[PATCH] perf/bpf: Don't call bpf_overflow_handler() for tracing events

[Kyle Huey]

The regressing commit is new in 6.10. It assumed that anytime event->prog
is set bpf_overflow_handler() should be invoked to execute the attached bpf
program. This assumption is false for tracing events, and as a result the
regressing commit broke bpftrace by invoking the bpf handler with garbage
inputs on overflow.

Prior to the regression the overflow handlers formed a chain (of length 0,
1, or 2) and perf_event_set_bpf_handler() (the !tracing case) added
bpf_overflow_handler() to that chain, while perf_event_attach_bpf_prog()
(the tracing case) did not. Both set event->prog. The chain of overflow
handlers was replaced by a single overflow handler slot and a fixed call to
bpf_overflow_handler() when appropriate. This modifies the condition there
to include !perf_event_is_tracing(), restoring the previous behavior and
fixing bpftrace.

Signed-off-by: Kyle Huey <khuey@xxxxxxxxxxxx>
Reported-by: Joe Damato <jdamato@xxxxxxxxxx>
Fixes: f11f10bfa1ca ("perf/bpf: Call BPF handler directly, not through overflow machinery")
Tested-by: Joe Damato <jdamato@xxxxxxxxxx> # bpftrace
Tested-by: Kyle Huey <khuey@xxxxxxxxxxxx> # bpf overflow handlers
---
 kernel/events/core.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 8f908f077935..f0d7119585dc 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -9666,6 +9666,8 @@ static inline void perf_event_free_bpf_handler(struct perf_event *event)
  * Generic event overflow handling, sampling.
  */
 
+static bool perf_event_is_tracing(struct perf_event *event);
+
 static int __perf_event_overflow(struct perf_event *event,
 				 int throttle, struct perf_sample_data *data,
 				 struct pt_regs *regs)
@@ -9682,7 +9684,9 @@ static int __perf_event_overflow(struct perf_event *event,
 
 	ret = __perf_event_account_interrupt(event, throttle);
 
-	if (event->prog && !bpf_overflow_handler(event, data, regs))
+	if (event->prog &&
+	    !perf_event_is_tracing(event) &&
+	    !bpf_overflow_handler(event, data, regs))
 		return ret;
 
 	/*
@@ -10612,6 +10616,11 @@ void perf_event_free_bpf_prog(struct perf_event *event)
 
 #else
 
+static inline bool perf_event_is_tracing(struct perf_event *event)
+{
+	return false;
+}
+
 static inline void perf_tp_register(void)
 {
 }
-- 
2.34.1