nbd: Flip qemu-nbd to prefer newstyle; add -O for old behavior

Oldstyle NBD negotiation cannot perform any of the extensions that we have recently been relying on. While you can always pass -x "" to get newstyle negotiation, these days, it is better to just default to newstyle (with an empty export name) and fall back to oldstyle only on an explicit request.

For comparison:

nbdkit 1.3 switched its default to newstyle (Jan 2018):
https://github.com/libguestfs/nbdkit/commit/b2a8aecc
https://github.com/libguestfs/nbdkit/commit/8158e773

nbd 3.10 dropped oldstyle long ago (Mar 2015):
https://github.com/NetworkBlockDevice/nbd/commit/36940193

Oldstyle NBD negotiation cannot perform any of the extensions that we have recently been relying on. While you can always pass -x "" to get newstyle negotiation, these days, it is better to just default to newstyle (with an empty export name) and fall back to oldstyle only on an explicit request.

For comparison:

nbdkit 1.3 switched its default to newstyle (Jan 2018):
https://github.com/libguestfs/nbdkit/commit/b2a8aecc
https://github.com/libguestfs/nbdkit/commit/8158e773

nbd 3.10 dropped oldstyle long ago (Mar 2015):
https://github.com/NetworkBlockDevice/nbd/commit/36940193

Signed-off-by Eric Blake <[email protected]>

---
  qemu-nbd.c | 27 +++++++++++++++++++--------
  1 file changed, 19 insertions(+), 8 deletions(-)

qemu-nbd.c +0 -0

index 51b9d38c727..8571a4f93d8 100644

qemu-nbd.c +15 -5

+"  -O, --oldstyle            force oldstyle negotiation\n"
  "\n"
  "Exposing part of the image:\n"
  "  -o, --offset=OFFSET       offset into the image\n"

@@ -529,6 +530,7 @@ int main(int argc, char **argv)
          { "object", required_argument, NULL, QEMU_NBD_OPT_OBJECT },
          { "export-name", required_argument, NULL, 'x' },
          { "description", required_argument, NULL, 'D' },
+        { "oldstyle", no_argument, NULL, 'O' },
          { "tls-creds", required_argument, NULL, QEMU_NBD_OPT_TLSCREDS },
          { "image-opts", no_argument, NULL, QEMU_NBD_OPT_IMAGE_OPTS },
          { "trace", required_argument, NULL, 'T' },

@@ -551,6 +553,7 @@ int main(int argc, char **argv)
      QDict *options = NULL;
      const char *export_name = NULL;
      const char *export_description = NULL;
+    bool oldstyle = false;
      const char *tlscredsid = NULL;
      bool imageOpts = false;
      bool writethrough = true;

@@ -723,6 +726,9 @@ int main(int argc, char **argv)
          case 'D':
              export_description = optarg;
              break;
+        case 'O':
+            oldstyle = true;
+            break;
          case 'v':
              verbose = 1;
              break;

@@ -799,7 +805,16 @@ int main(int argc, char **argv)
          }
      }
 
+    if (!oldstyle && !export_name) {
+        /* Set the default NBD protocol export name, to favor new style. */
+        export_name = "";
+    }
+
      if (tlscredsid) {
+        if (oldstyle) {
+            error_report("TLS is incompatible with oldstyle");
+            exit(EXIT_FAILURE);
+        }
          if (sockpath) {
              error_report("TLS is only supported with IPv4/IPv6");
              exit(EXIT_FAILURE);

@@ -808,11 +823,6 @@ int main(int argc, char **argv)
              error_report("TLS is not supported with a host device");
              exit(EXIT_FAILURE);
          }
-        if (!export_name) {
-            /* Set the default NBD protocol export name, since
-             * we *must* use new style protocol for TLS */
-            export_name = "";
-        }
          tlscreds = nbd_get_tls_creds(tlscredsid, &local_err);
          if (local_err) {
              error_report("Failed to get TLS creds %s",

@@ -1013,13 +1023,14 @@ int main(int argc, char **argv)
          error_report_err(local_err);

qemu-nbd.c +0 -3

-    } else if (export_description) {
-        error_report("Export description requires an export name");
-        exit(EXIT_FAILURE);
      }
 
      if (device) {

Quoted 7 lines · Eric Blake

Oldstyle NBD negotiation cannot perform any of the extensions that we have recently been relying on.  While you can always pass -x "" to get newstyle negotiation, these days, it is better to just default to newstyle (with an empty export name) and fall back to oldstyle only on an explicit request.

Quoted 8 lines

+    if (oldstyle && (export_name || export_description)) { +        error_report("oldstyle negotiation cannot set export details"); +        exit(EXIT_FAILURE);
+    }
      if (export_name) {
          nbd_export_set_name(exp, export_name); nbd_export_set_description(exp, export_description); newproto = true;

a bit strange, to have both newproto and oldstyle variables which are opposite by value.