libsystemd-dhcp: Fix checksum computation for buffer with odd size

Fix off-by-one error and notice that summing may need more than one
round for the result to be in the lower 16 bits.

diff --git a/src/libsystemd-dhcp/dhcp-client.c b/src/libsystemd-dhcp/dhcp-client.c
index 7dc154612816ac3902070cce9bbecdba76efe5f4..68a3b1a1b64c319db8d186091c88530c4c2f1605 100644 (file)
--- a/src/libsystemd-dhcp/dhcp-client.c
+++ b/src/libsystemd-dhcp/dhcp-client.c
@@ -382,10 +382,13 @@ static uint16_t client_checksum(void *buf, int len)
 
         if (len & 0x01) {
                 odd = buf;
-                sum += odd[len];
+                sum += odd[len - 1];
         }
 
-        return ~((sum & 0xffff) + (sum >> 16));
+        while (sum >> 16)
+                sum = (sum & 0xffff) + (sum >> 16);
+
+        return ~sum;
 }
 
 static void client_append_ip_headers(DHCPPacket *packet, uint16_t len)

diff --git a/src/libsystemd-dhcp/test-dhcp-client.c b/src/libsystemd-dhcp/test-dhcp-client.c
index d398510745f373049c4e122da48a882b04a377b1..7400cc6837e72b48fd22692d031f7beab871cab9 100644 (file)
--- a/src/libsystemd-dhcp/test-dhcp-client.c
+++ b/src/libsystemd-dhcp/test-dhcp-client.c
@@ -102,10 +102,13 @@ static uint16_t client_checksum(void *buf, int len)
 
         if (len & 0x01) {
                 odd = buf;
-                sum += odd[len];
+                sum += odd[len - 1];
         }
 
-        return ~((sum & 0xffff) + (sum >> 16));
+        while (sum >> 16)
+                sum = (sum & 0xffff) + (sum >> 16);
+
+        return ~sum;
 }
 
 static void test_checksum(void)