From: Patrik Flykt <patrik.flykt@linux.intel.com>
Date: Fri, 20 Dec 2013 15:16:15 +0000 (+0200)
Subject: libsystemd-dhcp: Fix checksum computation for buffer with odd size
X-Git-Tag: v209~727
X-Git-Url: https://www.chiark.greenend.org.uk/ucgi/~ianmdlvl/git?p=elogind.git;a=commitdiff_plain;h=0c6a3c888abd720b5ab28162d1ba4eadffde5272;hp=77e8d29dd2ecfa7f11c31c8d96021d323ab3fe3a;ds=sidebyside

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 7dc154612..68a3b1a1b 100644
--- 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 d39851074..7400cc683 100644
--- 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)