Merge branch 'selftests-bpf-introduce-experimental-bpf_in_interrupt'

Leon Hwang says:

====================
selftests/bpf: Introduce experimental bpf_in_interrupt()

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)'.

'bpf_in_interrupt()' runs well when PREEMPT_RT is enabled. But it's
difficult for me to test it well because I'm not familiar with
PREEMPT_RT.

Changes:
v2 -> v3:
* Address comments from Alexei:
  * Move bpf_in_interrupt() to bpf_experimental.h.
  * Add support for arm64.
v2: https://lore.kernel.org/bpf/20250825131502.54269-1-leon.hwang@linux.dev/

v1 -> v2:
* Fix a build error reported by test bot.
====================

Link: https://patch.msgid.link/20250903140438.59517-1-leon.hwang@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 30 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/prog_tests/timer.c b/tools/testing/selftests/bpf/prog_tests/timer.c
index d66687f1ee6a..049efb5e7823 100644
--- a/tools/testing/selftests/bpf/prog_tests/timer.c
+++ b/tools/testing/selftests/bpf/prog_tests/timer.c
@@ -3,6 +3,7 @@
 #include <test_progs.h>
 #include "timer.skel.h"
 #include "timer_failure.skel.h"
+#include "timer_interrupt.skel.h"
 
 #define NUM_THR 8
 
@@ -95,3 +96,32 @@ void serial_test_timer(void)
 
 	RUN_TESTS(timer_failure);
 }
+
+void test_timer_interrupt(void)
+{
+	struct timer_interrupt *skel = NULL;
+	int err, prog_fd;
+	LIBBPF_OPTS(bpf_test_run_opts, opts);
+
+	skel = timer_interrupt__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "timer_interrupt__open_and_load"))
+		return;
+
+	err = timer_interrupt__attach(skel);
+	if (!ASSERT_OK(err, "timer_interrupt__attach"))
+		goto out;
+
+	prog_fd = bpf_program__fd(skel->progs.test_timer_interrupt);
+	err = bpf_prog_test_run_opts(prog_fd, &opts);
+	if (!ASSERT_OK(err, "bpf_prog_test_run_opts"))
+		goto out;
+
+	usleep(50);
+
+	ASSERT_EQ(skel->bss->in_interrupt, 0, "in_interrupt");
+	if (skel->bss->preempt_count)
+		ASSERT_NEQ(skel->bss->in_interrupt_cb, 0, "in_interrupt_cb");
+
+out:
+	timer_interrupt__destroy(skel);
+}