sendmsg, descriptors and no content

sendmsg, descriptors and no content

[netdev]

Hi,


When I use sendmsg to send descriptors from one process to another using
unix-sockets I need to include at least one byte of normal data for the
descriptors to be send (using the iovec structure). The same code worked
fine on openbsd (ie. filedescriptors could be send without normal data).
If no normal data is included sendmsg will return 0, as if all data is
send correctly.


Is this difference on purpose? If so, why?


Regards,
Hylke


ps.
The current behaviour is because unix_stream_sendmsg in af_unix.c will
not do anything if len=0.

Re: sendmsg, descriptors and no content

[bert hubert]

On Tue, Oct 31, 2006 at 11:01:01PM +0100, netdev@weary.nl wrote:
> When I use sendmsg to send descriptors from one process to another using
> unix-sockets I need to include at least one byte of normal data for the
> descriptors to be send (using the iovec structure). The same code worked

W. R. Stevens, Unix Network Programming (2nd ed), vol 1, p. 389 recommends
sending at least a byte anyhow, which allows you to detect EOF.

Also see
http://www.cs-ipv6.lancs.ac.uk/ipv6/mail-archive/LinuxNetdev/1998-03/0144.html 
however.

I've attached an example that appears to work.

	Bert

#include <sys/socket.h>
#include <stdio.h>

int sfd(int passfd, int fd, int data)
{
    char cbuf[CMSG_SPACE(sizeof(int))];
    struct msghdr mh = { 0 };
    struct cmsghdr *cm;
    int *dp;
    struct iovec iov;

    if (fd >= 0) {
	mh.msg_control = cbuf;
	mh.msg_controllen = sizeof cbuf;
	cm = CMSG_FIRSTHDR(&mh);
	cm->cmsg_len = CMSG_LEN(sizeof(int));
	cm->cmsg_level = SOL_SOCKET;
	cm->cmsg_type = SCM_RIGHTS;

	dp = CMSG_DATA(cm);
	*dp = fd;
    }
    if (data != 0) {
	iov.iov_base = &data;
	iov.iov_len = sizeof data;
	mh.msg_iov = &iov;
	mh.msg_iovlen = 1;
    }

    return sendmsg(passfd, &mh, 0);
}

/* Only prepared to rcv one fd per message */
int rcvfd(int passfd, int *data, int *datalen)
{
    char cbuf[CMSG_SPACE(sizeof(int))];
    struct msghdr mh = { 0 };
    struct cmsghdr *cm;
    int *dp, ret;
    struct iovec iov;

    if (data) {
	mh.msg_iov = &iov;
	mh.msg_iovlen = 1;
	iov.iov_base = &data;
	iov.iov_len = sizeof(int);
    }

    mh.msg_control = cbuf;
    mh.msg_controllen = sizeof cbuf;
    cm = CMSG_FIRSTHDR(&mh);
    cm->cmsg_len = CMSG_LEN(sizeof(int));
    cm->cmsg_level = SOL_SOCKET;
    cm->cmsg_type = SCM_RIGHTS;

    *datalen = 0;
    ret = recvmsg(passfd, &mh, 0);
    if (ret < 0)
	return ret;
    if (datalen)
	*datalen = ret;

    dp = CMSG_DATA(cm);
    return *dp;
}

int main()
{
  int fd[2];
  int datalen;

  socketpair(AF_UNIX, SOCK_DGRAM, 0, fd);

  printf("Sending returned status: %d\n", sfd(fd[0], 0, 1));
  
  printf("Received fd: %d\n", rcvfd(fd[1], 0, &datalen));
  
}

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