Re: [PATCH 2/4] vhost-user: Convert slave channel to QIOChannelSocket

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

On Wed, Mar 10, 2021 at 02:45:25PM +0100, Greg Kurz wrote:
> On Wed, 10 Mar 2021 11:43:58 +0000
> Daniel P. Berrangé <berrange@redhat.com> wrote:
> 
> > On Tue, Mar 09, 2021 at 09:23:22PM +0100, Greg Kurz wrote:
> > > On Tue, 9 Mar 2021 15:17:21 +0000
> > > Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > > 
> > > > On Mon, Mar 08, 2021 at 01:31:39PM +0100, Greg Kurz wrote:
> > > > > +    g_autofree int *fd = NULL;
> > > > > +    size_t fdsize = 0;
> > > > > +    int i;
> > > > >  
> > > > >      /* Read header */
> > > > >      iov.iov_base = &hdr;
> > > > >      iov.iov_len = VHOST_USER_HDR_SIZE;
> > > > >  
> > > > >      do {
> > > > > -        size = recvmsg(u->slave_fd, &msgh, 0);
> > > > > -    } while (size < 0 && (errno == EINTR || errno == EAGAIN));
> > > > > +        size = qio_channel_readv_full(ioc, &iov, 1, &fd, &fdsize, NULL);
> > > > > +    } while (size == QIO_CHANNEL_ERR_BLOCK);
> > > > 
> > > > Is it possible to leak file descriptors and fd[] memory if we receive a
> > > > short message and then loop around? qio_channel_readv_full() will
> > > > overwrite &fd and that's how the leak occurs.
> > > > 
> > > 
> > > qio_channel_readv_full() only returns QIO_CHANNEL_ERR_BLOCK when the
> > > channel is non-blocking mode and no data is available. Under the hood,
> > > this translates to this code in qio_channel_socket_readv():
> > > 
> > >  retry:
> > >     ret = recvmsg(sioc->fd, &msg, sflags);
> > >     if (ret < 0) {
> > >         if (errno == EAGAIN) {
> > >             return QIO_CHANNEL_ERR_BLOCK;
> > >         }
> > >         if (errno == EINTR) {
> > >             goto retry;
> > >         }
> > > 
> > >         error_setg_errno(errp, errno,
> > >                          "Unable to read from socket");
> > >         return -1;
> > >     }
> > > 
> > > This is strictly equivalent to what we currently have. This cannot
> > > leak file descriptors because we would only loop until the first
> > > byte of real data is available and ancillary data cannot fly without
> > > real data, i.e. no file descriptors when recvmsg() returns EAGAIN.
> > 
> > Yep, EAGAIN will only happen if you have no data AND no FDs.
> > 
> > > 
> > > > On the other hand, it looks like ioc is in blocking mode. I'm not sure
> > > > QIO_CHANNEL_ERR_BLOCK can occur?
> > > > 
> > > 
> > > You're right but the existing code is ready to handle the non-blocking
> > > case, so I just kept this behavior.
> > 
> > The existing code is *not* handling the non-blocking case in any
> > useful way IMHO. It will block execution of this QEMU thread, and
> > sit in a tight loop burning CPU in the EAGAIN case.
> > 
> > Handling non-blocking means using an I/O watch with the event loop
> > to be notified when something becomes ready.
> > 
> 
> Hmm... slave_read() is a handler registered with qemu_set_fd_handler().
> Isn't it already the case then ? Can the first call to recvmsg() even
> return EAGAIN actually ?

IIUC it shouldn't be able to return EAGAIN in that scenario for stream
sockets (TCP, UNIX), only a datagram socket (UDP) would be at risk of
EAGAIN



Regards,
Daniel
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