Re: [PATCH v2 4/5] migration: Fix rare hang of migration_channel_read_peek()

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Jul 28, 2026 at 05:29:35PM -0400, Peter Xu wrote:
> On Tue, Jul 28, 2026 at 05:04:16PM -0400, Peter Xu wrote:
> > In an unlikely case, when a migration stream is attached to the destination
> > QEMU and only send <4 bytes to the channel as magic, it's possible that
> > migration_channel_read_peek() may spin forever.
> > 
> > Fix it by adding a manual sleep for partial read.
> > 
> > Since the path isn't attached to a coroutine, it means when partial read
> > happens, there's yet not much we can do but hang the main thread, it will
> > happen even for len==0 case.  It means monitors can hang due to this,
> > either partial read or no data arrived (but connection established).
> > 
> > Leave this for later, the hope is this is extremely rare in production.
> > 
> > Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3889
> > Reported-by: Feifan Qian <bea1e@proton.me>
> > Cc: Daniel P. Berrangé <berrange@redhat.com>
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> >  migration/channel.c | 11 +++++++++--
> >  1 file changed, 9 insertions(+), 2 deletions(-)
> > 
> > diff --git a/migration/channel.c b/migration/channel.c
> > index 1e2935f926..f446561b59 100644
> > --- a/migration/channel.c
> > +++ b/migration/channel.c
> > @@ -296,9 +296,16 @@ int migration_channel_read_peek(QIOChannel *ioc,
> >  
> >          if (len == buflen) {
> >              break;
> > +        } else if (len == 0) {
> 
> I think this should be QIO_CHANNEL_ERR_BLOCK, not 0..  I'll fix it when I
> post v3, and I'll do some more tests.

Yep, 0 == EOF, so you'll need ERR_BLOCK.

> 
> > +            qio_channel_wait_cond(ioc, G_IO_IN);
> > +        } else {
> > +            /*
> > +             * When partially ready, we can't use qio_channel_wait_cond()
> > +             * because it will return immediately.  Apply a manual wait.
> > +             */
> > +            assert(!qemu_in_coroutine());
> > +            g_usleep(1000);

So this will block the main loop, but this is OK because

 * If the network is untrusted, migration should have been configured
   to use TLS with a certificate allow-list. This peek takes place
   after the TLS handshake so is protected
 * On the dest host, it doesn't hugely matter that we block the main
   loop as there's no running guest yet that will stall. Just any
   use of QMP commands on the dest will stall.

> >          }
> > -
> > -        qio_channel_wait_cond(ioc, G_IO_IN);
> >      }
> >  
> >      return 0;
> > -- 
> > 2.54.0
> > 
> 
> -- 
> Peter Xu
> 

With regards,
Daniel
-- 
|: https://berrange.com       ~~        https://hachyderm.io/@berrange :|
|: https://libvirt.org          ~~          https://entangle-photo.org :|
|: https://pixelfed.art/berrange   ~~    https://fstop138.berrange.com :|