Make it possible to read from skb metadata area using the
bpf_dynptr_read() BPF helper.

This prepares ground for access to skb metadata from all BPF hooks
which operate on __sk_buff context.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
---
 net/core/filter.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 3c2948517838..f71b4b6b09fb 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -11981,9 +11981,15 @@ int bpf_dynptr_skb_read(const struct bpf_dynptr_kern *src, u32 offset,
 	case SKB_DYNPTR_PAYLOAD:
 		return ____bpf_skb_load_bytes(skb, offset, dst, len);
 
-	case SKB_DYNPTR_METADATA:
-		return -EOPNOTSUPP; /* not implemented */
+	case SKB_DYNPTR_METADATA: {
+		u32 meta_len = skb_metadata_len(skb);
+
+		if (len > meta_len || offset > meta_len - len)
+			return -E2BIG; /* out of bounds */
 
+		memmove(dst, skb_metadata_end(skb) - meta_len + offset, len);
+		return 0;
+	}
 	default:
 		WARN_ONCE(true, "%s: unknown skb dynptr offset %d\n", __func__, src->offset);
 		return -EFAULT;

-- 
2.43.0