[Qemu-devel] [PATCH] nbd: Tolerate more errors to structured reply request

[Eric Blake <eblake@redhat.com>]

A server may have a reason to reject a request for structured replies,
beyond just not recognizing them as a valid request.  It doesn't hurt
us to continue talking to such a server; otherwise 'qemu-nbd --list'
of such a server fails to display all possible details about the
export.

Encountered when temporarily tweaking nbdkit to reply with
NBD_REP_ERR_POLICY.  Present since structured reply support was first
added (commit d795299b reused starttls handling, but starttls has to
reject all errors).

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 nbd/client.c | 39 +++++++++++++++++++++++----------------
 1 file changed, 23 insertions(+), 16 deletions(-)

diff --git a/nbd/client.c b/nbd/client.c
index 8f524c3e3502..204f6e928d14 100644
--- a/nbd/client.c
+++ b/nbd/client.c
@@ -1,5 +1,5 @@
 /*
- *  Copyright (C) 2016-2018 Red Hat, Inc.
+ *  Copyright (C) 2016-2019 Red Hat, Inc.
  *  Copyright (C) 2005  Anthony Liguori <anthony@codemonkey.ws>
  *
  *  Network Block Device Client Side
@@ -141,17 +141,19 @@ static int nbd_receive_option_reply(QIOChannel *ioc, uint32_t opt,
     return 0;
 }

-/* If reply represents success, return 1 without further action.
- * If reply represents an error, consume the optional payload 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.
+/*
+ * If reply represents success, return 1 without further action.  If
+ * reply represents an error, consume the optional payload of the
+ * packet on ioc.  Then return 0 for unsupported (so the client can
+ * fall back to other approaches), where @strict determines if only
+ * ERR_UNSUP or all errors fit that category, or -1 with errp set for
+ * other errors.
  */
 static int nbd_handle_reply_err(QIOChannel *ioc, NBDOptionReply *reply,
-                                Error **errp)
+                                bool strict, Error **errp)
 {
     char *msg = NULL;
-    int result = -1;
+    int result = strict ? -1 : 0;

     if (!(reply->type & (1 << 31))) {
         return 1;
@@ -162,6 +164,7 @@ static int nbd_handle_reply_err(QIOChannel *ioc, NBDOptionReply *reply,
             error_setg(errp, "server error %" PRIu32
                        " (%s) message is too long",
                        reply->type, nbd_rep_lookup(reply->type));
+            result = -1;
             goto cleanup;
         }
         msg = g_malloc(reply->length + 1);
@@ -169,6 +172,7 @@ static int nbd_handle_reply_err(QIOChannel *ioc, NBDOptionReply *reply,
             error_prepend(errp, "Failed to read option error %" PRIu32
                           " (%s) message: ",
                           reply->type, nbd_rep_lookup(reply->type));
+            result = -1;
             goto cleanup;
         }
         msg[reply->length] = '\0';
@@ -257,7 +261,7 @@ static int nbd_receive_list(QIOChannel *ioc, char **name, char **description,
     if (nbd_receive_option_reply(ioc, NBD_OPT_LIST, &reply, errp) < 0) {
         return -1;
     }
-    error = nbd_handle_reply_err(ioc, &reply, errp);
+    error = nbd_handle_reply_err(ioc, &reply, true, errp);
     if (error <= 0) {
         return error;
     }
@@ -370,7 +374,7 @@ static int nbd_opt_info_or_go(QIOChannel *ioc, uint32_t opt,
         if (nbd_receive_option_reply(ioc, opt, &reply, errp) < 0) {
             return -1;
         }
-        error = nbd_handle_reply_err(ioc, &reply, errp);
+        error = nbd_handle_reply_err(ioc, &reply, true, errp);
         if (error <= 0) {
             return error;
         }
@@ -545,12 +549,15 @@ static int nbd_receive_query_exports(QIOChannel *ioc,
     }
 }

-/* nbd_request_simple_option: Send an option request, and parse the reply
+/*
+ * nbd_request_simple_option: Send an option request, and parse the reply.
+ * @strict controls whether ERR_UNSUP or all errors produce 0 status.
  * return 1 for successful negotiation,
  *        0 if operation is unsupported,
  *        -1 with errp set for any other error
  */
-static int nbd_request_simple_option(QIOChannel *ioc, int opt, Error **errp)
+static int nbd_request_simple_option(QIOChannel *ioc, int opt, bool strict,
+                                     Error **errp)
 {
     NBDOptionReply reply;
     int error;
@@ -562,7 +569,7 @@ static int nbd_request_simple_option(QIOChannel *ioc, int opt, Error **errp)
     if (nbd_receive_option_reply(ioc, opt, &reply, errp) < 0) {
         return -1;
     }
-    error = nbd_handle_reply_err(ioc, &reply, errp);
+    error = nbd_handle_reply_err(ioc, &reply, strict, errp);
     if (error <= 0) {
         return error;
     }
@@ -594,7 +601,7 @@ static QIOChannel *nbd_receive_starttls(QIOChannel *ioc,
     QIOChannelTLS *tioc;
     struct NBDTLSHandshakeData data = { 0 };

-    ret = nbd_request_simple_option(ioc, NBD_OPT_STARTTLS, errp);
+    ret = nbd_request_simple_option(ioc, NBD_OPT_STARTTLS, true, errp);
     if (ret <= 0) {
         if (ret == 0) {
             error_setg(errp, "Server don't support STARTTLS option");
@@ -694,7 +701,7 @@ static int nbd_receive_one_meta_context(QIOChannel *ioc,
         return -1;
     }

-    ret = nbd_handle_reply_err(ioc, &reply, errp);
+    ret = nbd_handle_reply_err(ioc, &reply, true, errp);
     if (ret <= 0) {
         return ret;
     }
@@ -950,7 +957,7 @@ static int nbd_start_negotiate(AioContext *aio_context, QIOChannel *ioc,
             if (structured_reply) {
                 result = nbd_request_simple_option(ioc,
                                                    NBD_OPT_STRUCTURED_REPLY,
-                                                   errp);
+                                                   false, errp);
                 if (result < 0) {
                     return -EINVAL;
                 }
-- 
2.20.1