net: Added checksum_adjust to partial adjust checksum

Author: AndreasAakesson

api/net/inet_common.hpp

@@ -53,6 +53,38 @@ namespace net {
     return checksum(0, data, len);
   }
 
+  /**
+   * @brief      Adjust the checksum according to the difference between old and new data.
+   *
+   * @note       Only supports even offsets (length needs to be even)
+   *             See: https://tools.ietf.org/html/rfc3022#page-9 (4.2) Checksum Adjustment
+   *
+   * @param      chksum  Pointer to the checksum to adjust
+   * @param      odata   The old data
+   * @param[in]  olen    The length of the old data
+   * @param      ndata   The new data
+   * @param[in]  nlen    The length of the new data
+   */
+  void checksum_adjust(uint8_t* chksum, const void* odata,
+                       int olen, const void* ndata, int nlen);
+
+  /**
+   * @brief      Helper function for adjusting checksum when only an object is changed,
+   *             e.g. IP address in a packet header
+   *
+   * @param      chksum   Pointer to the checksum to adjust
+   * @param[in]  old_obj  The old object
+   * @param[in]  new_obj  The new object
+   *
+   * @tparam     T        The type of object
+   */
+  template <typename T>
+  void checksum_adjust(uint16_t* chksum, const T* old_obj, const T* new_obj)
+  {
+    static_assert(sizeof(T) % 2 == 0, "Checksum adjust only supports even lengths");
+    checksum_adjust(reinterpret_cast<uint8_t*>(chksum), old_obj, sizeof(T), new_obj, sizeof(T));
+  }
+
   // View a packet differently based on context
   template <typename T, typename Packet>
   inline auto view_packet_as(Packet packet) noexcept {

src/net/inet_common.cpp

@@ -73,4 +73,34 @@ uint16_t checksum(uint32_t tsum, const void* data, size_t length) noexcept
   return ~a16;
 }
 
+// Taken from https://tools.ietf.org/html/rfc3022#page-9
+void checksum_adjust(uint8_t* chksum, const void* odata,
+   int olen, const void* ndata, int nlen)
+{
+  Expects(olen % 2 == 0 and nlen % 2 == 0);
+
+  const auto* optr = reinterpret_cast<const uint8_t*>(odata);
+  const auto* nptr = reinterpret_cast<const uint8_t*>(ndata);
+
+  int32_t x, o32, n32;
+  x=chksum[0]*256+chksum[1];
+  x=~x & 0xFFFF;
+  while (olen)
+  {
+    o32=optr[0]*256+optr[1]; optr+=2;
+    x-=o32 & 0xffff;
+    if (x<=0) { x--; x&=0xffff; }
+    olen-=2;
+  }
+  while (nlen)
+  {
+    n32=nptr[0]*256+nptr[1]; nptr+=2;
+    x+=n32 & 0xffff;
+    if (x & 0x10000) { x++; x&=0xffff; }
+    nlen-=2;
+  }
+  x=~x & 0xFFFF;
+  chksum[0]=x/256; chksum[1]=x & 0xff;
+}
+
 } //< namespace net

test/net/unit/checksum.cpp

@@ -70,3 +70,38 @@ CASE("Verify IP checksum using buffers of various lengths")
     EXPECT(net::checksum(buffer, sizeof(buffer)) == 0);
   }
 }
+
+CASE("Verify adjusting checksum")
+{
+  char buffer[2048];
+  for (size_t j = 0; j < 32; j++)
+  {
+    buffer[0] = 0;
+    buffer[1] = 0;
+    for (size_t i = 2; i < sizeof(buffer); i++)
+      buffer[i] = rand() & 0xff;
+
+    // Set checksum
+    auto csum = net::checksum(buffer, sizeof(buffer));
+    memcpy(buffer, &csum, sizeof(csum));
+
+    // Create some random data
+    char rndm[512];
+    for (size_t i = 0; i < sizeof(rndm); i++)
+      rndm[i] = rand() & 0xff;
+
+    // A fixed point inside the buffer
+    const size_t N = sizeof(buffer) / 2;
+
+    // Adjust the checksum
+    net::checksum_adjust((uint8_t*)buffer, &buffer[N], sizeof(rndm), rndm, sizeof(rndm));
+
+    // Make sure to add the random data at the same place
+    memcpy(&buffer[N], rndm, sizeof(rndm));
+
+    // Checksum the buffer (without the csum)
+    csum = net::checksum(&buffer[2], sizeof(buffer)-2);
+
+    EXPECT(csum == *(uint16_t*)buffer);
+  }
+}