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

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Martin KaFai Lau wrote:
> 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.

Is the only conflict when bpf_skb_set_tstamp is called for an skb from
a socket with sk_clockid set (and thus SO_TXTIME called)?

Interpreting skb->tstamp as mono if skb->mono_delivery_time is set and
skb->sk is NULL is fine. This is the ingress to egress redirect case.

I don't see an immediate use for this BPF function on egress where it
would overwrite an SO_TXTIME timestamp and now skb->tstamp is mono,
but skb->sk != NULL and skb->sk->sk_clockid != CLOCK_MONOTONIC.

Perhaps bpf_skb_set_tstamp() can just fail if another delivery time is
already explicitly programmed?

    skb->sk &&
    sock_flag(sk, SOCK_TXTIME) &&
    skb->sk->sk_clockid != CLOCK_MONOTONIC

Either that, or unset SOCK_TXTIME to make sk_clockid undefined and
fall back on interpreting as monotonic.