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: <ae17edd2-22af-4f0f-b130-bf2790bfd774@linux.dev>
Date: Wed, 23 Jul 2025 17:02:11 -0700
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Jakub Sitnicki <jakub@...udflare.com>
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 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;

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.

> +		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.

> +{
> +	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.

> +
> +	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.

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