netdev - Re: [RFC PATCH net-next v6 08/13] net-timestamp: support hw SCM_TSTAMP

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Message-ID: <7bf7110c-b978-45b8-9f74-4a37d6e98d5d@linux.dev>
Date: Fri, 24 Jan 2025 18:36:54 -0800
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Jason Xing <kerneljasonxing@...il.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
 pabeni@...hat.com, dsahern@...nel.org, willemdebruijn.kernel@...il.com,
 willemb@...gle.com, ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
 eddyz87@...il.com, song@...nel.org, yonghong.song@...ux.dev,
 john.fastabend@...il.com, kpsingh@...nel.org, sdf@...ichev.me,
 haoluo@...gle.com, jolsa@...nel.org, horms@...nel.org, bpf@...r.kernel.org,
 netdev@...r.kernel.org
Subject: Re: [RFC PATCH net-next v6 08/13] net-timestamp: support hw
 SCM_TSTAMP_SND for bpf extension

On 1/24/25 5:35 PM, Jason Xing wrote:
> On Sat, Jan 25, 2025 at 9:30 AM Martin KaFai Lau <martin.lau@...ux.dev> wrote:
>>
>> On 1/24/25 5:18 PM, Jason Xing wrote:
>>>>> @@ -5577,9 +5578,9 @@ static void skb_tstamp_tx_bpf(struct sk_buff *skb, struct sock *sk,
>>>>>                 op = BPF_SOCK_OPS_TS_SCHED_OPT_CB;
>>>>>                 break;
>>>>>         case SCM_TSTAMP_SND:
>>>>> +             op = sw ? BPF_SOCK_OPS_TS_SW_OPT_CB : BPF_SOCK_OPS_TS_HW_OPT_CB;
>>>>>                 if (!sw)
>>>>> -                     return;
>>>>> -             op = BPF_SOCK_OPS_TS_SW_OPT_CB;
>>>>> +                     *skb_hwtstamps(skb) = *hwtstamps;
>>>> hwtstamps may still be NULL, no?
>>> Right, it can be zero if something wrong happens.
>>
>> Then it needs a NULL check, no?
> 
> My original intention is passing whatever to the userspace, so the bpf
> program will be aware of what is happening in the kernel.

This is fine.

> Passing NULL to hwstamps is right which will not cause any problem, I think.
> 
> Do you mean the default value of hwstamps itself is NULL so in this
> case we don't need to re-init it to NULL again?
> 
> Like this:
> If (*hwtstamps)
   if (hwtstamps) instead ?

I don't know. If hwtstamps is NULL, doing *hwtstamps will be bad and oops....
May be my brain doesn't work well at the end of Friday. Please check.

>       *skb_hwtstamps(skb) = *hwtstamps;
> 
> But it looks no different actually.
> 
> Thanks,
> Jason