Make it possible to read from or write to skb metadata area using the
dynptr slices creates with bpf_dynptr_slice() or bpf_dynptr_slice_rdwr().

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>
---
 include/linux/filter.h |  6 ++++++
 kernel/bpf/helpers.c   |  2 +-
 net/core/filter.c      | 10 ++++++++++
 3 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/include/linux/filter.h b/include/linux/filter.h
index 7709e30ce2bb..a28c3a1593c9 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -1776,6 +1776,7 @@ int bpf_skb_meta_load_bytes(const struct sk_buff *src, u32 offset,
 			    void *dst, u32 len);
 int bpf_skb_meta_store_bytes(struct sk_buff *dst, u32 offset,
 			     const void *src, u32 len);
+void *bpf_skb_meta_pointer(struct sk_buff *skb, u32 offset, u32 len);
 #else /* CONFIG_NET */
 static inline int __bpf_skb_load_bytes(const struct sk_buff *skb, u32 offset,
 				       void *to, u32 len)
@@ -1822,6 +1823,11 @@ static inline int bpf_skb_meta_store_bytes(struct sk_buff *dst, u32 offset,
 {
 	return -EOPNOTSUPP;
 }
+
+static inline void *bpf_skb_meta_pointer(struct sk_buff *skb, u32 offset, u32 len)
+{
+	return NULL;
+}
 #endif /* CONFIG_NET */
 
 #endif /* __LINUX_FILTER_H__ */
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index ee057051db94..237fb5f9d625 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -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 3cbadee77492..6d9a462a0042 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -12002,6 +12002,16 @@ int bpf_skb_meta_store_bytes(struct sk_buff *dst, u32 offset,
 	return 0;
 }
 
+void *bpf_skb_meta_pointer(struct sk_buff *skb, u32 offset, u32 len)
+{
+	u32 meta_len = skb_metadata_len(skb);
+
+	if (len > meta_len || offset > meta_len - len)
+		return NULL; /* out of bounds */
+
+	return skb_metadata_end(skb) - meta_len + offset;
+}
+
 static int dynptr_from_skb_meta(struct __sk_buff *skb_, u64 flags,
 				struct bpf_dynptr *ptr_, bool rdonly)
 {

-- 
2.43.0