[PATCH RFC v2 5/8] 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>
Acked-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Marco Elver <elver@google.com>
---
v2:
* Use atomic_set(&event_count, 1), since it must always be 0 in
  perf_pending_event_disable().
* Implicitly restrict inheriting events if sigtrap, but the child was
  cloned with CLONE_CLEAR_SIGHAND, because it is not generally safe if
  the child cleared all signal handlers to continue sending SIGTRAP.
---
 include/uapi/linux/perf_event.h |  3 ++-
 kernel/events/core.c            | 28 +++++++++++++++++++++++++++-
 2 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index 8c5b9f5ad63f..3a4dbb1688f0 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -391,7 +391,8 @@ struct perf_event_attr {
 				build_id       :  1, /* use build id in mmap2 events */
 				inherit_thread :  1, /* children only inherit if cloned with CLONE_THREAD */
 				remove_on_exec :  1, /* event is removed from task on exec */
-				__reserved_1   : 27;
+				sigtrap        :  1, /* send synchronous SIGTRAP on event */
+				__reserved_1   : 26;
 
 	union {
 		__u32		wakeup_events;	  /* wakeup every n events */
diff --git a/kernel/events/core.c b/kernel/events/core.c
index bc9e6e35e414..e70c411b0b16 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -6328,6 +6328,17 @@ 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);
@@ -6337,6 +6348,13 @@ 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_set(&event->event_limit, 1); /* rearm event */
+			perf_sigtrap(event);
+			return;
+		}
+
 		perf_event_disable_local(event);
 		return;
 	}
@@ -11367,6 +11385,9 @@ 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;
 		/*
@@ -11645,6 +11666,9 @@ static int perf_copy_attr(struct perf_event_attr __user *uattr,
 	if (attr->remove_on_exec && attr->enable_on_exec)
 		return -EINVAL;
 
+	if (attr->sigtrap && !attr->remove_on_exec)
+		return -EINVAL;
+
 out:
 	return ret;
 
@@ -12874,7 +12898,9 @@ inherit_task_group(struct perf_event *event, struct task_struct *parent,
 	struct perf_event_context *child_ctx;
 
 	if (!event->attr.inherit ||
-	    (event->attr.inherit_thread && !(clone_flags & CLONE_THREAD))) {
+	    (event->attr.inherit_thread && !(clone_flags & CLONE_THREAD)) ||
+	    /* Do not inherit if sigtrap and signal handlers were cleared. */
+	    (event->attr.sigtrap && (clone_flags & CLONE_CLEAR_SIGHAND))) {
 		*inherited_all = 0;
 		return 0;
 	}
-- 
2.30.1.766.gb4fecdf3b7-goog