any observable difference, but it is best to be safe.

Additionally, a constant stack variable is converted to an equivalent
macro define for cleanliness.

Finally, print the error type encountered if reading data from an ARP response
fails with a read error.

ifchd/ifchd.c

@@ -1,5 +1,5 @@
 /* ifchd.c - interface change daemon
- * Time-stamp: <2011-05-01 19:03:48 njk>
+ * Time-stamp: <2011-05-30 10:30:20 njk>
  *
  * (C) 2004-2011 Nicholas J. Kain <njkain at gmail dot com>
  *
@@ -664,10 +664,12 @@ static void process_client_fd(int fd)
     memset(buf, '\0', sizeof buf);
 
     r = safe_read(sks[sqidx], buf, sizeof buf / 2 - 1);
-    if (r <= 0) {
-        if (r != 0)
-            log_line("error reading from client fd: %s", strerror(errno));
+    if (r == 0)
         goto fail;
+    else if (r < 0) {
+        if (errno == EAGAIN || errno == EWOULDBLOCK)
+            return;
+        log_line("error reading from client fd: %s", strerror(errno));
     }
 
     /* Discard everything and close connection if we risk overflow.

ndhc/arp.c

@@ -1,5 +1,5 @@
 /* arp.c - arp ping checking
- * Time-stamp: <2011-04-19 16:21:14 njk>
+ * Time-stamp: <2011-05-30 10:23:54 njk>
  *
  * Copyright 2010-2011 Nicholas J. Kain <njkain@gmail.com>
  *
@@ -233,7 +233,10 @@ void handle_arp_response(struct client_state_t *cs)
         int r = safe_read(cs->arpFd, (char *)&arpreply + arpreply_offset,
                           sizeof arpreply - arpreply_offset);
         if (r < 0) {
-            log_warning("handle_arp_response: short read");
+            if (errno == EWOULDBLOCK || errno == EAGAIN)
+                return;
+            log_warning("handle_arp_response: ARP response read failed: %s",
+                        strerror(errno));
             switch (cs->dhcpState) {
                 case DS_ARP_CHECK: arp_failed(cs); break;
                 case DS_ARP_GW_CHECK: arp_gw_failed(cs); break;

ndhc/dhcpmsg.c

@@ -1,5 +1,5 @@
 /* dhcpmsg.c - dhcp packet generation and sending functions
- * Time-stamp: <2011-03-30 23:57:43 nk>
+ * Time-stamp: <2011-05-30 10:43:28 njk>
  *
  * (c) 2004-2011 Nicholas J. Kain <njkain at gmail dot com>
  * (c) 2001 Russ Dill <Russ.Dill@asu.edu>
@@ -217,11 +217,12 @@ int get_raw_packet(struct dhcpMessage *payload, int fd)
 {
     struct ip_udp_dhcp_packet packet;
     uint16_t check;
-    const int packet_size = sizeof(struct ip_udp_dhcp_packet);
 
-    memset(&packet, 0, packet_size);
-    int len = safe_read(fd, (char *)&packet, packet_size);
-    if (len == -1 && errno != EAGAIN && errno != EWOULDBLOCK) {
+    memset(&packet, 0, IP_UPD_DHCP_SIZE);
+    int len = safe_read(fd, (char *)&packet, IP_UPD_DHCP_SIZE);
+    if (len == -1) {
+        if (errno == EAGAIN || errno == EWOULDBLOCK)
+            return -2;
         log_line("get_raw_packet: read error %s", strerror(errno));
         usleep(500000); /* possible down interface, looping condition */
         return -1;
@@ -252,7 +253,7 @@ int get_raw_packet(struct dhcpMessage *payload, int fd)
         sleep(1);
         return -2;
     }
-    if (len > packet_size) {
+    if (len > IP_UPD_DHCP_SIZE) {
         log_line("Data longer than that of a IP+UDP+DHCP message: %d", len);
         sleep(1);
         return -2;

ndhc/packet.c

@@ -1,5 +1,5 @@
 /* packet.c - send and react to DHCP message packets
- * Time-stamp: <2011-03-31 15:22:59 nk>
+ * Time-stamp: <2011-05-30 10:39:56 njk>
  *
  * (c) 2004-2011 Nicholas J. Kain <njkain at gmail dot com>
  * (c) 2001 Russ Dill <Russ.Dill@asu.edu>
@@ -48,6 +48,8 @@ int get_packet(struct dhcpMessage *packet, int fd)
     memset(packet, 0, DHCP_SIZE);
     bytes = safe_read(fd, (char *)packet, DHCP_SIZE);
     if (bytes == -1) {
+        if (errno == EAGAIN || errno == EWOULDBLOCK)
+            return -2;
         log_line("Read on listen socket failed: %s", strerror(errno));
         return -1;
     }
@@ -326,6 +328,11 @@ void handle_packet(struct client_state_t *cs)
     if (len < 0)
         return;
 
+    if (len < DHCP_SIZE) {
+        log_line("Received short DHCP packet -- ignoring");
+        return;
+    }
+
     if (packet.xid != cs->xid) {
         log_line("Ignoring XID %lx (our xid is %lx).",
                  (uint32_t) packet.xid, cs->xid);