linux-kernel - [PATCH bpf-next v2 1/2] selftests/bpf: Introduce execution context detection helpers

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Message-ID: <20260124132706.183681-2-changwoo@igalia.com>
Date: Sat, 24 Jan 2026 22:27:05 +0900
From: Changwoo Min <changwoo@...lia.com>
To: Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>
Cc: Martin KaFai Lau <martin.lau@...ux.dev>,
	Eduard Zingerman <eddyz87@...il.com>,
	Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	John Fastabend <john.fastabend@...il.com>,
	KP Singh <kpsingh@...nel.org>,
	Stanislav Fomichev <sdf@...ichev.me>,
	Hao Luo <haoluo@...gle.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Shuah Khan <shuah@...nel.org>,
	kernel-dev@...lia.com,
	bpf@...r.kernel.org,
	sched-ext@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org,
	Changwoo Min <changwoo@...lia.com>
Subject: [PATCH bpf-next v2 1/2] selftests/bpf: Introduce execution context detection helpers

Introduce bpf_in_nmi(), bpf_in_hardirq(), bpf_in_serving_softirq(), and
bpf_in_task() inline helpers in bpf_experimental.h. These allow BPF
programs to query the current execution context with higher granularity
than the existing bpf_in_interrupt() helper.

While BPF programs can often infer their context from attachment points,
subsystems like sched_ext may call the same BPF logic from multiple
contexts (e.g., task-to-task wake-ups vs. interrupt-to-task wake-ups).
These helpers provide a reliable way for logic to branch based on
the current CPU execution state.

Implementing these as BPF-native inline helpers wrapping
get_preempt_count() allows the compiler and JIT to inline the logic. The
implementation accounts for differences in preempt_count layout between
standard and PREEMPT_RT kernels.

Signed-off-by: Changwoo Min <changwoo@...lia.com>
---
 .../testing/selftests/bpf/bpf_experimental.h  | 38 +++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/tools/testing/selftests/bpf/bpf_experimental.h b/tools/testing/selftests/bpf/bpf_experimental.h
index 68a49b1f77ae..30abdc08366f 100644
--- a/tools/testing/selftests/bpf/bpf_experimental.h
+++ b/tools/testing/selftests/bpf/bpf_experimental.h
@@ -610,6 +610,8 @@ extern int bpf_cgroup_read_xattr(struct cgroup *cgroup, const char *name__str,
 #define HARDIRQ_MASK	(__IRQ_MASK(HARDIRQ_BITS) << HARDIRQ_SHIFT)
 #define NMI_MASK	(__IRQ_MASK(NMI_BITS)     << NMI_SHIFT)
 
+#define SOFTIRQ_OFFSET	(1UL << SOFTIRQ_SHIFT)
+
 extern bool CONFIG_PREEMPT_RT __kconfig __weak;
 #ifdef bpf_target_x86
 extern const int __preempt_count __ksym;
@@ -648,4 +650,40 @@ static inline int bpf_in_interrupt(void)
 	       (tsk->softirq_disable_cnt & SOFTIRQ_MASK);
 }
 
+static inline int bpf_in_nmi(void)
+{
+	return get_preempt_count() & NMI_MASK;
+}
+
+static inline int bpf_in_hardirq(void)
+{
+	return get_preempt_count() & HARDIRQ_MASK;
+}
+
+static inline int bpf_in_serving_softirq(void)
+{
+	struct task_struct___preempt_rt *tsk;
+	int pcnt;
+
+	pcnt = get_preempt_count();
+	if (!CONFIG_PREEMPT_RT)
+		return (pcnt & SOFTIRQ_MASK) & SOFTIRQ_OFFSET;
+
+	tsk = (void *) bpf_get_current_task_btf();
+	return (tsk->softirq_disable_cnt & SOFTIRQ_MASK) & SOFTIRQ_OFFSET;
+}
+
+static inline int bpf_in_task(void)
+{
+	struct task_struct___preempt_rt *tsk;
+	int pcnt;
+
+	pcnt = get_preempt_count();
+	if (!CONFIG_PREEMPT_RT)
+		return !(pcnt & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET));
+
+	tsk = (void *) bpf_get_current_task_btf();
+	return !((pcnt & (NMI_MASK | HARDIRQ_MASK)) |
+		 ((tsk->softirq_disable_cnt & SOFTIRQ_MASK) & SOFTIRQ_OFFSET));
+}
 #endif
-- 
2.52.0