Re: [PATCH v3 1/2] io: Add support for MSG_PEEK for socket channel

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

On Tue, Nov 22, 2022 at 03:10:53PM +0530, manish.mishra wrote:
> 
> On 22/11/22 2:59 pm, Daniel P. Berrangé wrote:
> > On Tue, Nov 22, 2022 at 02:38:53PM +0530, manish.mishra wrote:
> > > On 22/11/22 2:30 pm, Daniel P. Berrangé wrote:
> > > > On Sat, Nov 19, 2022 at 09:36:14AM +0000, manish.mishra wrote:
> > > > > MSG_PEEK reads from the peek of channel, The data is treated as
> > > > > unread and the next read shall still return this data. This
> > > > > support is currently added only for socket class. Extra parameter
> > > > > 'flags' is added to io_readv calls to pass extra read flags like
> > > > > MSG_PEEK.
> > > > > 
> > > > > Suggested-by: Daniel P. Berrangé <berrange@redhat.com
> > > > > Signed-off-by: manish.mishra <manish.mishra@nutanix.com>
> > > > > ---
> > > > >    chardev/char-socket.c               |  4 +-
> > > > >    include/io/channel.h                | 83 +++++++++++++++++++++++++++++
> > > > >    io/channel-buffer.c                 |  1 +
> > > > >    io/channel-command.c                |  1 +
> > > > >    io/channel-file.c                   |  1 +
> > > > >    io/channel-null.c                   |  1 +
> > > > >    io/channel-socket.c                 | 16 +++++-
> > > > >    io/channel-tls.c                    |  1 +
> > > > >    io/channel-websock.c                |  1 +
> > > > >    io/channel.c                        | 73 +++++++++++++++++++++++--
> > > > >    migration/channel-block.c           |  1 +
> > > > >    scsi/qemu-pr-helper.c               |  2 +-
> > > > >    tests/qtest/tpm-emu.c               |  2 +-
> > > > >    tests/unit/test-io-channel-socket.c |  1 +
> > > > >    util/vhost-user-server.c            |  2 +-
> > > > >    15 files changed, 179 insertions(+), 11 deletions(-)
> > > > 
> > > > > diff --git a/io/channel-socket.c b/io/channel-socket.c
> > > > > index b76dca9cc1..a06b24766d 100644
> > > > > --- a/io/channel-socket.c
> > > > > +++ b/io/channel-socket.c
> > > > > @@ -406,6 +406,8 @@ qio_channel_socket_accept(QIOChannelSocket *ioc,
> > > > >        }
> > > > >    #endif /* WIN32 */
> > > > > +    qio_channel_set_feature(QIO_CHANNEL(cioc), QIO_CHANNEL_FEATURE_READ_MSG_PEEK);
> > > > > +
> > > > This covers the incoming server side socket.
> > > > 
> > > > This also needs to be set in outgoing client side socket in
> > > > qio_channel_socket_connect_async
> > > 
> > > Yes sorry, i considered only current use-case, but as it is generic one both should be there. Thanks will update it.
> > > 
> > > > > @@ -705,7 +718,6 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
> > > > >    }
> > > > >    #endif /* WIN32 */
> > > > > -
> > > > >    #ifdef QEMU_MSG_ZEROCOPY
> > > > >    static int qio_channel_socket_flush(QIOChannel *ioc,
> > > > >                                        Error **errp)
> > > > Please remove this unrelated whitespace change.
> > > > 
> > > > 
> > > > > @@ -109,6 +117,37 @@ int qio_channel_readv_all_eof(QIOChannel *ioc,
> > > > >        return qio_channel_readv_full_all_eof(ioc, iov, niov, NULL, NULL, errp);
> > > > >    }
> > > > > +int qio_channel_readv_peek_all_eof(QIOChannel *ioc,
> > > > > +                                   const struct iovec *iov,
> > > > > +                                   size_t niov,
> > > > > +                                   Error **errp)
> > > > > +{
> > > > > +   ssize_t len = 0;
> > > > > +   ssize_t total = iov_size(iov, niov);
> > > > > +
> > > > > +   while (len < total) {
> > > > > +       len = qio_channel_readv_full(ioc, iov, niov, NULL,
> > > > > +                                    NULL, QIO_CHANNEL_READ_FLAG_MSG_PEEK, errp);
> > > > > +
> > > > > +       if (len == QIO_CHANNEL_ERR_BLOCK) {
> > > > > +            if (qemu_in_coroutine()) {
> > > > > +                qio_channel_yield(ioc, G_IO_IN);
> > > > > +            } else {
> > > > > +                qio_channel_wait(ioc, G_IO_IN);
> > > > > +            }
> > > > > +            continue;
> > > > > +       }
> > > > > +       if (len == 0) {
> > > > > +           return 0;
> > > > > +       }
> > > > > +       if (len < 0) {
> > > > > +           return -1;
> > > > > +       }
> > > > > +   }
> > > > This will busy wait burning CPU where there is a read > 0 and < total.
> > > > 
> > > Daniel, i could use MSG_WAITALL too if that works but then we will
> > > lose opportunity to yield. Or if you have some other idea.
> > I fear this is an inherant problem with the idea of using PEEK to
> > look at the magic data.
> > 
> > If we actually read the magic bytes off the wire, then we could have
> > the same code path for TLS and non-TLS. We would have to modify the
> > existing later code paths though to take account of fact that the
> > magic was already read by an earlier codepath.
> > 
> > With regards,
> > Daniel
> 
> 
> sure Daniel, I am happy to drop use of MSG_PEEK, but that way also we
> have issue with tls for reason we discussed in V2. Is it okay to send
> a patch with actual read ahead but not for tls case? tls anyway does
> not have this bug as it does handshake.

I've re-read the previous threads, but I don't see what the problem
with TLS is.  We already decided that TLS is not affected by the
race condition. So there should be no problem in reading the magic
bytes early on the TLS channels, while reading the bytes early on
a non-TLS channel will fix the race condition. 

With regards,
Daniel
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