[Qemu-devel] [PATCH v3 16/19] nbd/client: Add meta contexts to nbd_receive_export_list()

[Eric Blake <eblake@redhat.com>]

We want to be able to detect whether a given qemu NBD server is
exposing the right export(s) and dirty bitmaps, at least for
regression testing.  We could use 'nbd-client -l' from the upstream
NBD project to list exports, but it's annoying to rely on
out-of-tree binaries; furthermore, nbd-client doesn't necessarily
know about all of the qemu NBD extensions.  Thus, we plan on adding
a new mode to qemu-nbd that merely sniffs all possible information
from the server during handshake phase, then disconnects and dumps
the information.

This patch continues the work of the previous patch, by adding the
ability to track the list of available meta contexts into
NBDExportInfo.  It benefits from the recent refactoring patches
with a new nbd_list_meta_contexts() that reuses much of the same
framework as setting a meta context.

Note: a malicious server could exhaust memory of a client by feeding
an unending loop of contexts; perhaps we could place a limit on how
many we are willing to receive. But this is no different from our
earlier analysis on a server sending an unending list of exports,
and the death of a client due to memory exhaustion when the client
was going to exit soon anyways is not really a denial of service
attack.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20181215135324.152629-20-eblake@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>

---
v3: mention security (non-)issue in commit message [Rich]
---
 include/block/nbd.h |  2 ++
 nbd/client.c        | 41 +++++++++++++++++++++++++++++++++++++++--
 2 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/include/block/nbd.h b/include/block/nbd.h
index e9a442ce7e9..80ee3cc997e 100644
--- a/include/block/nbd.h
+++ b/include/block/nbd.h
@@ -284,6 +284,8 @@ struct NBDExportInfo {

     /* Set by server results during nbd_receive_export_list() */
     char *description;
+    int n_contexts;
+    char **contexts;
 };
 typedef struct NBDExportInfo NBDExportInfo;

diff --git a/nbd/client.c b/nbd/client.c
index a5a705a67f7..2001e6e8160 100644
--- a/nbd/client.c
+++ b/nbd/client.c
@@ -817,6 +817,36 @@ static int nbd_negotiate_simple_meta_context(QIOChannel *ioc,
     return received;
 }

+/*
+ * nbd_list_meta_contexts:
+ * Request the server to list all meta contexts for export @info->name.
+ * return 0 if list is complete (even if empty),
+ *        -1 with errp set for any other error
+ */
+static int nbd_list_meta_contexts(QIOChannel *ioc,
+                                  NBDExportInfo *info,
+                                  Error **errp)
+{
+    int ret;
+
+    if (nbd_send_one_meta_context(ioc, NBD_OPT_LIST_META_CONTEXT,
+                                  info->name, NULL, errp) < 0) {
+        return -1;
+    }
+
+    while (1) {
+        char *context;
+
+        ret = nbd_receive_one_meta_context(ioc, NBD_OPT_LIST_META_CONTEXT,
+                                           &context, NULL, errp);
+        if (ret <= 0) {
+            return ret;
+        }
+        info->contexts = g_renew(char *, info->contexts, ++info->n_contexts);
+        info->contexts[info->n_contexts - 1] = context;
+    }
+}
+
 /*
  * nbd_start_negotiate:
  * Start the handshake to the server.  After a positive return, the server
@@ -1063,7 +1093,7 @@ int nbd_receive_negotiate(QIOChannel *ioc, QCryptoTLSCreds *tlscreds,
 /* Clean up result of nbd_receive_export_list */
 void nbd_free_export_list(NBDExportInfo *info, int count)
 {
-    int i;
+    int i, j;

     if (!info) {
         return;
@@ -1072,6 +1102,10 @@ void nbd_free_export_list(NBDExportInfo *info, int count)
     for (i = 0; i < count; i++) {
         g_free(info[i].name);
         g_free(info[i].description);
+        for (j = 0; j < info[i].n_contexts; j++) {
+            g_free(info[i].contexts[j]);
+        }
+        g_free(info[i].contexts);
     }
     g_free(info);
 }
@@ -1139,7 +1173,10 @@ int nbd_receive_export_list(QIOChannel *ioc, QCryptoTLSCreds *tlscreds,
                 break;
             }

-            /* TODO: Grab meta contexts */
+            if (result == 3 &&
+                nbd_list_meta_contexts(ioc, &array[i], errp) < 0) {
+                goto out;
+            }
         }

         /* Send NBD_OPT_ABORT as a courtesy before hanging up */
-- 
2.20.1