netdev - Re: [PATCH bpf-next v4 2/8] bpf: Enable read/write access to skb metadata through a dynptr

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Message-ID: <87y0sdx6se.fsf@cloudflare.com>
Date: Thu, 24 Jul 2025 11:44:01 +0200
From: Jakub Sitnicki <jakub@...udflare.com>
To: Martin KaFai Lau <martin.lau@...ux.dev>
Cc: Alexei Starovoitov <ast@...nel.org>,  Andrii Nakryiko
 <andrii@...nel.org>,  Arthur Fabre <arthur@...hurfabre.com>,  Daniel
 Borkmann <daniel@...earbox.net>,  Eduard Zingerman <eddyz87@...il.com>,
  Eric Dumazet <edumazet@...gle.com>,  Jakub Kicinski <kuba@...nel.org>,
  Jesper Dangaard Brouer <hawk@...nel.org>,  Jesse Brandeburg
 <jbrandeburg@...udflare.com>,  Joanne Koong <joannelkoong@...il.com>,
  Lorenzo Bianconi <lorenzo@...nel.org>,  Toke Høiland-Jørgensen
 <thoiland@...hat.com>,  Yan Zhai <yan@...udflare.com>,
  kernel-team@...udflare.com,  netdev@...r.kernel.org,
  bpf@...r.kernel.org,  Stanislav Fomichev <sdf@...ichev.me>
Subject: Re: [PATCH bpf-next v4 2/8] bpf: Enable read/write access to skb
 metadata through a dynptr

On Wed, Jul 23, 2025 at 05:02 PM -07, Martin KaFai Lau wrote:
> On 7/23/25 10:36 AM, Jakub Sitnicki wrote:
>> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
>> index 9552b32208c5..237fb5f9d625 100644
>> --- a/kernel/bpf/helpers.c
>> +++ b/kernel/bpf/helpers.c
>> @@ -1781,7 +1781,7 @@ static int __bpf_dynptr_read(void *dst, u32 len, const struct bpf_dynptr_kern *s
>>   	case BPF_DYNPTR_TYPE_XDP:
>>   		return __bpf_xdp_load_bytes(src->data, src->offset + offset, dst, len);
>>   	case BPF_DYNPTR_TYPE_SKB_META:
>> -		return -EOPNOTSUPP; /* not implemented */
>> +		return bpf_skb_meta_load_bytes(src->data, src->offset + offset, dst, len);
>>   	default:
>>   		WARN_ONCE(true, "bpf_dynptr_read: unknown dynptr type %d\n", type);
>>   		return -EFAULT;
>> @@ -1839,7 +1839,7 @@ int __bpf_dynptr_write(const struct bpf_dynptr_kern *dst, u32 offset, void *src,
>>   			return -EINVAL;
>>   		return __bpf_xdp_store_bytes(dst->data, dst->offset + offset, src, len);
>>   	case BPF_DYNPTR_TYPE_SKB_META:
>> -		return -EOPNOTSUPP; /* not implemented */
>
> It needs to check the flags here such that the flags can be used in the future:
>
> 		if (flags)
> 			return -EINVAL;
>

Missed that. Thanks.

> pw-bot: cr
>
>> +		return bpf_skb_meta_store_bytes(dst->data, dst->offset + offset, src, len);
>>   	default:
>>   		WARN_ONCE(true, "bpf_dynptr_write: unknown dynptr type %d\n", type);
>>   		return -EFAULT;
>> @@ -2716,7 +2716,7 @@ __bpf_kfunc void *bpf_dynptr_slice(const struct bpf_dynptr *p, u32 offset,
>>   		return buffer__opt;
>>   	}
>>   	case BPF_DYNPTR_TYPE_SKB_META:
>> -		return NULL; /* not implemented */
>> +		return bpf_skb_meta_pointer(ptr->data, ptr->offset + offset, len);
>>   	default:
>>   		WARN_ONCE(true, "unknown dynptr type %d\n", type);
>>   		return NULL;
>> diff --git a/net/core/filter.c b/net/core/filter.c
>> index 0755dfc0fc2f..cf095897d4c1 100644
>> --- a/net/core/filter.c
>> +++ b/net/core/filter.c
>> @@ -11978,6 +11978,45 @@ bpf_sk_base_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>>   	return func;
>>   }
>>   +static void *skb_metadata_pointer(const struct sk_buff *skb, u32 off, u32
>> len)
>> +{
>> +	u32 meta_len = skb_metadata_len(skb);
>> +
>> +	if (len > meta_len || off > meta_len - len)
>
> A nit.
>
> After reading it again, I think this is a duplicated check. The
> bpf_dynptr_check_off_len() called in the kfunc should have already checked the
> same condition.

I took a better-safe-than-sorry approach. Can skb_metadata_len() change
after the call to bpf_dynptr_from_skb_meta()? Today it can't, but what
if we let TC BPF resize the metadata area in the future?

But you're right. It's duplicated code for now and can be dropped.

>
>> +		return ERR_PTR(-E2BIG); /* out of bounds */
>> +
>> +	return skb_metadata_end(skb) - meta_len + off;
>> +}
>> +
>> +int bpf_skb_meta_load_bytes(const struct sk_buff *src, u32 off, void *dst, u32 len)
>
> Since it needs a respin, I have a few nit comments that will be useful for
> reading filter.c.
>
> Change the "const struct sk_buff *src" to "const struct sk_buff *skb". It is how
> other places are naming the arg in filter.c.

No problem. I will unify it with:

int __bpf_skb_load_bytes(const struct sk_buff *skb, u32 offset, void *to, u32 len);
int __bpf_skb_store_bytes(struct sk_buff *skb, u32 offset, const void *from,
			  u32 len, u64 flags);

>
>> +{
>> +	const void *p = skb_metadata_pointer(src, off, len);
>
> Not sure if this variable is still needed if skb_metadata_pointer does not
> return err ptr.
>
> If it is still needed, use "const void *ptr" instead of "const void *p". The
> bpf_xdp_pointer and skb_header_pointer callers in filter.c also use that naming.
>

Will do.

>> +
>> +	if (IS_ERR(p))
>> +		return PTR_ERR(p);
>> +
>> +	memmove(dst, p, len);
>> +	return 0;
>> +}
>> +
>> +int bpf_skb_meta_store_bytes(struct sk_buff *dst, u32 off, const void *src, u32 len)
>> +{
>> +	void *p = skb_metadata_pointer(dst, off, len);
>
> Same for the "struct sk_buff *dst" and "void *p" in this function.
>

Will change.

>> +
>> +	if (IS_ERR(p))
>> +		return PTR_ERR(p);
>> +
>> +	memmove(p, src, len);
>> +	return 0;
>> +}

Thanks for reviewing.