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

From: Eric Dumazet
Date: Tue Jul 07 2020 - 13:33:57 EST




On 7/7/20 10:15 AM, 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>
> ---
> include/uapi/linux/net_dropmon.h | 3 ++
> net/core/drop_monitor.c | 79 +++++++++++++++++++++++++++++++-
> 2 files changed, 80 insertions(+), 2 deletions(-)
>
> 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..8049bff05abd 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
> *
> @@ -220,9 +222,8 @@ static void trace_drop_common(struct sk_buff *skb, void *location)
> struct per_cpu_dm_data *data;
> unsigned long flags;
>
> - local_irq_save(flags);
> + spin_lock_irqsave(&data->lock, flags);
> data = this_cpu_ptr(&dm_cpu_data);
> - spin_lock(&data->lock);

This change seems unrelated ?

And also buggy, since data is essentially garbage when you call spin_lock_irqsave(&data->lock, flags);

> dskb = data->skb;
>
> if (!dskb)
> @@ -255,6 +256,12 @@ static void trace_drop_common(struct sk_buff *skb, void *location)
>
> out:
> spin_unlock_irqrestore(&data->lock, flags);
> +
> + if (interface && interface != skb->dev) {
> + skb = skb_clone(skb, GFP_ATOMIC);

skb_clone() can return NULL

> + skb->dev = interface;
> + netif_receive_skb(skb);
> + }
> }
>
> static void trace_kfree_skb_hit(void *ignore, struct sk_buff *skb, void *location)
> @@ -1315,6 +1322,63 @@ 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;
> +
> + nd = dev_get_by_name(net, ifname);
> +
> + if (nd) {
> + interface = nd;
> + dev_hold(interface);
> + } else {
> + return -ENODEV;
> + }
> + return 0;
> +}
> +


What happens after this monitoring is started, then the admin does :

rmmod ifb