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:

quoted

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

quoted

+		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.

quoted

+		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);

quoted

+{
+	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.

quoted

+
+	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.

quoted

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

Thanks for reviewing.