1 files changed, 45 insertions, 10 deletions

diff --git a/nbd/client.c b/nbd/client.c
index 1593cd6c0e..6777e589d1 100644
--- a/nbd/client.c
+++ b/nbd/client.c
@@ -73,16 +73,46 @@ static QTAILQ_HEAD(, NBDExport) exports = QTAILQ_HEAD_INITIALIZER(exports);
 */
 
 
-static int nbd_handle_reply_err(uint32_t opt, uint32_t type, Error **errp)
+/* If type represents success, return 1 without further action.
+ * If type represents an error reply, consume the rest of the packet on ioc.
+ * Then return 0 for unsupported (so the client can fall back to
+ * other approaches), or -1 with errp set for other errors.
+ */
+static int nbd_handle_reply_err(QIOChannel *ioc, uint32_t opt, uint32_t type,
+                                Error **errp)
 {
+    uint32_t len;
+    char *msg = NULL;
+    int result = -1;
+
     if (!(type & (1 << 31))) {
-        return 0;
+        return 1;
+    }
+
+    if (read_sync(ioc, &len, sizeof(len)) != sizeof(len)) {
+        error_setg(errp, "failed to read option length");
+        return -1;
+    }
+    len = be32_to_cpu(len);
+    if (len) {
+        if (len > NBD_MAX_BUFFER_SIZE) {
+            error_setg(errp, "server's error message is too long");
+            goto cleanup;
+        }
+        msg = g_malloc(len + 1);
+        if (read_sync(ioc, msg, len) != len) {
+            error_setg(errp, "failed to read option error message");
+            goto cleanup;
+        }
+        msg[len] = '\0';
     }
 
     switch (type) {
     case NBD_REP_ERR_UNSUP:
-        error_setg(errp, "Unsupported option type %x", opt);
-        break;
+        TRACE("server doesn't understand request %d, attempting fallback",
+              opt);
+        result = 0;
+        goto cleanup;
 
     case NBD_REP_ERR_POLICY:
         error_setg(errp, "Denied by server for option %x", opt);
@@ -101,7 +131,13 @@ static int nbd_handle_reply_err(uint32_t opt, uint32_t type, Error **errp)
         break;
     }
 
-    return -1;
+    if (msg) {
+        error_append_hint(errp, "%s\n", msg);
+    }
+
+ cleanup:
+    g_free(msg);
+    return result;
 }
 
 static int nbd_receive_list(QIOChannel *ioc, char **name, Error **errp)
@@ -111,6 +147,7 @@ static int nbd_receive_list(QIOChannel *ioc, char **name, Error **errp)
     uint32_t type;
     uint32_t len;
     uint32_t namelen;
+    int error;
 
     *name = NULL;
     if (read_sync(ioc, &magic, sizeof(magic)) != sizeof(magic)) {
@@ -138,11 +175,9 @@ static int nbd_receive_list(QIOChannel *ioc, char **name, Error **errp)
         return -1;
     }
     type = be32_to_cpu(type);
-    if (type == NBD_REP_ERR_UNSUP) {
-        return 0;
-    }
-    if (nbd_handle_reply_err(opt, type, errp) < 0) {
-        return -1;
+    error = nbd_handle_reply_err(ioc, opt, type, errp);
+    if (error <= 0) {
+        return error;
     }
 
     if (read_sync(ioc, &len, sizeof(len)) != sizeof(len)) {