Add inet_csum_slice() to fix checksum for a sliced layer 4 payload

Padding for an odd number of bytes was not calculated properly.

sys/include/net/inet_csum.h

@@ -21,13 +21,15 @@
 #define INET_CSUM_H_
 
 #include <inttypes.h>
+#include <stddef.h>
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
 /**
- * @brief   Calculates the unnormalized Internet Checksum of @p buf.
+ * @brief   Calculates the unnormalized Internet Checksum of @p buf, where the
+ *          buffer provides a slice of the full checksum domain, calculated in order.
  *
  * @see <a href="https://tools.ietf.org/html/rfc1071">
  *          RFC 1071
@@ -35,14 +37,41 @@ extern "C" {
  *
  * @details The Internet Checksum is not normalized (i. e. its 1's complement
  *          was not taken of the result) to use it for further calculation.
+ *          This function handles padding an odd number of bytes across the full domain.
  *
- * @param[in] sum   An initial value for the checksum.
- * @param[in] buf   A buffer.
- * @param[in] len   Length of @p buf in byte.
+ * @param[in] sum       An initial value for the checksum.
+ * @param[in] buf       A buffer.
+ * @param[in] len       Length of @p buf in byte.
+ * @param[in] accum_len Accumulated length of checksum domain that has already
+ *                      been checksummed.
  *
  * @return  The unnormalized Internet Checksum of @p buf.
  */
-uint16_t inet_csum(uint16_t sum, const uint8_t *buf, uint16_t len);
+uint16_t inet_csum_slice(uint16_t sum, const uint8_t *buf, uint16_t len, size_t accum_len);
+
+/**
+ * @brief   Calculates the unnormalized Internet Checksum of @p buf, where the
+ *          buffer provides a standalone domain for the checksum.
+ *
+ * @see <a href="https://tools.ietf.org/html/rfc1071">
+ *          RFC 1071
+ *      </a>
+ *
+ * @details The Internet Checksum is not normalized (i. e. its 1's complement
+ *          was not taken of the result) to use it for further calculation.
+ *          This function, rather than inet_csum_slice(), has been used historically
+ *          when we are not concerned with padding for an odd number of bytes.
+ *
+ *
+ * @param[in] sum       An initial value for the checksum.
+ * @param[in] buf       A buffer.
+ * @param[in] len       Length of @p buf in byte.
+ *
+ * @return  The unnormalized Internet Checksum of @p buf.
+ */
+static inline uint16_t inet_csum(uint16_t sum, const uint8_t *buf, uint16_t len) {
+    return inet_csum_slice(sum, buf, len, 0);
+}
 
 #ifdef __cplusplus
 }

sys/net/crosslayer/inet_csum/inet_csum.c

@@ -20,7 +20,7 @@
 #define ENABLE_DEBUG    (0)
 #include "debug.h"
 
-uint16_t inet_csum(uint16_t sum, const uint8_t *buf, uint16_t len)
+uint16_t inet_csum_slice(uint16_t sum, const uint8_t *buf, uint16_t len, size_t accum_len)
 {
     uint32_t csum = sum;
 
@@ -34,14 +34,23 @@ uint16_t inet_csum(uint16_t sum, const uint8_t *buf, uint16_t len)
 #endif
 #endif
 
+    if (len == 0)
+        return csum;
+
+    if (accum_len & 1) {      /* if accumulated length is odd */
+        csum += *buf;         /* add first byte as bottom half of 16-byte word */
+        buf++;
+        len--;
+        accum_len++;
+    }
+
     for (int i = 0; i < (len >> 1); buf += 2, i++) {
         csum += (*buf << 8) + *(buf + 1);   /* group bytes by 16-byte words
                                              * and add them*/
     }
 
-    if (len & 1) {                      /* if len is odd */
-        csum += (*buf << 8);            /* add last byte as top half of 16-byte word */
-    }
+    if ((accum_len + len) & 1)      /* if accumulated length is odd */
+        csum += (*buf << 8);        /* add last byte as top half of 16-byte word */
 
     while (csum >> 16) {
         uint16_t carry = csum >> 16;