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

From: Martin KaFai Lau <[email-masked]>
Date: Wed, 13 Mar 2024 11:42:24 -0700

On 3/13/24 1:52 AM, Willem de Bruijn wrote:
> 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)?

Right, because skb->mono_delivery_time does not mean skb->tstamp is mono now and 
its interpretation depends on skb->sk->sk_clockid.

> 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.

skb->sk == NULL is fine. I tried something like this in 
bpf_convert_tstamp_type_read() for reading __sk_buff->tstamp_type:

__sk_buff->tstamp_type is BPF_SKB_TSTAMP_DELIVERY_MONO when:

	skb->mono_delivery_time == 1 &&
	(!skb->sk ||
	 !sk_fullsock(skb->sk) /* tcp tw or req sk */ ||
	 skb->sk->sk_protocol == IPPROTO_TCP)

Not a small bpf instruction addition to bpf_convert_tstamp_type_read() but doable.

> 
> 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.

The bpf prog may act as a traffic shaper that limits the bandwidth usage of all 
outgoing packets (tcp/udp/...) by setting the mono EDT in skb->tstamp before 
sending to the sch_fq.

I currently also don't have a use case for skb->sk->sk_clockid != 
CLOCK_MONOTONIC. However, it is something that bpf_skb_set_tstamp() can do now 
before queuing to sch_fq.

The container (in netns + veth) may use other sk_clockid/qdisc (e.g. sch_etf) 
setup and the non mono skb->tstamp is not cleared now during dev_forward_skb() 
between the veth pair.

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

This will change the existing bpf_skb_set_tstamp() behavior, so probably not 
acceptable.

> 
>      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.

Change sk->sk_flags in tc bpf prog? hmm... I am not sure this will work well also.

sock_valbool_flag(SOCK_TXTIME) should require a lock_sock() to make changes. The 
tc bpf prog cannot take the lock_sock, so bpf_skb_set_tstamp() currently only 
changes skb and does not change skb->sk.

I think changing sock_valbool_flag(SOCK_TXTIME) will also have a new user space 
visible side effect. The sendmsg for cmsg with SCM_TXTIME will start failing 
from looking at __sock_cmsg_send().

There may be a short period of disconnect between what is in sk->sk_flags and 
what is set in skb->tstamp. e.g. what if user space does setsockopt(SO_TXTIME) 
again after skb->tstamp is set by bpf. This could be considered a small glitch 
for some amount of skb(s) until the user space settled on setsockopt(SO_TXTIME).

I think all this is crying for another bit in skb to mean user_delivery_time 
(skb->tstamp depends on skb->sk->sk_clockid) while mono_delivery_time is the 
mono time either set by kernel-tcp or bpf. If we need to revert the 
mono_delivery_time reuse patch later, we will need to revert the netdev patch 
and the (to-be-made) bpf patch.