• Miklos Szeredi's avatar
    net: unix: fix sending fds in multiple buffers · 7d07d6b3
    Miklos Szeredi authored
    [ Upstream commit 8ba69ba6 ]
    
    Kalle Olavi Niemitalo reported that:
    
      "..., when one process calls sendmsg once to send 43804 bytes of
      data and one file descriptor, and another process then calls recvmsg
      three times to receive the 16032+16032+11740 bytes, each of those
      recvmsg calls returns the file descriptor in the ancillary data.  I
      confirmed this with strace.  The behaviour differs from Linux
      2.6.26, where reportedly only one of those recvmsg calls (I think
      the first one) returned the file descriptor."
    
    This bug was introduced by a patch from me titled "net: unix: fix inflight
    counting bug in garbage collector", commit 6209344f.
    
    And the reason is, quoting Kalle:
    
      "Before your patch, unix_attach_fds() would set scm->fp = NULL, so
      that if the loop in unix_stream_sendmsg() ran multiple iterations,
      it could not call unix_attach_fds() again.  But now,
      unix_attach_fds() leaves scm->fp unchanged, and I think this causes
      it to be called multiple times and duplicate the same file
      descriptors to each struct sk_buff."
    
    Fix this by introducing a flag that is cleared at the start and set
    when the fds attached to the first buffer.  The resulting code should
    work equivalently to the one on 2.6.26.
    Reported-by: default avatarKalle Olavi Niemitalo <kon@iki.fi>
    Signed-off-by: default avatarMiklos Szeredi <mszeredi@suse.cz>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@suse.de>
    7d07d6b3
af_unix.c 52.5 KB