Re: [PATCH bpf-next v3 1/2] net: bpf: Always call BPF cgroup filters for egress.

[Yonghong Song <yhs@meta.com>]

On 6/22/23 10:15 AM, Kui-Feng Lee wrote:
> 
> 
> On 6/21/23 20:37, Yonghong Song wrote:
>>
>>
>> On 6/20/23 10:14 AM, Kui-Feng Lee wrote:
>>> Always call BPF filters if CGROUP BPF is enabled for EGRESS without
>>> checking skb->sk against sk.
>>>
>>> The filters were called only if skb is owned by the sock that the
>>> skb is sent out through.  In another words, skb->sk should point to
>>> the sock that it is sending through its egress.  However, the filters 
>>> would
>>> miss SYNACK skbs that they are owned by a request_sock but sent through
>>> the listening sock, that is the socket listening incoming connections.
>>> This is an unnecessary restrict.
>>
>> The original patch which introduced 'sk == skb->sk' is
>>    3007098494be  cgroup: add support for eBPF programs
>> There are no mentioning in commit message why 'sk == skb->sk'
>> is needed. So it is possible that this is just restricted
>> for use cases at that moment. Now there are use cases
>> where 'sk != skb->sk' so removing this check can enable
>> the new use case. Maybe you can add this into your commit
>> message so people can understand the history of 'sk == skb->sk'.
> 
> After checking the code and the Alexei's comment[1] again, this check
> may be different from what I thought. In another post[2],
> Daniel Borkmann mentioned
> 
>      Wouldn't that mean however, when you go through stacked devices that
>      you'd run the same eBPF cgroup program for skb->sk multiple times?
> 
> I read this paragraph several times.
> This check ensures the filters are only called for the device on
> the top of a stack.  So, I probably should change the check to
> 
>      sk == skb_to_full_sk(skb)

I think this should work. It exactly covers your use case:
   they are owned by a request_sock but sent through
   the listening sock, that is the socket listening incoming connections
and sk == skb->sk for non request_sock/listening_sock case.

I originally though whether you could do
   sk == skb->sk || skb->sk->sk_state == TCP_NEW_SYN_RECV
but obviously your approach is better.

> 
> instead of removing it.  If we remove the check, egress filters
> could be called multiple times for a skb, just like what Daniel said.
> 
> Does that make sense?
> 
> [1] 
> https://lore.kernel.org/all/CAADnVQKi0c=Mf3b=z43=b6n2xBVhwPw4QoV_au5+pFE29iLkaQ@mail.gmail.com/
> [2] https://lore.kernel.org/all/58193E9D.7040201@iogearbox.net/
> 
>>
>>>
>>> Signed-off-by: Kui-Feng Lee <kuifeng@meta.com>
>>> ---
>>>   include/linux/bpf-cgroup.h | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/include/linux/bpf-cgroup.h b/include/linux/bpf-cgroup.h
>>> index 57e9e109257e..e656da531f9f 100644
>>> --- a/include/linux/bpf-cgroup.h
>>> +++ b/include/linux/bpf-cgroup.h
>>> @@ -199,7 +199,7 @@ static inline bool cgroup_bpf_sock_enabled(struct 
>>> sock *sk,
>>>   #define BPF_CGROUP_RUN_PROG_INET_EGRESS(sk, skb)                   \
>>>   ({                                           \
>>>       int __ret = 0;                                   \
>>> -    if (cgroup_bpf_enabled(CGROUP_INET_EGRESS) && sk && sk == 
>>> skb->sk) { \
>>> +    if (cgroup_bpf_enabled(CGROUP_INET_EGRESS) && sk) {               \
>>>           typeof(sk) __sk = sk_to_full_sk(sk);                   \
>>>           if (sk_fullsock(__sk) &&                       \
>>>               cgroup_bpf_sock_enabled(__sk, 
>>> CGROUP_INET_EGRESS))           \
>>