• Jason A. Donenfeld's avatar
    af_netlink: ensure that NLMSG_DONE never fails in dumps · 0642840b
    Jason A. Donenfeld authored
    The way people generally use netlink_dump is that they fill in the skb
    as much as possible, breaking when nla_put returns an error. Then, they
    get called again and start filling out the next skb, and again, and so
    forth. The mechanism at work here is the ability for the iterative
    dumping function to detect when the skb is filled up and not fill it
    past the brim, waiting for a fresh skb for the rest of the data.
    
    However, if the attributes are small and nicely packed, it is possible
    that a dump callback function successfully fills in attributes until the
    skb is of size 4080 (libmnl's default page-sized receive buffer size).
    The dump function completes, satisfied, and then, if it happens to be
    that this is actually the last skb, and no further ones are to be sent,
    then netlink_dump will add on the NLMSG_DONE part:
    
      nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE, sizeof(len), NLM_F_MULTI);
    
    It is very important that netlink_dump does this, of course. However, in
    this example, that call to nlmsg_put_answer will fail, because the
    previous filling by the dump function did not leave it enough room. And
    how could it possibly have done so? All of the nla_put variety of
    functions simply check to see if the skb has enough tailroom,
    independent of the context it is in.
    
    In order to keep the important assumptions of all netlink dump users, it
    is therefore important to give them an skb that has this end part of the
    tail already reserved, so that the call to nlmsg_put_answer does not
    fail. Otherwise, library authors are forced to find some bizarre sized
    receive buffer that has a large modulo relative to the common sizes of
    messages received, which is ugly and buggy.
    
    This patch thus saves the NLMSG_DONE for an additional message, for the
    case that things are dangerously close to the brim. This requires
    keeping track of the errno from ->dump() across calls.
    Signed-off-by: default avatarJason A. Donenfeld <Jason@zx2c4.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    0642840b
af_netlink.c 63.8 KB