On Wed, 23 Feb 2005 01:07:47 -0800
Andrew Morton <[EMAIL PROTECTED]> wrote:
> Guillaume Thouvenin <[EMAIL PROTECTED]> wrote:
> >
> > Hello,
> >
> > This patch replaces the relay_fork module and it implements a fork
> > connector in the kernel/fork.c:do_fork() routine. The connector sends
> > information about parent PID and child PID over a netlink interface. It
> > allows to several user space applications to be informed when a fork
> > occurs in the kernel. The main drawback is that even if nobody listens,
> > message is send. I don't know how to avoid that.
>
> We really should find a way to fix that. Especially if we want all the
> distributors to enable the connector in their builds (we do).
Mesage is never reached anyone if there are no listeners, skb will be just
freed,
even without any linking.
do_one_broadcast() in net/netlink/af_netlink.c takes care of it.
Unicast message also will be discarded in cn_rx_skb().
These operations are quite cheap - just link/unlink skb to/from appropriate
queues.
> What happened to the idea of sending an on/off message down the netlink
> socket?
?
> > +#ifdef CONFIG_FORK_CONNECTOR
> > +#define FORK_CN_INFO_SIZE 64
> > +static inline void fork_connector(pid_t parent, pid_t child)
> > +{
> > + struct cb_id fork_id = {CN_IDX_FORK, CN_VAL_FORK};
>
> This can be static const, which will save some stack and cycles.
>
> > + static __u32 seq; /* used to test if we lost message */
> > +
> > + if (cn_already_initialized) {
> > + struct cn_msg *msg;
> > + size_t size;
> > +
> > + size = sizeof(*msg) + FORK_CN_INFO_SIZE;
> > + msg = kmalloc(size, GFP_KERNEL);
> > + if (msg) {
> > + memset(msg, '\0', size);
>
> Do we really need to memset the whole thing?
Yes, to not leak kernel memory context.
> > + memcpy(&msg->id, &fork_id, sizeof(msg->id));
> > + msg->seq = seq++;
>
> `seq' needs a lock to protect it. Or use atomic_add_return(), maybe.
Not necessary, I doubt fork userspace listener uses protocol described in
connector.c and relies on seq field since it is not needed to have acks/replays.
Although it can be used as a flag that it is new fork, but message itself
is already such an event.
> > + msg->ack = 0; /* not used */
> > + /*
> > + * size of data is the number of characters
> > + * printed plus one for the trailing '\0'
> > + */
> > + msg->len = snprintf(msg->data, FORK_CN_INFO_SIZE-1,
> > + "%i %i", parent, child) + 1;
>
> scnprintf() would be more appropriate here, even though it should be a "can't
> happen".
>
> > + cn_netlink_send(msg, CN_IDX_FORK);
> > +
> > + kfree(msg);
>
> `msg' is only 84 bytes and do_fork() has a shallow call graph. Make `msg'
> a local?
Yes it can be local.
Evgeniy Polyakov
Only failure makes us experts. -- Theo de Raadt
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