Filtering pid_tgid is meanlingless when the current task is preempted by
an interrupt.

To address this, introduce 'bpf_in_interrupt()' helper function, which
allows BPF programs to determine whether they are executing in interrupt
context.

'get_preempt_count()':

* On x86, '*(int *) bpf_this_cpu_ptr(&__preempt_count)'.
* On arm64, 'bpf_get_current_task_btf()->thread_info.preempt.count'.

Then 'bpf_in_interrupt()' will be:

* If !PREEMPT_RT, 'get_preempt_count() & (NMI_MASK | HARDIRQ_MASK
  | SOFTIRQ_MASK)'.
* If PREEMPT_RT, '(get_preempt_count() & (NMI_MASK | HARDIRQ_MASK))
  | (bpf_get_current_task_btf()->softirq_disable_cnt & SOFTIRQ_MASK)'.

As for other archs, it can be added support by updating
'get_preempt_count()'.

Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 .../testing/selftests/bpf/bpf_experimental.h  | 54 +++++++++++++++++++
 1 file changed, 54 insertions(+)

diff --git a/tools/testing/selftests/bpf/bpf_experimental.h b/tools/testing/selftests/bpf/bpf_experimental.h
index da7e230f2781..d89eda3fd8a3 100644
--- a/tools/testing/selftests/bpf/bpf_experimental.h
+++ b/tools/testing/selftests/bpf/bpf_experimental.h
@@ -599,4 +599,58 @@ extern void bpf_iter_dmabuf_destroy(struct bpf_iter_dmabuf *it) __weak __ksym;
 extern int bpf_cgroup_read_xattr(struct cgroup *cgroup, const char *name__str,
 				 struct bpf_dynptr *value_p) __weak __ksym;

+#define PREEMPT_BITS	8
+#define SOFTIRQ_BITS	8
+#define HARDIRQ_BITS	4
+#define NMI_BITS	4
+
+#define PREEMPT_SHIFT	0
+#define SOFTIRQ_SHIFT	(PREEMPT_SHIFT + PREEMPT_BITS)
+#define HARDIRQ_SHIFT	(SOFTIRQ_SHIFT + SOFTIRQ_BITS)
+#define NMI_SHIFT	(HARDIRQ_SHIFT + HARDIRQ_BITS)
+
+#define __IRQ_MASK(x)	((1UL << (x))-1)
+
+#define SOFTIRQ_MASK	(__IRQ_MASK(SOFTIRQ_BITS) << SOFTIRQ_SHIFT)
+#define HARDIRQ_MASK	(__IRQ_MASK(HARDIRQ_BITS) << HARDIRQ_SHIFT)
+#define NMI_MASK	(__IRQ_MASK(NMI_BITS)     << NMI_SHIFT)
+
+extern bool CONFIG_PREEMPT_RT __kconfig __weak;
+#ifdef bpf_target_x86
+extern const int __preempt_count __ksym;
+#endif
+
+struct task_struct___preempt_rt {
+	int softirq_disable_cnt;
+} __attribute__((preserve_access_index));
+
+static inline int get_preempt_count(void)
+{
+#if defined(bpf_target_x86)
+	return *(int *) bpf_this_cpu_ptr(&__preempt_count);
+#elif defined(bpf_target_arm64)
+	return bpf_get_current_task_btf()->thread_info.preempt.count;
+#endif
+	return 0;
+}
+
+/* Description
+ *	Report whether it is in interrupt context. Only works on the following archs:
+ *	* x86
+ *	* arm64
+ */
+static inline int bpf_in_interrupt(void)
+{
+	struct task_struct___preempt_rt *tsk;
+	int pcnt;
+
+	pcnt = get_preempt_count();
+	if (!CONFIG_PREEMPT_RT)
+		return pcnt & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_MASK);
+
+	tsk = (void *) bpf_get_current_task_btf();
+	return (pcnt & (NMI_MASK | HARDIRQ_MASK)) |
+	       (tsk->softirq_disable_cnt & SOFTIRQ_MASK);
+}
+
 #endif
--
2.51.0