Re: [PATCHv2 net-next] dropwatch: Support monitoring of dropped frames

[Izabela Bakollari]

I have worked on this feature as part of the Linux Kernel Mentorship
Program. Your review would really help me in this learning process.

Thanks,
Izabela

On Tue, Aug 4, 2020 at 6:09 PM <izabela.bakollari@xxxxxxxxx> wrote:
>
> From: Izabela Bakollari <izabela.bakollari@xxxxxxxxx>
>
> Dropwatch is a utility that monitors dropped frames by having userspace
> record them over the dropwatch protocol over a file. This augument
> allows live monitoring of dropped frames using tools like tcpdump.
>
> With this feature, dropwatch allows two additional commands (start and
> stop interface) which allows the assignment of a net_device to the
> dropwatch protocol. When assinged, dropwatch will clone dropped frames,
> and receive them on the assigned interface, allowing tools like tcpdump
> to monitor for them.
>
> With this feature, create a dummy ethernet interface (ip link add dev
> dummy0 type dummy), assign it to the dropwatch kernel subsystem, by using
> these new commands, and then monitor dropped frames in real time by
> running tcpdump -i dummy0.
>
> Signed-off-by: Izabela Bakollari <izabela.bakollari@xxxxxxxxx>
> ---
> Changes in v2:
> - protect the dummy ethernet interface from being changed by another
> thread/cpu
> ---
>  include/uapi/linux/net_dropmon.h |  3 ++
>  net/core/drop_monitor.c          | 84 ++++++++++++++++++++++++++++++++
>  2 files changed, 87 insertions(+)
>
> diff --git a/include/uapi/linux/net_dropmon.h b/include/uapi/linux/net_dropmon.h
> index 67e31f329190..e8e861e03a8a 100644
> --- a/include/uapi/linux/net_dropmon.h
> +++ b/include/uapi/linux/net_dropmon.h
> @@ -58,6 +58,8 @@ enum {
>         NET_DM_CMD_CONFIG_NEW,
>         NET_DM_CMD_STATS_GET,
>         NET_DM_CMD_STATS_NEW,
> +       NET_DM_CMD_START_IFC,
> +       NET_DM_CMD_STOP_IFC,
>         _NET_DM_CMD_MAX,
>  };
>
> @@ -93,6 +95,7 @@ enum net_dm_attr {
>         NET_DM_ATTR_SW_DROPS,                   /* flag */
>         NET_DM_ATTR_HW_DROPS,                   /* flag */
>         NET_DM_ATTR_FLOW_ACTION_COOKIE,         /* binary */
> +       NET_DM_ATTR_IFNAME,                     /* string */
>
>         __NET_DM_ATTR_MAX,
>         NET_DM_ATTR_MAX = __NET_DM_ATTR_MAX - 1
> diff --git a/net/core/drop_monitor.c b/net/core/drop_monitor.c
> index 8e33cec9fc4e..781e69876d2f 100644
> --- a/net/core/drop_monitor.c
> +++ b/net/core/drop_monitor.c
> @@ -30,6 +30,7 @@
>  #include <net/genetlink.h>
>  #include <net/netevent.h>
>  #include <net/flow_offload.h>
> +#include <net/sock.h>
>
>  #include <trace/events/skb.h>
>  #include <trace/events/napi.h>
> @@ -46,6 +47,7 @@
>   */
>  static int trace_state = TRACE_OFF;
>  static bool monitor_hw;
> +struct net_device *interface;
>
>  /* net_dm_mutex
>   *
> @@ -54,6 +56,8 @@ static bool monitor_hw;
>   */
>  static DEFINE_MUTEX(net_dm_mutex);
>
> +static DEFINE_SPINLOCK(interface_lock);
> +
>  struct net_dm_stats {
>         u64 dropped;
>         struct u64_stats_sync syncp;
> @@ -255,6 +259,21 @@ static void trace_drop_common(struct sk_buff *skb, void *location)
>
>  out:
>         spin_unlock_irqrestore(&data->lock, flags);
> +       spin_lock_irqsave(&interface_lock, flags);
> +       if (interface && interface != skb->dev) {
> +               skb = skb_clone(skb, GFP_ATOMIC);
> +               if (skb) {
> +                       skb->dev = interface;
> +                       spin_unlock_irqrestore(&interface_lock, flags);
> +                       netif_receive_skb(skb);
> +               } else {
> +                       spin_unlock_irqrestore(&interface_lock, flags);
> +                       pr_err("dropwatch: Not enough memory to clone dropped skb\n");
> +                       return;
> +               }
> +       } else {
> +               spin_unlock_irqrestore(&interface_lock, flags);
> +       }
>  }
>
>  static void trace_kfree_skb_hit(void *ignore, struct sk_buff *skb, void *location)
> @@ -1315,6 +1334,53 @@ static int net_dm_cmd_trace(struct sk_buff *skb,
>         return -EOPNOTSUPP;
>  }
>
> +static int net_dm_interface_start(struct net *net, const char *ifname)
> +{
> +       struct net_device *nd = dev_get_by_name(net, ifname);
> +
> +       if (nd)
> +               interface = nd;
> +       else
> +               return -ENODEV;
> +
> +       return 0;
> +}
> +
> +static int net_dm_interface_stop(struct net *net, const char *ifname)
> +{
> +       dev_put(interface);
> +       interface = NULL;
> +
> +       return 0;
> +}
> +
> +static int net_dm_cmd_ifc_trace(struct sk_buff *skb, struct genl_info *info)
> +{
> +       struct net *net = sock_net(skb->sk);
> +       char ifname[IFNAMSIZ];
> +
> +       if (net_dm_is_monitoring())
> +               return -EBUSY;
> +
> +       memset(ifname, 0, IFNAMSIZ);
> +       nla_strlcpy(ifname, info->attrs[NET_DM_ATTR_IFNAME], IFNAMSIZ - 1);
> +
> +       switch (info->genlhdr->cmd) {
> +       case NET_DM_CMD_START_IFC:
> +               if (!interface)
> +                       return net_dm_interface_start(net, ifname);
> +               else
> +                       return -EBUSY;
> +       case NET_DM_CMD_STOP_IFC:
> +               if (interface)
> +                       return net_dm_interface_stop(net, interface->name);
> +               else
> +                       return -ENODEV;
> +       }
> +
> +       return 0;
> +}
> +
>  static int net_dm_config_fill(struct sk_buff *msg, struct genl_info *info)
>  {
>         void *hdr;
> @@ -1503,6 +1569,7 @@ static int dropmon_net_event(struct notifier_block *ev_block,
>         struct net_device *dev = netdev_notifier_info_to_dev(ptr);
>         struct dm_hw_stat_delta *new_stat = NULL;
>         struct dm_hw_stat_delta *tmp;
> +       unsigned long flags;
>
>         switch (event) {
>         case NETDEV_REGISTER:
> @@ -1529,6 +1596,12 @@ static int dropmon_net_event(struct notifier_block *ev_block,
>                                 }
>                         }
>                 }
> +               spin_lock_irqsave(&interface_lock, flags);
> +               if (interface && interface == dev) {
> +                       dev_put(interface);
> +                       interface = NULL;
> +               }
> +               spin_unlock_irqrestore(&interface_lock, flags);
>                 mutex_unlock(&net_dm_mutex);
>                 break;
>         }
> @@ -1543,6 +1616,7 @@ static const struct nla_policy net_dm_nl_policy[NET_DM_ATTR_MAX + 1] = {
>         [NET_DM_ATTR_QUEUE_LEN] = { .type = NLA_U32 },
>         [NET_DM_ATTR_SW_DROPS]  = {. type = NLA_FLAG },
>         [NET_DM_ATTR_HW_DROPS]  = {. type = NLA_FLAG },
> +       [NET_DM_ATTR_IFNAME] = {. type = NLA_STRING, .len = IFNAMSIZ },
>  };
>
>  static const struct genl_ops dropmon_ops[] = {
> @@ -1570,6 +1644,16 @@ static const struct genl_ops dropmon_ops[] = {
>                 .cmd = NET_DM_CMD_STATS_GET,
>                 .doit = net_dm_cmd_stats_get,
>         },
> +       {
> +               .cmd = NET_DM_CMD_START_IFC,
> +               .validate = GENL_DONT_VALIDATE_STRICT | GENL_DONT_VALIDATE_DUMP,
> +               .doit = net_dm_cmd_ifc_trace,
> +       },
> +       {
> +               .cmd = NET_DM_CMD_STOP_IFC,
> +               .validate = GENL_DONT_VALIDATE_STRICT | GENL_DONT_VALIDATE_DUMP,
> +               .doit = net_dm_cmd_ifc_trace,
> +       },
>  };
>
>  static int net_dm_nl_pre_doit(const struct genl_ops *ops,
> --
> 2.18.4
>