net: Fix checksum algorithm

The recent commit to improve the checksum algorithm actually broke it,
as shown by the NTP example not working anymore.

Rework the algorithm to make it work again, and improve its performance
by always processing two bytes at a time.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>

Author: pcercuei

kernel/net/net_ipv4.c

@@ -27,37 +27,30 @@
 
 static net_ipv4_stats_t ipv4_stats = { 0 };
 
-/* Perform an IP-style checksum on a block of data */
-uint16_t __pure net_ipv4_checksum(const uint8_t *data, size_t bytes, uint16_t start) {
-    uint32_t sum = start;
-    uintptr_t end = (uintptr_t)data + bytes;
+static inline uint16_t checksum_one(uint16_t val, uint16_t sum) {
+    uint16_t result;
 
-    /* Make sure we don't do any unaligned memory accesses */
-    if(((uint32_t)data) & 0x01) {
-        const uint8_t *ptr = data;
+    return __builtin_add_overflow(val, sum, &result) + result;
+}
 
-        while((size_t)(&ptr[2]) <= end) {
-            sum += ptr[0] | ((ptr[1]) << 8);
-            ptr += 2;
-        }
-    }
-    /* Fast path assumes at least 2-byte alignment. */
-    else {
-        const uint16_t *ptr = (const uint16_t *)data;
-        while((size_t)(&ptr[1]) <= end) {
-            sum += *ptr++;
-        }
+/* Perform an IP-style checksum on a block of data */
+uint16_t __pure net_ipv4_checksum(const uint8_t *data, size_t bytes, uint16_t sum) {
+    /* Make sure we don't do any unaligned memory accesses */
+    if((uintptr_t)data & 1) {
+        sum = checksum_one(*data, sum);
+        bytes--;
+        data++;
     }
 
-    /* Handle the last byte, if we have an odd byte count */
-    if(bytes & 0x1)
-        sum += data[bytes - 1];
+    /* Compute checksum two bytes at a time */
+    for(; bytes > 1; bytes -= 2, data += 2)
+        sum = checksum_one(*(const uint16_t *)data, sum);
 
-    /* Take care of any carry bits */
-    while(sum >> 16)
-        sum = (sum >> 16) + (sum & 0xFFFF);
+    /* Handle the last byte, if we have an odd byte count */
+    if(bytes)
+        sum = checksum_one(*data, sum);
 
-    return (uint16_t)~sum;
+    return ~sum;
 }
 
 /* Determine if a given IP is in the current network */