Hi David,
On 2/7/22 9:03 PM, David Ahern wrote:
> On 2/7/22 7:55 PM, Dongli Zhang wrote:
>> The TUN can be used as vhost-net backend. E.g, the tun_net_xmit() is the
>> interface to forward the skb from TUN to vhost-net/virtio-net.
>>
>> However, there are many "goto drop" in the TUN driver. Therefore, the
>> kfree_skb_reason() is involved at each "goto drop" to help userspace
>> ftrace/ebpf to track the reason for the loss of packets.
>>
>> Cc: Joao Martins <joao.m.martins@xxxxxxxxxx>
>> Cc: Joe Jin <joe.jin@xxxxxxxxxx>
>> Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
>> ---
>> drivers/net/tun.c | 33 +++++++++++++++++++++++++--------
>> include/linux/skbuff.h | 6 ++++++
>> include/trace/events/skb.h | 6 ++++++
>> 3 files changed, 37 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>> index fed85447701a..d67f2419dbb4 100644
>> --- a/drivers/net/tun.c
>> +++ b/drivers/net/tun.c
>> @@ -1062,13 +1062,16 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
>> struct netdev_queue *queue;
>> struct tun_file *tfile;
>> int len = skb->len;
>> + int drop_reason = SKB_DROP_REASON_NOT_SPECIFIED;
>
I will avoid initializing here.
>
>>
>> rcu_read_lock();
>> tfile = rcu_dereference(tun->tfiles[txq]);
>>
>> /* Drop packet if interface is not attached */
>> - if (!tfile)
>> + if (!tfile) {
>> + drop_reason = SKB_DROP_REASON_DEV_NOT_ATTACHED;
Initially I was using TUN_NOT_ATTACHED. I used a more generic DEV_NOT_ATTACHED
in order to re-use the reason in the future.
How about TUN specific TUN_NOT_ATTACHED, as the core issue is because the below
is not hit.
rcu_assign_pointer(tun->tfiles[tun->numqueues], tfile);
>
> That is going to be a confusing reason code (tap device existed to get
> here) and does not really explain this error.
>
>
>> goto drop;
>> + }
>>
>> if (!rcu_dereference(tun->steering_prog))
>> tun_automq_xmit(tun, skb);
>> @@ -1078,19 +1081,27 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
>> /* Drop if the filter does not like it.
>> * This is a noop if the filter is disabled.
>> * Filter can be enabled only for the TAP devices. */
>> - if (!check_filter(&tun->txflt, skb))
>> + if (!check_filter(&tun->txflt, skb)) {
>> + drop_reason = SKB_DROP_REASON_TAP_RUN_FILTER;
>
> just SKB_DROP_REASON_TAP_FILTER
I will use SKB_DROP_REASON_TAP_FILTER.
>
>> goto drop;
>> + }
>>
>> if (tfile->socket.sk->sk_filter &&
>> - sk_filter(tfile->socket.sk, skb))
>> + sk_filter(tfile->socket.sk, skb)) {
>> + drop_reason = SKB_DROP_REASON_SKB_TRIM;
>
> SKB_DROP_REASON_SOCKET_FILTER
Sorry for my mistake, I should have re-used this SKB_DROP_REASON_SOCKET_FILTER.
>
> The remainder of your changes feels like another variant of your
> previous "function / line" reason code. You are creating new reason
> codes for every goto failure with a code based name. The reason needs to
> be the essence of the failure in a user friendly label.
>
The remainder are:
- SKB_DROP_REASON_SKB_TRIM
- SKB_DROP_REASON_SKB_ORPHAN_FRAGS
- SKB_DROP_REASON_SKB_PULL
- SKB_DROP_REASON_DEV_DOWN
- SKB_DROP_REASON_SKB_COPY_DATA (introduced by Patch 1/2)
I tried to make them self-explaining and re-usable to other developers.
Yes, I am creating new reason codes for every goto failure with a code based
name because each function might be failed due to many reasons. In addition, I
need to avoid duplicate 'drop_reason' returned by a function in order to help
developer identify the specific line of code that the sk_buff is dropped.
Thank you very much!
Dongli Zhang
