bpf: add bpf_skb_change_tail helper

This work adds a bpf_skb_change_tail() helper for tc BPF programs. The
basic idea is to expand or shrink the skb in a controlled manner. The
eBPF program can then rewrite the rest via helpers like bpf_skb_store_bytes(),
bpf_lX_csum_replace() and others rather than passing a raw buffer for
writing here.

bpf_skb_change_tail() is really a slow path helper and intended for
replies with f.e. ICMP control messages. Concept is similar to other
helpers like bpf_skb_change_proto() helper to keep the helper without
protocol specifics and let the BPF program mangle the remaining parts.
A flags field has been added and is reserved for now should we extend
the helper in future.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 53 insertions, 1 deletions

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 0f665cb26b50..7047448e8129 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -2295,7 +2295,7 @@ static inline int pskb_network_may_pull(struct sk_buff *skb, unsigned int len)
 
 int ___pskb_trim(struct sk_buff *skb, unsigned int len);
 
-static inline void __skb_trim(struct sk_buff *skb, unsigned int len)
+static inline void __skb_set_length(struct sk_buff *skb, unsigned int len)
 {
 	if (unlikely(skb_is_nonlinear(skb))) {
 		WARN_ON(1);
@@ -2305,6 +2305,11 @@ static inline void __skb_trim(struct sk_buff *skb, unsigned int len)
 	skb_set_tail_pointer(skb, len);
 }
 
+static inline void __skb_trim(struct sk_buff *skb, unsigned int len)
+{
+	__skb_set_length(skb, len);
+}
+
 void skb_trim(struct sk_buff *skb, unsigned int len);
 
 static inline int __pskb_trim(struct sk_buff *skb, unsigned int len)
@@ -2335,6 +2340,20 @@ static inline void pskb_trim_unique(struct sk_buff *skb, unsigned int len)
 	BUG_ON(err);
 }
 
+static inline int __skb_grow(struct sk_buff *skb, unsigned int len)
+{
+	unsigned int diff = len - skb->len;
+
+	if (skb_tailroom(skb) < diff) {
+		int ret = pskb_expand_head(skb, 0, diff - skb_tailroom(skb),
+					   GFP_ATOMIC);
+		if (ret)
+			return ret;
+	}
+	__skb_set_length(skb, len);
+	return 0;
+}
+
 /**
  *	skb_orphan - orphan a buffer
  *	@skb: buffer to orphan
@@ -2938,6 +2957,21 @@ static inline int pskb_trim_rcsum(struct sk_buff *skb, unsigned int len)
 	return __pskb_trim(skb, len);
 }
 
+static inline int __skb_trim_rcsum(struct sk_buff *skb, unsigned int len)
+{
+	if (skb->ip_summed == CHECKSUM_COMPLETE)
+		skb->ip_summed = CHECKSUM_NONE;
+	__skb_trim(skb, len);
+	return 0;
+}
+
+static inline int __skb_grow_rcsum(struct sk_buff *skb, unsigned int len)
+{
+	if (skb->ip_summed == CHECKSUM_COMPLETE)
+		skb->ip_summed = CHECKSUM_NONE;
+	return __skb_grow(skb, len);
+}
+
 #define skb_queue_walk(queue, skb) \
 		for (skb = (queue)->next;					\
 		     skb != (struct sk_buff *)(queue);				\
@@ -3726,6 +3760,13 @@ static inline bool skb_is_gso_v6(const struct sk_buff *skb)
 	return skb_shinfo(skb)->gso_type & SKB_GSO_TCPV6;
 }
 
+static inline void skb_gso_reset(struct sk_buff *skb)
+{
+	skb_shinfo(skb)->gso_size = 0;
+	skb_shinfo(skb)->gso_segs = 0;
+	skb_shinfo(skb)->gso_type = 0;
+}
+
 void __skb_warn_lro_forwarding(const struct sk_buff *skb);
 
 static inline bool skb_warn_if_lro(const struct sk_buff *skb)
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 866d53c33298..e4c5a1baa993 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -386,6 +386,17 @@ enum bpf_func_id {
 	 */
 	BPF_FUNC_current_task_under_cgroup,
 
+	/**
+	 * bpf_skb_change_tail(skb, len, flags)
+	 * The helper will resize the skb to the given new size,
+	 * to be used f.e. with control messages.
+	 * @skb: pointer to skb
+	 * @len: new skb length
+	 * @flags: reserved
+	 * Return: 0 on success or negative error
+	 */
+	BPF_FUNC_skb_change_tail,
+
 	__BPF_FUNC_MAX_ID,
 };