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.
Signed-off-by: Changwoo Min <changwoo@igalia.com>
---
.../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 ++
4 files changed, 143 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/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 */
--
2.52.0On Sat, Jan 24, 2026 at 5:27 AM Changwoo Min <changwoo@igalia.com> wrote:
>
> 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.
>
> Signed-off-by: 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);
Nicely done! selftests should work in PREEMPT_RT too
though we don't enable it in bpf CI.
I was about to apply it, but the new tests fails on s390:
test_exe_ctx:FAIL:hardirq_ok unexpected hardirq_ok: actual 0 <= expected 0
test_exe_ctx:FAIL:softirq_ok unexpected softirq_ok: actual 0 <= expected 0
The existing bpf_in_interrupt() also works on x86 and arm64 only.
When it was introduced it came with pretty weak selftest
commit 31329b6 ("selftests/bpf: Introduce experimental bpf_in_interrupt()")
so it's not really testing anything on s390 and non-x86/arm architectures.
So just add your strong selftests to DENYLIST.s390x.
get_preempt_count() on s390 looks like this:
static __always_inline int preempt_count(void)
{
return READ_ONCE(get_lowcore()->preempt_count) & ~PREEMPT_NEED_RESCHED;
}
but get_lowcore() needs asm.
So it's not going to be easy to make it work purely in bpf.
Let's punt it to people that care about s390.
pw-bot: cr
Hi Alexei,
Thank you for the review.
On 1/25/26 12:37 PM, Alexei Starovoitov wrote:
> On Sat, Jan 24, 2026 at 5:27 AM Changwoo Min <changwoo@igalia.com> wrote:
>>
>> 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.
>>
>> Signed-off-by: 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);
>
> Nicely done! selftests should work in PREEMPT_RT too
> though we don't enable it in bpf CI.
>
> I was about to apply it, but the new tests fails on s390:
>
> test_exe_ctx:FAIL:hardirq_ok unexpected hardirq_ok: actual 0 <= expected 0
> test_exe_ctx:FAIL:softirq_ok unexpected softirq_ok: actual 0 <= expected 0
>
> The existing bpf_in_interrupt() also works on x86 and arm64 only.
> When it was introduced it came with pretty weak selftest
> commit 31329b6 ("selftests/bpf: Introduce experimental bpf_in_interrupt()")
> so it's not really testing anything on s390 and non-x86/arm architectures.
>
> So just add your strong selftests to DENYLIST.s390x.
> get_preempt_count() on s390 looks like this:
> static __always_inline int preempt_count(void)
> {
> return READ_ONCE(get_lowcore()->preempt_count) & ~PREEMPT_NEED_RESCHED;
> }
> but get_lowcore() needs asm.
> So it's not going to be easy to make it work purely in bpf.
> Let's punt it to people that care about s390.
Regarding the s390 failure: that makes sense. Since the
get_preempt_count() logic relies on architecture-specific register/
memory access that hasn't been implemented for s390 BPF yet, I will add
exe_ctx to DENYLIST.s390x in v3.
I'll send out the v3 with the denylist update shortly. Also, I will add
comments to those helpers mentioning that only x86 and arm64 are
supported.
Regards,
Changwoo
>
> pw-bot: cr
>
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