From: Andre Nathan <andre@...>
Date: 2008-08-26T04:30:18+09:00
Subject: Calling sendmsg() from Ruby

Hello

I'm trying to call the sendmsg() system call from ruby in a way that
allocates space for the kernel to set the user credentials in the
appropriate structures (this is in FreeBSD i386).

I'm using the code below, which gives me an EFAULT (bad address) error,
and I cannot see what I'm doing wrong here... any help would be awesome.

The equivalent C code (which works) follows after the ruby attempt.

  def pointer(buf)
    [buf].pack("P").unpack("L_").first
  end

  def sendmsg(fd, buf)
    iov = [buf, buf.length].pack("PI_")

    cmsg_len = 96       # CMSG_LEN(sizeof(struct cmsgcred))
    cmsg_space = 96     # CMSG_SPACE(sizeof(struct cmsgcred))
    cmsg_level = 0xffff # SOL_SOCKET
    cmsg_type = 0x03    # SCM_CREDS
    cmsg_data = ["\x00" * cmsg_space].pack("P")

    cmsghdr = [
      cmsg_len,
      cmsg_level,
      cmsg_type,
      cmsg_data
    ].pack("I_i_i_P")

    msg_control_ptr = pointer(cmsghdr)
    msg_controllen = cmsg_space

    msghdr = [
      0,                # msg_name
      0,                # msg_namelen
      pointer(iov),     # msg_iov
      1,                # msg_iovlen
      pointer(cmsghdr), # msg_control
      cmsg_space,       # msg_controllen
      0                 # msg_flags
    ].pack("L_i_L_i_L_i_i_")

    syscall(28, pointer(msghdr), 0) # 28 is sendmsg()
  end

Here's the C snippet:

  union {
    struct cmsghdr hdr;
    char cred[CMSG_SPACE(sizeof(struct cmsgcred))];
  } cmsg;
  struct iovec iov[1];

  struct msghdr msg;
  iov[0].iov_base = (void *)buf;
  iov[0].iov_len = count;

  memset(&msg, 0, sizeof(msg));
  msg.msg_iov = iov;
  msg.msg_iovlen = 1;

  msg.msg_control = (caddr_t)&cmsg;
  msg.msg_controllen = CMSG_SPACE(sizeof(struct cmsgcred));
  memset(&cmsg, 0, sizeof(cmsg));
  cmsg.hdr.cmsg_len = CMSG_LEN(sizeof(struct cmsgcred));
  cmsg.hdr.cmsg_level = SOL_SOCKET;
  cmsg.hdr.cmsg_type = SCM_CREDS;
   
  sendmsg(fd, &msg, 0);

Thanks in advance,
Andre