From: Andre Nathan <andre@...>
Date: 2008-08-29T09:20:44+09:00
Subject: Re: Calling sendmsg() from Ruby

On Tue, 2008-08-26 at 04:30 +0900, Andre Nathan wrote:
> 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).

For the record, this is how I got it to work. I was wrong in the way I
was packing cmsg_data, and I also had forgotten passing an argument to
the syscall (duh).

  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_len = 84

    cmsghdr = ([
      cmsg_len,
      cmsg_level,
      cmsg_type
    ] + [0] * cmsg_data_len).pack("I_i_i_C#{cmsg_data_len}")

    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, fd.fileno, pointer(msghdr), 0)
  end