[PATCH v1 bpf-next/net 3/5] bpf: Add bpf_skb_set_hwtstamp().

[Kuniyuki Iwashima <kuniyu@google.com>]

We have some hosts where packets come from special hardware
and are provided directly to userspace, bypassing the kernel
networking stack.

When standard socket applications are run on these hosts,
a userspace proxy is required to mediate traffic between the
hardware and the applications.

            +---------+                 +----------------------+
            |  proxy  |                 |  socket application  |
            +---------+                 +----------------------+
              ^     ^                               ^
  userspace   |     |                               |
  -----------| |-----------------------------------------------
             | |    |    +---------------------+    | skb
             | |    `--->|  virtual interface  |<---'
  kernel     | |   skb   +---------------------+
  -----------| |-----------------------------------------------
              |
              v
       +------------+
       |  hardware  |
       +------------+

However, even though the hardware fully supports timestamping,
the HW timestamps are not directly accessible to the socket
applications because the skb is consumed/injected by the proxy.

For RX flow, let's add a kfunc to update skb_hwtstamps(skb)->hwtstamp
at tc/ingress.

With this kfunc, the proxy can carry the RX hardware timestamp
via encapsulated packets (e.g. in GENEVE option) and BPF prog
can extract it into skb_hwtstamps(skb)->hwtstamp at tc/ingress
of the virtual interface above.

            +---------+                 +----------------------+
            |  proxy  |                 |  socket application  |
            +---------+                 +----------------------+
              ^     | encap packets                 ^ recv payload
  userspace   |     |  w/ RX hwtstamp               |  w/ RX hwtstamp
  -----------| |-----------------------------------------------
             | |    |    +---------------------+    | skb
             | |    `--->|       geneve0       |----'
  kernel     | |   skb   +---------------------+
             | |             |             ^
             | |             v             |
             | |          +------------------+  extract RX hwtstamp
             | |          |  BPF@tc/ingress  |   and set it to skb
             | |          +------------------+
  -----------| |-----------------------------------------------
              |
              |
       +------------+
       |  hardware  |
       +------------+

This allows transparently proxying RX hardware timestamps to
the socket applications via SCM_TIMESTAMPING.

Note that bpf_skb_set_hwtstamp() calls skb_header_unclone() and
bpf_compute_data_pointers(), so it is marked as a packet-changing
kfunc.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 include/linux/skbuff.h |  5 +++++
 kernel/bpf/verifier.c  |  9 ++++++++-
 net/core/filter.c      | 23 +++++++++++++++++++++++
 3 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 115db8c44db2..b4ac1180f5a8 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -4701,6 +4701,11 @@ static inline bool skb_defer_rx_timestamp(struct sk_buff *skb)
 
 #endif /* !CONFIG_NETWORK_PHY_TIMESTAMPING */
 
+struct bpf_hwtstamp {
+	ktime_t hwtstamp;
+	u64 reserved;
+} __packed;
+
 /**
  * skb_complete_tx_timestamp() - deliver cloned skb with tx timestamps
  *
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 7fb88e1cd7c4..6b23577d001a 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -11191,6 +11191,7 @@ enum special_kfunc_type {
 	KF_bpf_session_is_return,
 	KF_bpf_stream_vprintk,
 	KF_bpf_stream_print_stack,
+	KF_bpf_skb_set_hwtstamp,
 };
 
 BTF_ID_LIST(special_kfunc_list)
@@ -11283,6 +11284,11 @@ BTF_ID_UNUSED
 #endif
 BTF_ID(func, bpf_stream_vprintk)
 BTF_ID(func, bpf_stream_print_stack)
+#ifdef CONFIG_NET
+BTF_ID(func, bpf_skb_set_hwtstamp)
+#else
+BTF_ID_UNUSED
+#endif
 
 static bool is_bpf_obj_new_kfunc(u32 func_id)
 {
@@ -11364,7 +11370,8 @@ static bool is_kfunc_bpf_preempt_enable(struct bpf_kfunc_call_arg_meta *meta)
 
 bool bpf_is_kfunc_pkt_changing(struct bpf_kfunc_call_arg_meta *meta)
 {
-	return meta->func_id == special_kfunc_list[KF_bpf_xdp_pull_data];
+	return meta->func_id == special_kfunc_list[KF_bpf_xdp_pull_data] ||
+	       meta->func_id == special_kfunc_list[KF_bpf_skb_set_hwtstamp];
 }
 
 static enum kfunc_ptr_arg_type
diff --git a/net/core/filter.c b/net/core/filter.c
index acdc66aa4f27..ab7adef9c015 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -12372,6 +12372,28 @@ __bpf_kfunc int bpf_sock_ops_enable_tx_tstamp(struct bpf_sock_ops_kern *skops,
 	return 0;
 }
 
+__bpf_kfunc int bpf_skb_set_hwtstamp(struct __sk_buff *s,
+				     struct bpf_hwtstamp *attrs, int attrs__sz)
+{
+	struct sk_buff *skb = (struct sk_buff *)s;
+
+	if (attrs__sz != sizeof(*attrs) || attrs->reserved)
+		return -EINVAL;
+
+	if (!skb_at_tc_ingress(skb))
+		return -EINVAL;
+
+	if (skb_header_unclone(skb, GFP_ATOMIC))
+		return -ENOMEM;
+
+	skb_clear_tstamp(skb);
+	skb_hwtstamps(skb)->hwtstamp = attrs->hwtstamp;
+
+	bpf_compute_data_pointers(skb);
+
+	return 0;
+}
+
 /**
  * bpf_xdp_pull_data() - Pull in non-linear xdp data.
  * @x: &xdp_md associated with the XDP buffer
@@ -12500,6 +12522,7 @@ BTF_KFUNCS_END(bpf_kfunc_check_set_sock_addr)
 
 BTF_KFUNCS_START(bpf_kfunc_check_set_sched_cls)
 BTF_ID_FLAGS(func, bpf_sk_assign_tcp_reqsk)
+BTF_ID_FLAGS(func, bpf_skb_set_hwtstamp)
 BTF_KFUNCS_END(bpf_kfunc_check_set_sched_cls)
 
 BTF_KFUNCS_START(bpf_kfunc_check_set_sock_ops)
-- 
2.54.0.1136.gdb2ca164c4-goog