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

[Eric Blake]

On Fri, Feb 11, 2022 at 07:14:30AM +0100, Laszlo Ersek wrote:
> On 02/10/22 16:38, Eric Blake wrote:
> > 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.
> > ---
> > 

> > +#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
> > 
> 
> hahaha, this was real quick ;)

I don't know if the speedy pun was intentional ;)

Now pushed as 31997b25.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org