Make it possible to write to skb metadata area using the
bpf_dynptr_write() 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 de0d1ce34f0d..7709e30ce2bb 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -1774,6 +1774,8 @@ 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);
+int bpf_skb_meta_store_bytes(struct sk_buff *dst, u32 offset,
+			     const void *src, u32 len);
 #else /* CONFIG_NET */
 static inline int __bpf_skb_load_bytes(const struct sk_buff *skb, u32 offset,
 				       void *to, u32 len)
@@ -1814,6 +1816,12 @@ static inline int bpf_skb_meta_load_bytes(const struct sk_buff *src, u32 offset,
 {
 	return -EOPNOTSUPP;
 }
+
+static inline int bpf_skb_meta_store_bytes(struct sk_buff *dst, u32 offset,
+					   const void *src, u32 len)
+{
+	return -EOPNOTSUPP;
+}
 #endif /* CONFIG_NET */
 
 #endif /* __LINUX_FILTER_H__ */
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 54b0e8dd747e..a264a6a3b4e4 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -1834,7 +1834,7 @@ 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 */
+		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;
diff --git a/net/core/filter.c b/net/core/filter.c
index 93524839a49f..86b1572e8424 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -11990,6 +11990,18 @@ int bpf_skb_meta_load_bytes(const struct sk_buff *skb, u32 offset,
 	return 0;
 }
 
+int bpf_skb_meta_store_bytes(struct sk_buff *dst, u32 offset,
+			     const void *src, u32 len)
+{
+	u32 meta_len = skb_metadata_len(dst);
+
+	if (len > meta_len || offset > meta_len - len)
+		return -E2BIG; /* out of bounds */
+
+	memmove(skb_metadata_end(dst) - meta_len + offset, src, len);
+	return 0;
+}
+
 static int dynptr_from_skb_meta(struct __sk_buff *skb_, u64 flags,
 				struct bpf_dynptr *ptr_, bool rdonly)
 {

-- 
2.43.0