Re: [PATCH net-next v4] net: Re-use and set mono_delivery_time bit for userspace tstamp packets

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

On 3/1/24 12:13 PM, Abhishek Chauhan wrote:
> Bridge driver today has no support to forward the userspace timestamp
> packets and ends up resetting the timestamp. ETF qdisc checks the
> packet coming from userspace and encounters to be 0 thereby dropping
> time sensitive packets. These changes will allow userspace timestamps
> packets to be forwarded from the bridge to NIC drivers.
> 
> Setting the same bit (mono_delivery_time) to avoid dropping of
> userspace tstamp packets in the forwarding path.
> 
> Existing functionality of mono_delivery_time remains unaltered here,
> instead just extended with userspace tstamp support for bridge
> forwarding path.

The patch currently broke the bpf selftest test_tc_dtime: 
https://github.com/kernel-patches/bpf/actions/runs/8242487344/job/22541746675

In particular, there is a uapi field __sk_buff->tstamp_type which currently has 
BPF_SKB_TSTAMP_DELIVERY_MONO to mean skb->tstamp has the MONO "delivery" time. 
BPF_SKB_TSTAMP_UNSPEC means everything else (this could be a rx timestamp at 
ingress or a delivery time set by user space).

__sk_buff->tstamp_type depends on skb->mono_delivery_time which does not 
necessarily mean mono after this patch. I thought about fixing it on the bpf 
side such that reading __sk_buff->tstamp_type only returns 
BPF_SKB_TSTAMP_DELIVERY_MONO when the skb->mono_delivery_time is set and skb->sk 
is IPPROTO_TCP. However, it won't work because of bpf_skb_set_tstamp().

There is a bpf helper, bpf_skb_set_tstamp(skb, tstamp, 
BPF_SKB_TSTAMP_DELIVERY_MONO). This helper changes both the skb->tstamp and the 
skb->mono_delivery_time. The expectation is this could change skb->tstamp in the 
ingress skb and redirect to egress sch_fq. It could also set a mono time to 
skb->tstamp where the udp sk->sk_clockid may not be necessary in mono and then 
bpf_redirect to egress sch_fq. When bpf_skb_set_tstamp(skb, tstamp, 
BPF_SKB_TSTAMP_DELIVERY_MONO) succeeds, reading __sk_buff->tstamp_type expects 
BPF_SKB_TSTAMP_DELIVERY_MONO also.

I ran out of idea to solve this uapi breakage.

I am afraid it may need to go back to v1 idea and use another bit 
(user_delivery_time) in the skb.