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

[Kui-Feng Lee <sinquersw@gmail.com>]

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)

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))           \
>