nbd/server: add nbd_read_opt_name helper

Add helper to read name in format:

  uint32 len       (<= NBD_MAX_NAME_SIZE)
  len bytes string (not 0-terminated)

The helper will be reused in following patch.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20180312152126.286890-3-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: grammar fixes, actually check error]
Signed-off-by: Eric Blake <eblake@redhat.com>

nbd/server.c

@@ -273,6 +273,48 @@ static int nbd_opt_read(NBDClient *client, void *buffer, size_t size,
     return qio_channel_read_all(client->ioc, buffer, size, errp) < 0 ? -EIO : 1;
 }
 
+/* nbd_opt_read_name
+ *
+ * Read a string with the format:
+ *   uint32_t len     (<= NBD_MAX_NAME_SIZE)
+ *   len bytes string (not 0-terminated)
+ *
+ * @name should be enough to store NBD_MAX_NAME_SIZE+1.
+ * If @length is non-null, it will be set to the actual string length.
+ *
+ * Return -errno on I/O error, 0 if option was completely handled by
+ * sending a reply about inconsistent lengths, or 1 on success.
+ */
+static int nbd_opt_read_name(NBDClient *client, char *name, uint32_t *length,
+                             Error **errp)
+{
+    int ret;
+    uint32_t len;
+
+    ret = nbd_opt_read(client, &len, sizeof(len), errp);
+    if (ret <= 0) {
+        return ret;
+    }
+    cpu_to_be32s(&len);
+
+    if (len > NBD_MAX_NAME_SIZE) {
+        return nbd_opt_invalid(client, errp,
+                               "Invalid name length: %" PRIu32, len);
+    }
+
+    ret = nbd_opt_read(client, name, len, errp);
+    if (ret <= 0) {
+        return ret;
+    }
+    name[len] = '\0';
+
+    if (length) {
+        *length = len;
+    }
+
+    return 1;
+}
+
 /* Send a single NBD_REP_SERVER reply to NBD_OPT_LIST, including payload.
  * Return -errno on error, 0 on success. */
 static int nbd_negotiate_send_rep_list(NBDClient *client, NBDExport *exp,
@@ -455,19 +497,10 @@ static int nbd_negotiate_handle_info(NBDClient *client, uint16_t myflags,
         2 bytes: N, number of requests (can be 0)
         N * 2 bytes: N requests
     */
-    rc = nbd_opt_read(client, &namelen, sizeof(namelen), errp);
-    if (rc <= 0) {
-        return rc;
-    }
-    be32_to_cpus(&namelen);
-    if (namelen >= sizeof(name)) {
-        return nbd_opt_invalid(client, errp, "name too long for qemu");
-    }
-    rc = nbd_opt_read(client, name, namelen, errp);
+    rc = nbd_opt_read_name(client, name, &namelen, errp);
     if (rc <= 0) {
         return rc;
     }
-    name[namelen] = '\0';
     trace_nbd_negotiate_handle_export_name_request(name);
 
     rc = nbd_opt_read(client, &requests, sizeof(requests), errp);