Re: [PATCH bpf-next v8 10/12] bpf: make TCP tx timestamp bpf extension work

[Martin KaFai Lau <martin.lau@linux.dev>]

On 2/4/25 5:57 PM, Jakub Kicinski wrote:
> On Wed,  5 Feb 2025 02:30:22 +0800 Jason Xing wrote:
>> +	if (cgroup_bpf_enabled(CGROUP_SOCK_OPS) &&
>> +	    SK_BPF_CB_FLAG_TEST(sk, SK_BPF_CB_TX_TIMESTAMPING) && skb) {
>> +		struct skb_shared_info *shinfo = skb_shinfo(skb);
>> +		struct tcp_skb_cb *tcb = TCP_SKB_CB(skb);
>> +
>> +		tcb->txstamp_ack_bpf = 1;
>> +		shinfo->tx_flags |= SKBTX_BPF;
>> +		shinfo->tskey = TCP_SKB_CB(skb)->seq + skb->len - 1;
>> +	}
> 
> If BPF program is attached we'll timestamp all skbs? Am I reading this
> right?

If the attached bpf program explicitly turns on the SK_BPF_CB_TX_TIMESTAMPING 
bit of a sock, then all skbs of this sock will be tx timestamp-ed.

> 
> Wouldn't it be better to let BPF_SOCK_OPS_TS_SND_CB return whether it's
> interested in tracing current packet all the way thru the stack?

I like this idea. It can give the BPF prog a chance to do skb sampling on a 
particular socket.

The return value of BPF_SOCK_OPS_TS_SND_CB (or any cgroup BPF prog return value) 
already has another usage, which its return value is currently enforced by the 
verifier. It is better not to convolute it further.

I don't prefer to add more use cases to skops->reply either, which is an union 
of args[4], such that later progs (in the cgrp prog array) may lose the args value.

Jason, instead of always setting SKBTX_BPF and txstamp_ack_bpf in the kernel, a 
new BPF kfunc can be added so that the BPF prog can call it to selectively set 
SKBTX_BPF and txstamp_ack_bpf in some skb.