netdev - Re: [PATCH bpf-next,v4 2/4] xfrm: interface: Add unstable helpers for setting/getting XFRM metadata from TC-BPF

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Date:   Fri, 2 Dec 2022 12:27:01 -0800
From:   Martin KaFai Lau <martin.lau@...ux.dev>
To:     Eyal Birger <eyal.birger@...il.com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        linux-kselftest@...r.kernel.org, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
        steffen.klassert@...unet.com, herbert@...dor.apana.org.au,
        andrii@...nel.org, daniel@...earbox.net, nicolas.dichtel@...nd.com,
        razor@...ckwall.org, mykolal@...com, ast@...nel.org,
        song@...nel.org, yhs@...com, john.fastabend@...il.com,
        kpsingh@...nel.org, sdf@...gle.com, haoluo@...gle.com,
        jolsa@...nel.org, shuah@...nel.org, liuhangbin@...il.com,
        lixiaoyan@...gle.com
Subject: Re: [PATCH bpf-next,v4 2/4] xfrm: interface: Add unstable helpers for
 setting/getting XFRM metadata from TC-BPF

On 12/2/22 11:42 AM, Eyal Birger wrote:
> Hi Martin,
> 
> On Fri, Dec 2, 2022 at 9:08 PM Martin KaFai Lau <martin.lau@...ux.dev> wrote:
>>
>> On 12/2/22 1:59 AM, Eyal Birger wrote:
>>> +__used noinline
>>> +int bpf_skb_set_xfrm_info(struct __sk_buff *skb_ctx,
>>> +                       const struct bpf_xfrm_info *from)
>>> +{
>>> +     struct sk_buff *skb = (struct sk_buff *)skb_ctx;
>>> +     struct metadata_dst *md_dst;
>>> +     struct xfrm_md_info *info;
>>> +
>>> +     if (unlikely(skb_metadata_dst(skb)))
>>> +             return -EINVAL;
>>> +
>>> +     md_dst = this_cpu_ptr(xfrm_md_dst);
>>> +
>>> +     info = &md_dst->u.xfrm_info;
>>> +
>>> +     info->if_id = from->if_id;
>>> +     info->link = from->link;
>>> +     skb_dst_force(skb);
>>> +     info->dst_orig = skb_dst(skb);
>>> +
>>> +     dst_hold((struct dst_entry *)md_dst);
>>> +     skb_dst_set(skb, (struct dst_entry *)md_dst);
>>
>>
>> I may be missed something obvious and this just came to my mind,
>>
>> What stops cleanup_xfrm_interface_bpf() being run while skb is still holding the
>> md_dst?
>>
> Oh I think you're right. I missed this.
> 
> In order to keep this implementation I suppose it means that the module would
> not be allowed to be removed upon use of this kfunc. but this could be seen as
> annoying from the configuration user experience.
> 
> Alternatively the metadata dsts can be separately allocated from the kfunc,
> which is probably the simplest approach to maintain, so I'll work on that
> approach.

If it means dst_alloc on every skb, it will not be cheap.

Another option is to metadata_dst_alloc_percpu() once during the very first 
bpf_skb_set_xfrm_info() call and the xfrm_md_dst memory will never be freed.  It 
is a tradeoff but likely the correct one.  You can take a look at 
bpf_get_skb_set_tunnel_proto().