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>

3 files changed, 132 insertions, 0 deletions

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
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);
+}
diff --git a/tools/testing/selftests/bpf/progs/timer_interrupt.c b/tools/testing/selftests/bpf/progs/timer_interrupt.c
new file mode 100644
index 000000000000..19180a455f40
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/timer_interrupt.c
@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include "bpf_experimental.h"
+
+char _license[] SEC("license") = "GPL";
+
+#define CLOCK_MONOTONIC		1
+
+int preempt_count;
+int in_interrupt;
+int in_interrupt_cb;
+
+struct elem {
+	struct bpf_timer t;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, int);
+	__type(value, struct elem);
+} array SEC(".maps");
+
+static int timer_in_interrupt(void *map, int *key, struct bpf_timer *timer)
+{
+	preempt_count = get_preempt_count();
+	in_interrupt_cb = bpf_in_interrupt();
+	return 0;
+}
+
+SEC("fentry/bpf_fentry_test1")
+int BPF_PROG(test_timer_interrupt)
+{
+	struct bpf_timer *timer;
+	int key = 0;
+
+	timer = bpf_map_lookup_elem(&array, &key);
+	if (!timer)
+		return 0;
+
+	in_interrupt = bpf_in_interrupt();
+	bpf_timer_init(timer, &array, CLOCK_MONOTONIC);
+	bpf_timer_set_callback(timer, timer_in_interrupt);
+	bpf_timer_start(timer, 0, 0);
+	return 0;
+}