[PATCH RFC 3/4] perf/core: Add support for SIGTRAP on perf events

[Marco Elver <elver@google.com>]

Adds bit perf_event_attr::sigtrap, which can be set to cause events to
send SIGTRAP (with si_code TRAP_PERF) to the task where the event
occurred. To distinguish perf events and allow user space to decode
si_perf (if set), the event type is set in si_errno.

The primary motivation is to support synchronous signals on perf events
in the task where an event (such as breakpoints) triggered.

Link: https://lore.kernel.org/lkml/YBv3rAT566k+6zjg@hirez.programming.kicks-ass.net/
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Marco Elver <elver@google.com>
---
 include/uapi/linux/perf_event.h |  3 ++-
 kernel/events/core.c            | 21 +++++++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index ad15e40d7f5d..b9cc6829a40c 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -389,7 +389,8 @@ struct perf_event_attr {
 				cgroup         :  1, /* include cgroup events */
 				text_poke      :  1, /* include text poke events */
 				build_id       :  1, /* use build id in mmap2 events */
-				__reserved_1   : 29;
+				sigtrap        :  1, /* send synchronous SIGTRAP on event */
+				__reserved_1   : 28;
 
 	union {
 		__u32		wakeup_events;	  /* wakeup every n events */
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 37a8297be164..8718763045fd 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -6288,6 +6288,17 @@ void perf_event_wakeup(struct perf_event *event)
 	}
 }
 
+static void perf_sigtrap(struct perf_event *event)
+{
+	struct kernel_siginfo info;
+
+	clear_siginfo(&info);
+	info.si_signo = SIGTRAP;
+	info.si_code = TRAP_PERF;
+	info.si_errno = event->attr.type;
+	force_sig_info(&info);
+}
+
 static void perf_pending_event_disable(struct perf_event *event)
 {
 	int cpu = READ_ONCE(event->pending_disable);
@@ -6297,6 +6308,13 @@ static void perf_pending_event_disable(struct perf_event *event)
 
 	if (cpu == smp_processor_id()) {
 		WRITE_ONCE(event->pending_disable, -1);
+
+		if (event->attr.sigtrap) {
+			atomic_inc(&event->event_limit); /* rearm event */
+			perf_sigtrap(event);
+			return;
+		}
+
 		perf_event_disable_local(event);
 		return;
 	}
@@ -11325,6 +11343,9 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu,
 
 	event->state		= PERF_EVENT_STATE_INACTIVE;
 
+	if (event->attr.sigtrap)
+		atomic_set(&event->event_limit, 1);
+
 	if (task) {
 		event->attach_state = PERF_ATTACH_TASK;
 		/*
-- 
2.30.0.617.g56c4b15f3c-goog