Re: [RFC PATCH bpf-next] bpf: add bpf_get_skb_hash helper function

a local skb is allocated and we populate the fields needed in the skb
before calling skb_get_hash. To avoid memory allocations for each packet,
we allocate an skb per CPU and use the same buffer for subsequent hash
calculations on the same CPU.

Signed-off-by: Harshitha Ramamurthy <redacted>
---
 include/uapi/linux/bpf.h       |  8 ++++++
 net/core/filter.c              | 50 ++++++++++++++++++++++++++++++++++
 tools/include/uapi/linux/bpf.h |  8 ++++++
 3 files changed, 66 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index b134e679e9db..25aa850c8a40 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h

@@ -3394,6 +3394,13 @@ union bpf_attr {
  *		A non-negative value equal to or less than *size* on success,
  *		or a negative error in case of failure.
  *
+ * u32 bpf_get_skb_hash(struct xdp_buff *xdp_md)
+ *	Description
+ *		Return the skb hash for the xdp context passed. This function
+ *		allocates a temporary skb and populates the fields needed. It
+ *		then calls skb_get_hash to calculate the skb hash for the packet.
+ *	Return
+ *		The 32-bit hash.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\

@@ -3538,6 +3545,7 @@ union bpf_attr {
 	FN(skc_to_tcp_request_sock),	\
 	FN(skc_to_udp6_sock),		\
 	FN(get_task_stack),		\
+	FN(get_skb_hash),		\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper

diff --git a/net/core/filter.c b/net/core/filter.c
index 7124f0fe6974..9f6ad7209b44 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c

@@ -3765,6 +3765,54 @@ static const struct bpf_func_proto bpf_xdp_redirect_map_proto = {
 	.arg3_type      = ARG_ANYTHING,
 };
 
+static DEFINE_PER_CPU(struct sk_buff *, hash_skb);
+
+BPF_CALL_1(bpf_get_skb_hash, struct xdp_buff *, xdp)
+{
+	void *data_end = xdp->data_end;
+	struct ethhdr *eth = xdp->data;
+	void *data = xdp->data;
+	unsigned long flags;
+	struct sk_buff *skb;
+	int nh_off, len;
+	u32 ret = 0;
+
+	/* disable interrupts to get the correct skb pointer */
+	local_irq_save(flags);
+
+	len = data_end - data;
+	skb = this_cpu_read(hash_skb);
+	if (!skb) {
+		skb = alloc_skb(len, GFP_ATOMIC);
+		if (!skb)
+			goto out;
+		this_cpu_write(hash_skb, skb);
+	}
+
+	nh_off = sizeof(*eth);