server/client: harden reading of initialization data, and make async

* Client terminal initialization data is now received
  asynchronously. To facilitate this, a couple of minor changes to the
  protocol was made:
    - The client now starts with sending a 4-byte unsigned integer
      with the *total* size of the initialization data (*not*
      including the size field itself.
    - Strings (TERM variable + argv) are now sent NULL-terminated

* The server allocates a single buffer, and fills it
  asynchronously. When full (as indicated by the initial 'total size'
  integer), we parse it (taking care not to read outside boundaries
  etc, and verifies the lengths (of the TERM variable and argv array)
  indicated by the client matches the actual lengths of the strings
  received.

* The server now ignores 'unexpected' data received from the client,
  after the terminal has been instantiated.

server.c

@@ -34,6 +34,12 @@ struct client {
     struct server *server;
     int fd;
 
+    struct {
+        uint8_t *data;
+        size_t left;
+        size_t idx;
+    } buffer;
+
     struct terminal *term;
 };
 
@@ -60,6 +66,7 @@ client_destroy(struct client *client)
         }
     }
 
+    free(client->buffer.data);
     free(client);
 }
 
@@ -93,7 +100,6 @@ fdm_client(struct fdm *fdm, int fd, int events, void *data)
     struct client *client = data;
     struct server *server = client->server;
 
-    char *term_env = NULL;
     char **argv = NULL;
     int argc = 0;
 
@@ -104,47 +110,107 @@ fdm_client(struct fdm *fdm, int fd, int events, void *data)
 
     if (client->term != NULL) {
         uint8_t dummy[128];
-        read(fd, dummy, sizeof(dummy));
+        ssize_t count = read(fd, dummy, sizeof(dummy));
+        LOG_WARN("client unexpectedly sent %zd bytes", count);
+        return true;  /* TODO: shutdown instead? */
+    }
+
+    if (client->buffer.data == NULL) {
+        /*
+         * We haven't received any data yet - the first thing the
+         * client sends is the total size of the initialization
+         * data.
+         */
+        uint32_t total_len;
+        if (recv(fd, &total_len, sizeof(total_len), 0) != sizeof(total_len)) {
+            LOG_ERRNO("failed to read total length");
+            goto shutdown;
+        }
+
+        LOG_DBG("total len: %u", total_len);
+        client->buffer.data = malloc(total_len + 1);
+        client->buffer.left = total_len;
+        client->buffer.idx = 0;
+
+        /* Prevent our strlen() calls to run outside */
+        client->buffer.data[total_len] = '\0';
+        return true;  /* Let FDM trigger again when we have more data */
+    }
+
+    /* Keep filling our buffer of initialization data */
+    ssize_t count = recv(
+        fd, &client->buffer.data[client->buffer.idx], client->buffer.left, 0);
+
+    if (count < 0) {
+        LOG_ERRNO("failed to read");
+        goto shutdown;
+    }
+
+    client->buffer.idx += count;
+    client->buffer.left -= count;
+
+    if (client->buffer.left > 0) {
+        /* Not done yet */
         return true;
     }
 
-    uint16_t term_env_len;
-    if (recv(fd, &term_env_len, sizeof(term_env_len), 0) != sizeof(term_env_len))
-        goto shutdown;
-
-    term_env = malloc(term_env_len + 1);
-    term_env[term_env_len] = '\0';
-    if (term_env_len > 0) {
-        if (recv(fd, term_env, term_env_len, 0) != term_env_len)
-            goto shutdown;
-    }
-
-    if (recv(fd, &argc, sizeof(argc), 0) != sizeof(argc))
-        goto shutdown;
-
-    LOG_DBG("argc = %d", argc);
-
-    argv = calloc(argc + 1, sizeof(argv[0]));
-    for (int i = 0; i < argc; i++) {
-        uint16_t len;
-        if (recv(fd, &len, sizeof(len), 0) != sizeof(len))
-            goto shutdown;
-
-        argv[i] = malloc(len + 1);
-        argv[i][len] = '\0';
-        if (len == 0)
-            continue;
-
-        if (recv(fd, argv[i], len, 0) != len)
-            goto shutdown;
-
-        LOG_DBG("argv[%d] = %s (%hu)", i, argv[i], len);
-    }
+    /* All initialization data received - time to instantiate a terminal! */
 
     assert(client->term == NULL);
+    assert(client->buffer.data != NULL);
+    assert(client->buffer.left == 0);
+
+    /*
+     * Parse the received buffer, verifying lengths etc
+     */
+
+#define CHECK_BUF(sz) do {                      \
+        if (p + (sz) > end)                     \
+            goto shutdown;                      \
+    } while (0)
+
+    uint8_t *p = client->buffer.data;
+    const uint8_t *end = &client->buffer.data[client->buffer.idx];
+
+    CHECK_BUF(sizeof(uint16_t));
+    uint16_t term_env_len = *(uint16_t *)p; p += sizeof(term_env_len);
+
+    CHECK_BUF(term_env_len);
+    const char *term_env = (const char *)p; p += strlen(term_env) + 1;
+    LOG_DBG("TERM = %.*s", term_env_len, term_env);
+
+    if (term_env_len != strlen(term_env) + 1) {
+        LOG_ERR("TERM length mismatch: indicated = %hu, actual = %zu",
+                term_env_len - 1, strlen(term_env));
+        goto shutdown;
+    }
+
+    CHECK_BUF(sizeof(argc));
+    argc = *(int *)p; p += sizeof(argc);
+    argv = calloc(argc + 1, sizeof(argv[0]));
+    LOG_DBG("argc = %d", argc);
+
+    for (int i = 0; i < argc; i++) {
+        CHECK_BUF(sizeof(uint16_t));
+        uint16_t len = *(uint16_t *)p; p += sizeof(len);
+
+        CHECK_BUF(len);
+        argv[i] = (char *)p; p += strlen(argv[i]) + 1;
+        LOG_DBG("argv[%d] = %s", i, argv[i]);
+
+        if (len != strlen(argv[i]) + 1) {
+            LOG_ERR("argv[%d] length mismatch: indicated = %hu, actual = %zu",
+                    i, len - 1, strlen(argv[i]));
+            goto shutdown;
+        }
+    }
+    argv[argc] = NULL;
+
+#undef CHECK_BUF
+
     client->term = term_init(
         server->conf, server->fdm, server->wayl,
-        term_env_len > 0 ? term_env : server->conf->term,
+        strlen(term_env) > 0 ? term_env : server->conf->term,
         argc, argv, &term_shutdown_handler, client);
 
     if (client->term == NULL) {
@@ -152,20 +218,13 @@ fdm_client(struct fdm *fdm, int fd, int events, void *data)
         goto shutdown;
     }
 
-    for (int i = 0; i < argc; i++)
-        free(argv[i]);
     free(argv);
-    free(term_env);
     return true;
 
 shutdown:
     LOG_DBG("client FD=%d: disconnected", client->fd);
 
-    for (int i = 0; i < argc; i++)
-        free(argv[i]);
     free(argv);
-    free(term_env);
-
     fdm_del(fdm, fd);
     client->fd = -1;