[Spice-devel] [PATCH spice 5/9] reds-stream: add send_msgfd()

[Frediano Ziglio]

> 
> Hi
> 
> ----- Original Message -----
> > > 
> > > From: Marc-André Lureau <mlureau at redhat.com>
> > > 
> > > A new function to send fd with unix socket anciliary data.
> > > 
> > > Signed-off-by: Marc-André Lureau <marcandre.lureau at gmail.com>
> > > ---
> > >  server/reds-stream.c | 42 ++++++++++++++++++++++++++++++++++++++++++
> > >  server/reds-stream.h |  1 +
> > >  2 files changed, 43 insertions(+)
> > > 
> > > diff --git a/server/reds-stream.c b/server/reds-stream.c
> > > index d87cb23..4829b43 100644
> > > --- a/server/reds-stream.c
> > > +++ b/server/reds-stream.c
> > > @@ -28,6 +28,7 @@
> > >  #include <netdb.h>
> > >  #include <unistd.h>
> > >  #include <sys/socket.h>
> > > +#include <fcntl.h>
> > >  
> > >  #include <glib.h>
> > >  
> > > @@ -254,6 +255,47 @@ int reds_stream_is_plain_unix(const RedsStream *s)
> > >  
> > >  }
> > >  
> > > +int reds_stream_send_msgfd(RedsStream *stream, int fd)
> > > +{
> > > +    struct msghdr msgh;
> > > +    struct iovec iov;
> > > +    int r;
> > > +
> > > +    size_t fd_size = 1 * sizeof(int);
> > 
> > const here?
> 
> Yeah, I am not that strict about internal variables. Sure why not?
> 
> > 
> > > +    char control[CMSG_SPACE(fd_size)];
> > 
> > This would be better to be an union of data and struct cmsghdr, something
> > like
> > 
> > union {
> >    struct cmsghdr hdr;
> >    char data[CMSG_SPACE(fd_size)];
> > } control;
> > 
> > (of course this needs some other changes in the code).
> 
> Ok
> 
> > 
> > > +    struct cmsghdr *cmsg;
> > > +
> > > +    spice_return_val_if_fail(reds_stream_is_plain_unix(stream), -1);
> > > +
> > > +    memset(&msgh, 0, sizeof(msgh));
> > > +    memset(control, 0, sizeof(control));
> > > +    /* set the payload */
> > > +    iov.iov_base = (char*)"@";
> > 
> > is the cast required?
> 
> yes, "assignment discards 'const' qualifier from pointer target type"
> 
> > 
> > > +    iov.iov_len = 1;
> > > +
> > > +    if (fd != -1) {
> > > +        msgh.msg_iovlen = 1;
> > > +        msgh.msg_iov = &iov;
> > > +        msgh.msg_control = control;
> > > +        msgh.msg_controllen = sizeof(control);
> > > +
> > > +        cmsg = CMSG_FIRSTHDR(&msgh);
> > > +
> > > +        cmsg->cmsg_len = CMSG_LEN(fd_size);
> > > +        cmsg->cmsg_level = SOL_SOCKET;
> > > +        cmsg->cmsg_type = SCM_RIGHTS;
> > > +        memcpy(CMSG_DATA(cmsg), &fd, fd_size);
> > > +    } else {
> > > +        msgh.msg_iovlen = 0;
> > 
> > So you don't send nothing if no file descriptor is passes?
> > How do you handle the read on the other end?
> 
> The read of one byte takes place, but no fd get through. The client should
> not always expect a fd with the msg, depending on the msg. For
> SPICE_MSG_DISPLAY_GL_SCANOUT_UNIX, it's possible to send no fd to "unset"
> the current scanout.
> 

My doubt is: don't you lose sync if client read one byte while server does
not send any data? Did you try appending an integer after the file descriptor
and see what's happen?

> > > +    }
> > > +
> > > +    do {
> > > +        r = sendmsg(stream->socket, &msgh, 0);
> > 
> > MSG_NOSIGNAL as flag?
> 
> Why not? does that really change something here?
> 

I think if you try to send data over a closed socket system will
send a SIGPIPE to the process. If SIGPIPE is not blocked/ignored
your program will exit. Actually Qemu and our software ignore the
SIGPIPE so it's not a problem... but adding the flag make sure
signal is not raised (an error is returned which is much easier
to handle in a library).

> > 
> > > +    } while (r < 0 && (errno == EINTR || errno == EAGAIN));
> > > +
> > > +    return r;
> > > +}
> > > +
> > >  ssize_t reds_stream_writev(RedsStream *s, const struct iovec *iov, int
> > >  iovcnt)
> > >  {
> > >      int i;
> > > diff --git a/server/reds-stream.h b/server/reds-stream.h
> > > index 9e53b22..72e5dd1 100644
> > > --- a/server/reds-stream.h
> > > +++ b/server/reds-stream.h
> > > @@ -74,6 +74,7 @@ int reds_stream_enable_ssl(RedsStream *stream, SSL_CTX
> > > *ctx);
> > >  void reds_stream_set_info_flag(RedsStream *stream, unsigned int flag);
> > >  int reds_stream_get_family(const RedsStream *stream);
> > >  int reds_stream_is_plain_unix(const RedsStream *stream);
> > > +int reds_stream_send_msgfd(RedsStream *stream, int fd);
> > >  
> > >  typedef enum {
> > >      REDS_SASL_ERROR_OK,
> 
> 
> thanks for the review
> 

Frediano