[PATCH net-next 1/6] bpf: Add bpf_ktime_get_real_ns() kfunc

[Jason Xing <kerneljasonxing@gmail.com>]

From: Jason Xing <kernelxing@tencent.com>

Currently BPF programs can obtain timestamps via bpf_ktime_get_ns(),
which returns CLOCK_MONOTONIC time. However, the skb->tstamp field
populated by the network stack uses ktime_get_real() which is in the
CLOCK_REALTIME domain.

In the series, kernel reports the software/hardware timestamps through
sockopt and then userspace bpf application gets them and calculate the
delta only through an unified time unit.

However, prior to this, when a BPF program tries to measure RX packet
delay by comparing skb->tstamp with bpf_ktime_get_ns(), the result
is incorrect because the two clocks have different epochs.

Introduce a new BPF kfunc bpf_ktime_get_real_ns() that returns the
current CLOCK_REALTIME time. This allows BPF programs to perform
accurate delay calculations without clock domain mismatch issue.

Signed-off-by: Jason Xing <kernelxing@tencent.com>
---
 kernel/bpf/helpers.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 2bb60200c266..863645d096ef 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2317,6 +2317,11 @@ void bpf_rb_root_free(const struct btf_field *field, void *rb_root,
 
 __bpf_kfunc_start_defs();
 
+__bpf_kfunc u64 bpf_ktime_get_real_ns(void)
+{
+	return ktime_get_real_fast_ns();
+}
+
 /**
  * bpf_obj_new() - allocate an object described by program BTF
  * @local_type_id__k: type ID in program BTF
@@ -4859,6 +4864,7 @@ BTF_ID_FLAGS(func, bpf_task_work_schedule_resume, KF_IMPLICIT_ARGS)
 BTF_ID_FLAGS(func, bpf_dynptr_from_file)
 BTF_ID_FLAGS(func, bpf_dynptr_file_discard)
 BTF_ID_FLAGS(func, bpf_timer_cancel_async)
+BTF_ID_FLAGS(func, bpf_ktime_get_real_ns)
 BTF_KFUNCS_END(common_btf_ids)
 
 static const struct btf_kfunc_id_set common_kfunc_set = {
-- 
2.43.7