Using recvmsg() with non-unicast datagram (UDP) socket: inconsistent results

Using recvmsg() with non-unicast datagram (UDP) socket: inconsistent results

[Lavrentiev, Anton (NIH/NLM/NCBI) [C]]

Hi,

Observed is an inconsistent result when recvmsg() is used on a
UDP socket with or without ancillary control message parameters.

Here's is an example:

    static char buf[1 << 16];
    struct sockaddr_in sin;
    struct msghdr msg;
    struct iovec iov;
#ifndef CYGWIN_BUG
    char cmsgbuf[32];
#endif /*CYGWIN_BUG*/
    int n;

    iov.iov_base       = buf;
    iov.iov_len        = sizeof(buf);
    memset(&sin, 0, sizeof(sin));
    memset(&msg, 0, sizeof(msg));
    msg.msg_name       = (caddr_t) &sin;
    msg.msg_namelen    = sizeof(sin);
    msg.msg_iov        = &iov;
    msg.msg_iovlen     = 1;
#ifndef CYGWIN_BUG
    memset(cmsgbuf, 0, sizeof(cmsgbuf));
    msg.msg_control    = cmsgbuf;
    msg.msg_controllen = sizeof(cmsgbuf);
#endif /*CYGWIN_BUG*/

    if ((n = recvmsg(sock, &msg, 0/*no special flags*/)) <= 0) {
	   /* Handle error */
        return;
    }

    /* HERE: msg.msg_flags is inconsistent here for non-unicast UDP sockets */

For a non-unicast socket "sock", the returned msg.msg_flags will have proper WSA
bits (MSG_BCAST or MSG_MCAST) set if CYGWIN_BUG is not defined (i.e. the recvmag()
call is passed non-zero msg.msg_controllen).  Otherwise, msg.msg_flags will always
get returned 0.

The relevant code from CYGWIN source suggests that the native WSA flags should
have got preserved (fhandler_socket.cc, fhandler_socket::recvmsg):

	msg->msg_flags = wsamsg.dwFlags;

yet apparently this had slipped away in fhandler_socket::recv_internal, which
branches on the presence of msg_controllen (and opts out for use other WSA API
call if that is not set, even for non-stream sockets):

     if (wsamsg->Control.len > 0)
       {
         ...
         use_recvmsg = true;
       }

Can this please be rectified?

Thanks,

Anton Lavrentiev
Contractor NIH/NLM/NCBI


--
Problem reports:       http://cygwin.com/problems.html
FAQ:                   http://cygwin.com/faq/
Documentation:         http://cygwin.com/docs.html
Unsubscribe info:      http://cygwin.com/ml/#unsubscribe-simple

Re: Using recvmsg() with non-unicast datagram (UDP) socket: inconsistent results

[Corinna Vinschen]

On Jul 31 12:43, Lavrentiev, Anton (NIH/NLM/NCBI) [C] wrote:
> Hi,
> 
> Observed is an inconsistent result when recvmsg() is used on a
> UDP socket with or without ancillary control message parameters.
> 
> Here's is an example:
> 
>     static char buf[1 << 16];
>     struct sockaddr_in sin;
>     struct msghdr msg;
>     struct iovec iov;
> #ifndef CYGWIN_BUG
>     char cmsgbuf[32];
> #endif /*CYGWIN_BUG*/
>     int n;
> 
>     iov.iov_base       = buf;
>     iov.iov_len        = sizeof(buf);
>     memset(&sin, 0, sizeof(sin));
>     memset(&msg, 0, sizeof(msg));
>     msg.msg_name       = (caddr_t) &sin;
>     msg.msg_namelen    = sizeof(sin);
>     msg.msg_iov        = &iov;
>     msg.msg_iovlen     = 1;
> #ifndef CYGWIN_BUG
>     memset(cmsgbuf, 0, sizeof(cmsgbuf));
>     msg.msg_control    = cmsgbuf;
>     msg.msg_controllen = sizeof(cmsgbuf);
> #endif /*CYGWIN_BUG*/
> 
>     if ((n = recvmsg(sock, &msg, 0/*no special flags*/)) <= 0) {
> 	   /* Handle error */
>         return;
>     }
> 
>     /* HERE: msg.msg_flags is inconsistent here for non-unicast UDP sockets */
> 
> For a non-unicast socket "sock", the returned msg.msg_flags will have proper WSA
> bits (MSG_BCAST or MSG_MCAST) set if CYGWIN_BUG is not defined (i.e. the recvmag()
> call is passed non-zero msg.msg_controllen).  Otherwise, msg.msg_flags will always
> get returned 0.
> 
> The relevant code from CYGWIN source suggests that the native WSA flags should
> have got preserved (fhandler_socket.cc, fhandler_socket::recvmsg):
> 
> 	msg->msg_flags = wsamsg.dwFlags;
> 
> yet apparently this had slipped away in fhandler_socket::recv_internal, which
> branches on the presence of msg_controllen (and opts out for use other WSA API
> call if that is not set, even for non-stream sockets):
> 
>      if (wsamsg->Control.len > 0)
>        {
>          ...
>          use_recvmsg = true;
>        }
> 
> Can this please be rectified?

I applied a patch which should fix this problem.  It always uses
WSARecvMsg now, if called from recvmsg, still provided it is possible,
of course (No SOCK_STREAM, no AF_LOCAL).

I'm just generating a new developer snapshot.  Please give the today's
snapshot from http://cygwin.com/snapshots/ a try.  Should be up in about
half an hour.


Thanks,
Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

--
Problem reports:       http://cygwin.com/problems.html
FAQ:                   http://cygwin.com/faq/
Documentation:         http://cygwin.com/docs.html
Unsubscribe info:      http://cygwin.com/ml/#unsubscribe-simple

RE: Using recvmsg() with non-unicast datagram (UDP) socket: inconsistent results

[Lavrentiev, Anton (NIH/NLM/NCBI) [C]]

> I applied a patch which should fix this problem.  It always uses
> WSARecvMsg now, if called from recvmsg, still provided it is possible,
> of course (No SOCK_STREAM, no AF_LOCAL).
> 
> I'm just generating a new developer snapshot.  Please give the today's
> snapshot from http://cygwin.com/snapshots/ a try.  Should be up in
> about
> half an hour.

The patched DLL now returns consistent msg_flags regardless of whether
or not the control message has been requested.  Thanks!

Could I ask for another favor that since the native WSA msg_flags get
propagated to the CYGWIN environment, the definitions for the additional
flag bits, MSG_MCAST and MSG_BCAST (as defined by WSA) to be added to
<cygwin/socket.h> please?

http://msdn.microsoft.com/en-us/library/windows/desktop/ms741645%28v=vs.85%29.aspx

Anton Lavrentiev
Contractor NIH/NLM/NCBI

Re: Using recvmsg() with non-unicast datagram (UDP) socket: inconsistent results

[Corinna Vinschen]

On Aug  1 14:35, Lavrentiev, Anton (NIH/NLM/NCBI) [C] wrote:
> > I applied a patch which should fix this problem.  It always uses
> > WSARecvMsg now, if called from recvmsg, still provided it is possible,
> > of course (No SOCK_STREAM, no AF_LOCAL).
> > 
> > I'm just generating a new developer snapshot.  Please give the today's
> > snapshot from http://cygwin.com/snapshots/ a try.  Should be up in
> > about
> > half an hour.
> 
> The patched DLL now returns consistent msg_flags regardless of whether
> or not the control message has been requested.  Thanks!
> 
> Could I ask for another favor that since the native WSA msg_flags get
> propagated to the CYGWIN environment, the definitions for the additional
> flag bits, MSG_MCAST and MSG_BCAST (as defined by WSA) to be added to
> <cygwin/socket.h> please?

Done.  These are Windows-specific values not docuemnted by POSIX.
But Linux defines its own return values as well, so, never mind.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

--
Problem reports:       http://cygwin.com/problems.html
FAQ:                   http://cygwin.com/faq/
Documentation:         http://cygwin.com/docs.html
Unsubscribe info:      http://cygwin.com/ml/#unsubscribe-simple