cvs commit: src/sys/net if_ethersubr.c

[Luigi Rizzo]

luigi       2006-12-08 10:36:45 UTC

  FreeBSD src repository

  Modified files:
    sys/net              if_ethersubr.c 
  Log:
  Fix an oscure bug triggered by a recent change in kern_socket.c.
  The symptoms were that outgoing DHCP requests for diskless kernels
  had the IP header corrupt. After long investigations, the source of
  the problem was found in ether_output() - for SIMPLEX interfaces
  and broadcast traffic, a copy of the packet is passed back to the kernel
  through if_simloop(). However if_simloop() modifies the mbuf, while
  the copy obtained through m_copym() is a readonly one.
  
  The bug has been there forever, but it has been triggered only recently
  by a change in sosend_dgram() which passed down mbufs with sufficient
  space to prepend the header.
  
  This fix is trivial - use m_dup() instead of m_copy() to create
  the copy. As an alternative, we could try and modify if_simloop()
  to play safely with readonly mbufs, but i don't think it is worthwhile
  because 1) this is a relatively infrequent code path so we do not need
  to worry too much about performance, and 2) the cost of doing an
  extra m_pullup in if_simloop() is probably the same as doing the
  copy of the cluster, anyways.
  
  MFC after: 1 week
  
  Revision  Changes    Path
  1.221     +9 -1      src/sys/net/if_ethersubr.c