[PATCH bpf-next v3 0/2] selftests/bpf: Introduce execution context detection helpers

[PATCH bpf-next v3 0/2] selftests/bpf: Introduce execution context detection helpers

[Changwoo Min]

This series introduces four new BPF-native inline helpers -- bpf_in_nmi(),
bpf_in_hardirq(), bpf_in_serving_softirq(), and bpf_in_task() -- to allow
BPF programs to query the current execution context.

Following the feedback on v1, these are implemented in bpf_experimental.h
as inline helpers wrapping get_preempt_count(). This approach allows the
logic to be JIT-inlined for better performance compared to a kfunc call,
while providing the granular context detection (e.g., hardirq vs. softirq)
required by subsystems like sched_ext.

The series includes a new selftest suite, exe_ctx, which uses bpf_testmod
to verify context detection across Task, HardIRQ, and SoftIRQ boundaries
via irq_work and tasklets. NMI context testing is omitted as NMIs cannot
be triggered deterministically within software-only BPF CI environments.

ChangeLog v2 -> v3:
- Added exe_ctx to DENYLIST.s390x since new helpers are supported only
  on x86 and arm64 (patch 2).
- Added comments to helpers describing supported architectures (patch 1).

ChangeLog v1 -> v2:
- Dropped the core kernel kfunc implementations, and implemented context
  detection as inline BPF helpers in bpf_experimental.h.
- Renamed the selftest suite from ctx_kfunc to exe_ctx to reflect the
  change from kfuncs to helpers.
- Updated BPF programs to use the new inline helpers.
- Swapped clean-up order between tasklet and irqwork in bpf_testmod to 
  avoid re-scheduling the already-killed tasklet (reported by bot+bpf-ci).

Changwoo Min (2):
  selftests/bpf: Introduce execution context detection helpers
  selftests/bpf: Add tests for execution context helpers

 tools/testing/selftests/bpf/DENYLIST.s390x    |  1 +
 .../testing/selftests/bpf/bpf_experimental.h  | 58 ++++++++++++++++++
 .../selftests/bpf/prog_tests/exe_ctx.c        | 59 +++++++++++++++++++
 tools/testing/selftests/bpf/progs/test_ctx.c  | 48 +++++++++++++++
 .../selftests/bpf/test_kmods/bpf_testmod.c    | 32 ++++++++++
 .../bpf/test_kmods/bpf_testmod_kfunc.h        |  4 ++
 6 files changed, 202 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/exe_ctx.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_ctx.c

-- 
2.52.0

[PATCH bpf-next v3 1/2] selftests/bpf: Introduce execution context detection helpers

[Changwoo Min]

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.

Reviewed-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Changwoo Min <changwoo@igalia.com>
---
 .../testing/selftests/bpf/bpf_experimental.h  | 58 +++++++++++++++++++
 1 file changed, 58 insertions(+)

diff --git a/tools/testing/selftests/bpf/bpf_experimental.h b/tools/testing/selftests/bpf/bpf_experimental.h
index 68a49b1f77ae..a39576c8ba04 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 +650,60 @@ static inline int bpf_in_interrupt(void)
 	       (tsk->softirq_disable_cnt & SOFTIRQ_MASK);
 }
 
+/* Description
+ *	Report whether it is in NMI context. Only works on the following archs:
+ *	* x86
+ *	* arm64
+ */
+static inline int bpf_in_nmi(void)
+{
+	return get_preempt_count() & NMI_MASK;
+}
+
+/* Description
+ *	Report whether it is in hard IRQ context. Only works on the following archs:
+ *	* x86
+ *	* arm64
+ */
+static inline int bpf_in_hardirq(void)
+{
+	return get_preempt_count() & HARDIRQ_MASK;
+}
+
+/* Description
+ *	Report whether it is in softirq context. Only works on the following archs:
+ *	* x86
+ *	* arm64
+ */
+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;
+}
+
+/* Description
+ *	Report whether it is in task context. Only works on the following archs:
+ *	* x86
+ *	* arm64
+ */
+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

[PATCH bpf-next v3 2/2] selftests/bpf: Add tests for execution context helpers

[Changwoo Min]

Add a new selftest suite `exe_ctx` to verify the accuracy of the
bpf_in_task(), bpf_in_hardirq(), and bpf_in_serving_softirq() helpers
introduced in bpf_experimental.h.

Testing these execution contexts deterministically requires crossing
context boundaries within a single CPU. To achieve this, the test
implements a "Trigger-Observer" pattern using bpf_testmod:

1. Trigger: A BPF syscall program calls a new bpf_testmod kfunc
   bpf_kfunc_trigger_ctx_check().
2. Task to HardIRQ: The kfunc uses irq_work_queue() to trigger a
   self-IPI on the local CPU.
3. HardIRQ to SoftIRQ: The irq_work handler calls a dummy function
   (observed by BPF fentry) and then schedules a tasklet to
   transition into SoftIRQ context.

The user-space runner ensures determinism by pinning itself to CPU 0
before execution, forcing the entire interrupt chain to remain on a
single core. Dummy noinline functions with compiler barriers are
added to bpf_testmod.c to serve as stable attachment points for
fentry programs. A retry loop is used in user-space to wait for the
asynchronous SoftIRQ to complete.

Note that testing on s390x is avoided because supporting those helpers
purely in BPF on s390x is not possible at this point.

Reviewed-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Changwoo Min <changwoo@igalia.com>
---
 tools/testing/selftests/bpf/DENYLIST.s390x    |  1 +
 .../selftests/bpf/prog_tests/exe_ctx.c        | 59 +++++++++++++++++++
 tools/testing/selftests/bpf/progs/test_ctx.c  | 48 +++++++++++++++
 .../selftests/bpf/test_kmods/bpf_testmod.c    | 32 ++++++++++
 .../bpf/test_kmods/bpf_testmod_kfunc.h        |  4 ++
 5 files changed, 144 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/exe_ctx.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_ctx.c

diff --git a/tools/testing/selftests/bpf/DENYLIST.s390x b/tools/testing/selftests/bpf/DENYLIST.s390x
index a17baf8c6fd7..f7e1e5f5511c 100644
--- a/tools/testing/selftests/bpf/DENYLIST.s390x
+++ b/tools/testing/selftests/bpf/DENYLIST.s390x
@@ -1,4 +1,5 @@
 # TEMPORARY
 # Alphabetical order
+exe_ctx                                  # execution context check (e.g., hardirq, softirq, etc)
 get_stack_raw_tp                         # user_stack corrupted user stack                                             (no backchain userspace)
 stacktrace_build_id                      # compare_map_keys stackid_hmap vs. stackmap err -2 errno 2                   (?)
diff --git a/tools/testing/selftests/bpf/prog_tests/exe_ctx.c b/tools/testing/selftests/bpf/prog_tests/exe_ctx.c
new file mode 100644
index 000000000000..aed6a6ef0876
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/exe_ctx.c
@@ -0,0 +1,59 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2026 Valve Corporation.
+ * Author: Changwoo Min <changwoo@igalia.com>
+ */
+
+#include <test_progs.h>
+#include <sys/syscall.h>
+#include "test_ctx.skel.h"
+
+void test_exe_ctx(void)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, opts);
+	cpu_set_t old_cpuset, target_cpuset;
+	struct test_ctx *skel;
+	int err, prog_fd;
+
+	/* 1. Pin the current process to CPU 0. */
+	if (sched_getaffinity(0, sizeof(old_cpuset), &old_cpuset) == 0) {
+		CPU_ZERO(&target_cpuset);
+		CPU_SET(0, &target_cpuset);
+		ASSERT_OK(sched_setaffinity(0, sizeof(target_cpuset),
+					    &target_cpuset), "setaffinity");
+	}
+
+	skel = test_ctx__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel_load"))
+		goto restore_affinity;
+
+	err = test_ctx__attach(skel);
+	if (!ASSERT_OK(err, "skel_attach"))
+		goto cleanup;
+
+	/* 2. When we run this, the kernel will execute the BPF prog on CPU 0. */
+	prog_fd = bpf_program__fd(skel->progs.trigger_all_contexts);
+	err = bpf_prog_test_run_opts(prog_fd, &opts);
+	ASSERT_OK(err, "test_run_trigger");
+
+	/* 3. Wait for the local CPU's softirq/tasklet to finish. */
+	for (int i = 0; i < 1000; i++) {
+		if (skel->bss->count_task > 0 &&
+		    skel->bss->count_hardirq > 0 &&
+		    skel->bss->count_softirq > 0)
+			break;
+		usleep(1000); /* Wait 1ms per iteration, up to 1 sec total */
+	}
+
+	/* On CPU 0, these should now all be non-zero. */
+	ASSERT_GT(skel->bss->count_task, 0, "task_ok");
+	ASSERT_GT(skel->bss->count_hardirq, 0, "hardirq_ok");
+	ASSERT_GT(skel->bss->count_softirq, 0, "softirq_ok");
+
+cleanup:
+	test_ctx__destroy(skel);
+
+restore_affinity:
+	ASSERT_OK(sched_setaffinity(0, sizeof(old_cpuset), &old_cpuset),
+		  "restore_affinity");
+}
diff --git a/tools/testing/selftests/bpf/progs/test_ctx.c b/tools/testing/selftests/bpf/progs/test_ctx.c
new file mode 100644
index 000000000000..7d4995506717
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_ctx.c
@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2026 Valve Corporation.
+ * Author: Changwoo Min <changwoo@igalia.com>
+ */
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include "bpf_experimental.h"
+
+char _license[] SEC("license") = "GPL";
+
+extern void bpf_kfunc_trigger_ctx_check(void) __ksym;
+
+int count_hardirq;
+int count_softirq;
+int count_task;
+
+/* Triggered via bpf_prog_test_run from user-space */
+SEC("syscall")
+int trigger_all_contexts(void *ctx)
+{
+	if (bpf_in_task())
+		__sync_fetch_and_add(&count_task, 1);
+
+	/* Trigger the firing of a hardirq and softirq for test. */
+	bpf_kfunc_trigger_ctx_check();
+	return 0;
+}
+
+/* Observer for HardIRQ */
+SEC("fentry/bpf_testmod_test_hardirq_fn")
+int BPF_PROG(on_hardirq)
+{
+	if (bpf_in_hardirq())
+		__sync_fetch_and_add(&count_hardirq, 1);
+	return 0;
+}
+
+/* Observer for SoftIRQ */
+SEC("fentry/bpf_testmod_test_softirq_fn")
+int BPF_PROG(on_softirq)
+{
+	if (bpf_in_serving_softirq())
+		__sync_fetch_and_add(&count_softirq, 1);
+	return 0;
+}
diff --git a/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c b/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
index d425034b72d3..1b04022859b7 100644
--- a/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
+++ b/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
@@ -1164,6 +1164,33 @@ __bpf_kfunc int bpf_kfunc_implicit_arg(int a, struct bpf_prog_aux *aux);
 __bpf_kfunc int bpf_kfunc_implicit_arg_legacy(int a, int b, struct bpf_prog_aux *aux);
 __bpf_kfunc int bpf_kfunc_implicit_arg_legacy_impl(int a, int b, struct bpf_prog_aux *aux);
 
+/* hook targets */
+noinline void bpf_testmod_test_hardirq_fn(void) { barrier(); }
+noinline void bpf_testmod_test_softirq_fn(void) { barrier(); }
+
+/* Tasklet for SoftIRQ context */
+static void ctx_check_tasklet_fn(struct tasklet_struct *t)
+{
+	bpf_testmod_test_softirq_fn();
+}
+
+DECLARE_TASKLET(ctx_check_tasklet, ctx_check_tasklet_fn);
+
+/* IRQ Work for HardIRQ context */
+static void ctx_check_irq_fn(struct irq_work *work)
+{
+	bpf_testmod_test_hardirq_fn();
+	tasklet_schedule(&ctx_check_tasklet);
+}
+
+static struct irq_work ctx_check_irq = IRQ_WORK_INIT_HARD(ctx_check_irq_fn);
+
+/* The kfunc trigger */
+__bpf_kfunc void bpf_kfunc_trigger_ctx_check(void)
+{
+	irq_work_queue(&ctx_check_irq);
+}
+
 BTF_KFUNCS_START(bpf_testmod_check_kfunc_ids)
 BTF_ID_FLAGS(func, bpf_testmod_test_mod_kfunc)
 BTF_ID_FLAGS(func, bpf_kfunc_call_test1)
@@ -1209,6 +1236,7 @@ BTF_ID_FLAGS(func, bpf_kfunc_multi_st_ops_test_1_assoc, KF_IMPLICIT_ARGS)
 BTF_ID_FLAGS(func, bpf_kfunc_implicit_arg, KF_IMPLICIT_ARGS)
 BTF_ID_FLAGS(func, bpf_kfunc_implicit_arg_legacy, KF_IMPLICIT_ARGS)
 BTF_ID_FLAGS(func, bpf_kfunc_implicit_arg_legacy_impl)
+BTF_ID_FLAGS(func, bpf_kfunc_trigger_ctx_check)
 BTF_KFUNCS_END(bpf_testmod_check_kfunc_ids)
 
 static int bpf_testmod_ops_init(struct btf *btf)
@@ -1840,6 +1868,10 @@ static void bpf_testmod_exit(void)
 	while (refcount_read(&prog_test_struct.cnt) > 1)
 		msleep(20);
 
+	/* Clean up irqwork and tasklet */
+	irq_work_sync(&ctx_check_irq);
+	tasklet_kill(&ctx_check_tasklet);
+
 	bpf_kfunc_close_sock();
 	sysfs_remove_bin_file(kernel_kobj, &bin_attr_bpf_testmod_file);
 	unregister_bpf_testmod_uprobe();
diff --git a/tools/testing/selftests/bpf/test_kmods/bpf_testmod_kfunc.h b/tools/testing/selftests/bpf/test_kmods/bpf_testmod_kfunc.h
index 10f89f06245f..d5c5454e257e 100644
--- a/tools/testing/selftests/bpf/test_kmods/bpf_testmod_kfunc.h
+++ b/tools/testing/selftests/bpf/test_kmods/bpf_testmod_kfunc.h
@@ -169,4 +169,8 @@ extern int bpf_kfunc_multi_st_ops_test_1_assoc(struct st_ops_args *args) __weak
 struct prog_test_member *bpf_kfunc_get_default_trusted_ptr_test(void) __ksym;
 void bpf_kfunc_put_default_trusted_ptr_test(struct prog_test_member *trusted_ptr) __ksym;
 
+void bpf_testmod_test_hardirq_fn(void);
+void bpf_testmod_test_softirq_fn(void);
+void bpf_kfunc_trigger_ctx_check(void) __ksym;
+
 #endif /* _BPF_TESTMOD_KFUNC_H */
-- 
2.52.0

Re: [PATCH bpf-next v3 0/2] selftests/bpf: Introduce execution context detection helpers

[patchwork-bot+netdevbpf]

Hello:

This series was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Sun, 25 Jan 2026 20:54:11 +0900 you wrote:
> This series introduces four new BPF-native inline helpers -- bpf_in_nmi(),
> bpf_in_hardirq(), bpf_in_serving_softirq(), and bpf_in_task() -- to allow
> BPF programs to query the current execution context.
> 
> Following the feedback on v1, these are implemented in bpf_experimental.h
> as inline helpers wrapping get_preempt_count(). This approach allows the
> logic to be JIT-inlined for better performance compared to a kfunc call,
> while providing the granular context detection (e.g., hardirq vs. softirq)
> required by subsystems like sched_ext.
> 
> [...]

Here is the summary with links:
  - [bpf-next,v3,1/2] selftests/bpf: Introduce execution context detection helpers
    https://git.kernel.org/bpf/bpf-next/c/c31df36bd26a
  - [bpf-next,v3,2/2] selftests/bpf: Add tests for execution context helpers
    https://git.kernel.org/bpf/bpf-next/c/221b5e76c1c6

You are awesome, thank you!
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