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

This patch adds a helper function called bpf_get_skb_hash to calculate
the skb hash for a packet at the XDP layer. In the 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);
+	if (data + nh_off > data_end)
+		goto out;
+
+	skb->data = data;
+	skb->head = data;
+	skb->network_header = nh_off;
+	skb->protocol = eth->h_proto;
+	skb->len = len;
+	skb->dev = xdp->rxq->dev;
+
+	ret = skb_get_hash(skb);
+out:
+	local_irq_restore(flags);
+	return ret;
+}
+
+const struct bpf_func_proto bpf_get_skb_hash_proto = {
+	.func		= bpf_get_skb_hash,
+	.gpl_only	= false,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_PTR_TO_CTX,
+};
+
 static unsigned long bpf_skb_copy(void *dst_buff, const void *skb,
 				  unsigned long off, unsigned long len)
 {

@@ -6503,6 +6551,8 @@ xdp_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_xdp_adjust_tail_proto;
 	case BPF_FUNC_fib_lookup:
 		return &bpf_xdp_fib_lookup_proto;
+	case BPF_FUNC_get_skb_hash:
+		return &bpf_get_skb_hash_proto;
 #ifdef CONFIG_INET
 	case BPF_FUNC_sk_lookup_udp:
 		return &bpf_xdp_sk_lookup_udp_proto;

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

-- 
2.20.1

On 8/10/20 12:28 PM, Harshitha Ramamurthy wrote:

This patch adds a helper function called bpf_get_skb_hash to calculate
the skb hash for a packet at the XDP layer. In the helper function,

Why? i.e., expected use case?

Pulling this from hardware when possible is better. e.g., Saeed's
hardware hints proposal includes it.

vlans?

+	if (data + nh_off > data_end)
+		goto out;
+
+	skb->data = data;
+	skb->head = data;
+	skb->network_header = nh_off;
+	skb->protocol = eth->h_proto;
+	skb->len = len;
+	skb->dev = xdp->rxq->dev;
+
+	ret = skb_get_hash(skb);

static inline __u32 skb_get_hash(struct sk_buff *skb)
{
        if (!skb->l4_hash && !skb->sw_hash)
                __skb_get_hash(skb);

        return skb->hash;
}

__skb_get_hash -> __skb_set_sw_hash -> __skb_set_hash which sets
sw_hash as a minimum, so it seems to me you will always be returning the
hash of the first packet since you do not clear relevant fields of the skb.

On Mon, 2020-08-10 at 13:02 -0600, David Ahern wrote:

On 8/10/20 12:28 PM, Harshitha Ramamurthy wrote:

quoted

This patch adds a helper function called bpf_get_skb_hash to
calculate
the skb hash for a packet at the XDP layer. In the helper function,

Why? i.e., expected use case?

Pulling this from hardware when possible is better. e.g., Saeed's
hardware hints proposal includes it.

Thanks for the review and comments.

So, when possible, it might be better to pull the HW hash but not all
HW provides it so this function would be useful in those cases. Also,
this is a precursor to potentially calling the in-kernel flow dissector
from a helper function.

quoted

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 <harshitha.ramamurthy@intel.com

quoted

---
 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);

vlans?

Yes, need to factor for vlans. Will fix it.

quoted

+	if (data + nh_off > data_end)
+		goto out;
+
+	skb->data = data;
+	skb->head = data;
+	skb->network_header = nh_off;
+	skb->protocol = eth->h_proto;
+	skb->len = len;
+	skb->dev = xdp->rxq->dev;
+
+	ret = skb_get_hash(skb);

static inline __u32 skb_get_hash(struct sk_buff *skb)
{
        if (!skb->l4_hash && !skb->sw_hash)
                __skb_get_hash(skb);

        return skb->hash;
}

__skb_get_hash -> __skb_set_sw_hash -> __skb_set_hash which sets
sw_hash as a minimum, so it seems to me you will always be returning
the
hash of the first packet since you do not clear relevant fields of
the skb.

yes, that's true. Calling skb_clear_hash first should fix this. I will
try it out.

Thanks,
Harshitha.