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>
---
 include/linux/filter.h |  8 ++++++++
 kernel/bpf/helpers.c   |  2 +-
 net/core/filter.c      | 12 ++++++++++++
 3 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/include/linux/filter.h b/include/linux/filter.h
index eca229752cbe..de0d1ce34f0d 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -1772,6 +1772,8 @@ int __bpf_xdp_store_bytes(struct xdp_buff *xdp, u32 offset, void *buf, u32 len);
 void *bpf_xdp_pointer(struct xdp_buff *xdp, u32 offset, u32 len);
 void bpf_xdp_copy_buf(struct xdp_buff *xdp, unsigned long off,
 		      void *buf, unsigned long len, bool flush);
+int bpf_skb_meta_load_bytes(const struct sk_buff *src, u32 offset,
+			    void *dst, u32 len);
 #else /* CONFIG_NET */
 static inline int __bpf_skb_load_bytes(const struct sk_buff *skb, u32 offset,
 				       void *to, u32 len)
@@ -1806,6 +1808,12 @@ static inline void bpf_xdp_copy_buf(struct xdp_buff *xdp, unsigned long off, voi
 				    unsigned long len, bool flush)
 {
 }
+
+static inline int bpf_skb_meta_load_bytes(const struct sk_buff *src, u32 offset,
+					  void *dst, u32 len)
+{
+	return -EOPNOTSUPP;
+}
 #endif /* CONFIG_NET */
 
 #endif /* __LINUX_FILTER_H__ */
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index e25a6d44efd6..54b0e8dd747e 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -1776,7 +1776,7 @@ 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;
diff --git a/net/core/filter.c b/net/core/filter.c
index e4a1f50904db..93524839a49f 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -11978,6 +11978,18 @@ bpf_sk_base_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 	return func;
 }
 
+int bpf_skb_meta_load_bytes(const struct sk_buff *skb, u32 offset,
+			    void *dst, u32 len)
+{
+	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;
+}
+
 static int dynptr_from_skb_meta(struct __sk_buff *skb_, u64 flags,
 				struct bpf_dynptr *ptr_, bool rdonly)
 {

-- 
2.43.0