[Libguestfs] [nbdkit PATCH] nbd: Opt in to libnbd pread_initialize speedup

[Eric Blake]

Our nbd plugin has always properly checked for asynch errors (and thus
has never been at risk of a vulnerability similar to CVE-2022-0485
just fixed in nbdcopy).  What's more, the nbdkit core guarantees
(since commit b1ce255e in 2019, v1.13.1) that the buffer handed to a
.pread callback has been pre-sanitized to not leak heap contents,
regardless of how buggy a plugin might be (see
server/protocol.c:protocol_recv_request_send_reply(), which uses
server/threadlocal.c:threadlocal_buffer() for a safe buffer).  Thus,
we do not need libnbd to spend time re-initializing the buffer, if
libnbd is new enough to give us that knob.
---

I know that Laszlo was sceptical whether any real client might want to
use the libnbd knob [1], but I'm actually comfortable with this one.

[1] https://listman.redhat.com/archives/libguestfs/2022-February/msg00144.html

 plugins/nbd/nbd.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/plugins/nbd/nbd.c b/plugins/nbd/nbd.c
index ae595ea7..2e154e8d 100644
--- a/plugins/nbd/nbd.c
+++ b/plugins/nbd/nbd.c
@@ -1,5 +1,5 @@
 /* nbdkit
- * Copyright (C) 2017-2020, 2022 Red Hat Inc.
+ * Copyright (C) 2017-2022 Red Hat Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
@@ -637,6 +637,15 @@ nbdplug_open_handle (int readonly, const char *client_export)
 #if LIBNBD_HAVE_NBD_SET_FULL_INFO
   if (nbd_set_full_info (h->nbd, 1) == -1)
     goto errnbd;
+#endif
+#if LIBNBD_HAVE_NBD_SET_PREAD_INITIALIZE
+  /* nbdkit guarantees that the buffers passed to our .pread callback
+   * are pre-initialized; and we in turn ensure that the buffer is not
+   * dereferenced if the NBD server replied with an error.  Thus, we
+   * are safe opting in to this libnbd speedup.
+   */
+  if (nbd_set_pread_initialize (h->nbd, false) == -1)
+    goto errnbd;
 #endif
   if (dynamic_export && uri) {
 #if LIBNBD_HAVE_NBD_SET_OPT_MODE
-- 
2.34.1