Add mg_crc16(), make PPP use it

mongoose.c

@@ -5948,30 +5948,14 @@ uint8_t *mg_l2_pppoe_header(struct mg_tcpip_if *ifp, enum mg_l2proto proto,
                                            s_id, src, dst, frame));
 }
 
-// Calculate FCS/CRC for PPP frames. Could be implemented faster using lookup
-// tables.
-static uint16_t fcs_do(uint8_t *frame, size_t len) {
-  uint32_t fcs = 0xffff;
-  unsigned int j;
-  for (j = 0; j < len; j++) {
-    unsigned int i;
-    uint8_t x = frame[j];
-    for (i = 0; i < 8; i++) {
-      fcs = ((fcs ^ x) & 1) ? (fcs >> 1) ^ 0x8408 : fcs >> 1;
-      x >>= 1;
-    }
-  }
-  return (uint16_t) (fcs & 0xffff);
-}
-
 size_t mg_l2_ppp_trailer(struct mg_tcpip_if *ifp, size_t len, uint8_t *cur) {
   uint16_t crc;
   uint8_t *frame;
   len += sizeof(struct ppp) + sizeof(struct hdlc_);
   frame = cur - len;
-  crc = fcs_do(frame, len);
-  *cur++ = (uint8_t) ~crc;  // add CRC, note the byte order
-  *cur++ = (uint8_t) (~crc >> 8);
+  crc = mg_crc16(0, (const char *) frame, len);
+  *cur++ = (uint8_t) crc;  // add CRC, note the byte order
+  *cur++ = (uint8_t) (crc >> 8);
   // there is no len field in PPP
   (void) ifp;
   return len + 2;
@@ -26075,6 +26059,20 @@ uint32_t mg_crc32(uint32_t crc, const char *buf, size_t len) {
   return ~crc;
 }
 
+uint16_t mg_crc16(uint16_t crc, const char *buf, size_t len) {
+  static const uint16_t crclut[16] = {
+      // table for polynomial 0x8408 (reflected)
+      0x0000, 0x1081, 0x2102, 0x3183, 0x4204, 0x5285, 0x6306, 0x7387,
+      0x8408, 0x9489, 0xA50A, 0xB58B, 0xC60C, 0xD68D, 0xE70E, 0xF78F};
+  unsigned int c = (unsigned int) ~crc & 0xffff;
+  while (len--) {
+    uint8_t b = *(uint8_t *) buf++;
+    c = crclut[(c ^ b) & 0x0F] ^ (c >> 4);
+    c = crclut[(c ^ (b >> 4)) & 0x0F] ^ (c >> 4);
+  }
+  return (uint16_t) ~c;
+}
+
 static int isbyte(int n) {
   return n >= 0 && n <= 255;
 }

mongoose.h

@@ -1960,6 +1960,10 @@ char *mg_random_str(char *buf, size_t len);
 // a new checksum; pass the result of a prior call to extend over more data.
 uint32_t mg_crc32(uint32_t crc, const char *buf, size_t len);
 
+// Computes CRC16 (HDLC, polynomial 0x8408) over buf/len. Pass crc=0 to start
+// a new checksum; pass the result of a prior call to extend over more data.
+uint16_t mg_crc16(uint16_t crc, const char *buf, size_t len);
+
 // Returns true if path is safe to serve from the filesystem. Rejects paths
 // that start with '~' or '..', or contain a '/../' component, to prevent
 // directory traversal attacks.

src/l2_ppp.c

@@ -169,30 +169,14 @@ uint8_t *mg_l2_pppoe_header(struct mg_tcpip_if *ifp, enum mg_l2proto proto,
                                            s_id, src, dst, frame));
 }
 
-// Calculate FCS/CRC for PPP frames. Could be implemented faster using lookup
-// tables.
-static uint16_t fcs_do(uint8_t *frame, size_t len) {
-  uint32_t fcs = 0xffff;
-  unsigned int j;
-  for (j = 0; j < len; j++) {
-    unsigned int i;
-    uint8_t x = frame[j];
-    for (i = 0; i < 8; i++) {
-      fcs = ((fcs ^ x) & 1) ? (fcs >> 1) ^ 0x8408 : fcs >> 1;
-      x >>= 1;
-    }
-  }
-  return (uint16_t) (fcs & 0xffff);
-}
-
 size_t mg_l2_ppp_trailer(struct mg_tcpip_if *ifp, size_t len, uint8_t *cur) {
   uint16_t crc;
   uint8_t *frame;
   len += sizeof(struct ppp) + sizeof(struct hdlc_);
   frame = cur - len;
-  crc = fcs_do(frame, len);
-  *cur++ = (uint8_t) ~crc;  // add CRC, note the byte order
-  *cur++ = (uint8_t) (~crc >> 8);
+  crc = mg_crc16(0, (const char *) frame, len);
+  *cur++ = (uint8_t) crc;  // add CRC, note the byte order
+  *cur++ = (uint8_t) (crc >> 8);
   // there is no len field in PPP
   (void) ifp;
   return len + 2;

src/util.c

@@ -128,6 +128,20 @@ uint32_t mg_crc32(uint32_t crc, const char *buf, size_t len) {
   return ~crc;
 }
 
+uint16_t mg_crc16(uint16_t crc, const char *buf, size_t len) {
+  static const uint16_t crclut[16] = {
+      // table for polynomial 0x8408 (reflected)
+      0x0000, 0x1081, 0x2102, 0x3183, 0x4204, 0x5285, 0x6306, 0x7387,
+      0x8408, 0x9489, 0xA50A, 0xB58B, 0xC60C, 0xD68D, 0xE70E, 0xF78F};
+  unsigned int c = (unsigned int) ~crc & 0xffff;
+  while (len--) {
+    uint8_t b = *(uint8_t *) buf++;
+    c = crclut[(c ^ b) & 0x0F] ^ (c >> 4);
+    c = crclut[(c ^ (b >> 4)) & 0x0F] ^ (c >> 4);
+  }
+  return (uint16_t) ~c;
+}
+
 static int isbyte(int n) {
   return n >= 0 && n <= 255;
 }

src/util.h

@@ -40,6 +40,10 @@ char *mg_random_str(char *buf, size_t len);
 // a new checksum; pass the result of a prior call to extend over more data.
 uint32_t mg_crc32(uint32_t crc, const char *buf, size_t len);
 
+// Computes CRC16 (HDLC, polynomial 0x8408) over buf/len. Pass crc=0 to start
+// a new checksum; pass the result of a prior call to extend over more data.
+uint16_t mg_crc16(uint16_t crc, const char *buf, size_t len);
+
 // Returns true if path is safe to serve from the filesystem. Rejects paths
 // that start with '~' or '..', or contain a '/../' component, to prevent
 // directory traversal attacks.

test/ppp_test.c

@@ -61,6 +61,17 @@ static void *ppp_payload(uint8_t *frame) {
   return (struct ppp *) ((struct hdlc_ *) frame + 1) + 1;
 }
 
+static void test_fcs(void) {
+  struct mg_tcpip_if ifp;
+  uint8_t frame[] = {0xff, 0x03, 0x00, 0x21, 1, 2, 3, 4, 0, 0};
+  uint8_t fcs[] = {0xae, 0x77};
+  init_if(&ifp, MG_TCPIP_L2_PPP);
+  // PPP trailer stores the FCS bytes in the proper order
+  ASSERT(mg_l2_ppp_trailer(&ifp, 4, frame + sizeof(frame) - sizeof(fcs)) ==
+         sizeof(frame));
+  ASSERT(memcmp(frame + sizeof(frame) - sizeof(fcs), fcs, sizeof(fcs)) == 0);
+}
+
 static size_t mk_pppoe_disc(uint8_t *buf, uint8_t code, uint16_t id,
                             const void *payload, size_t len) {
   struct eth *eth = (struct eth *) buf;
@@ -385,6 +396,10 @@ static void test_pppoe(void) {
   printf("HEALTH_DASHBOARD\t\"%s\": %s,\n", x, s_error ? "false" : "true")
 
 int main(void) {
+  s_error = false;
+  test_fcs();
+  DASHBOARD("fcs");
+
   s_error = false;
   test_lcp();
   DASHBOARD("lcp");

test/unit_test.c

@@ -3039,6 +3039,13 @@ static void test_crc32(void) {
   ASSERT(mg_crc32(mg_crc32(0, "ab", 2), "c", 1) == 891568578);
 }
 
+static void test_crc16(void) {
+  ASSERT(mg_crc16(0, 0, 0) == 0);
+  ASSERT(mg_crc16(0, "a", 1) == 0x82f7);
+  ASSERT(mg_crc16(0, "abc", 3) == 0x9e25);
+  ASSERT(mg_crc16(mg_crc16(0, "ab", 2), "c", 1) == 0x9e25);
+}
+
 static void us(struct mg_connection *c, int ev, void *ev_data) {
   struct mg_http_message *hm = (struct mg_http_message *) ev_data;
   if (ev == MG_EV_HTTP_MSG && mg_match(hm->uri, mg_str("/upload"), NULL)) {
@@ -5512,6 +5519,7 @@ int main(void) {
   test_str();
   test_match();
   test_crc32();
+  test_crc16();
   DASHBOARD("misc");
 
   s_error = false;