perf: Make perf_event_output() propagate the output() return

For the original mode of operation it isn't needed, since we report back
errors via PERF_RECORD_LOST records in the ring buffer, but for use in
bpf_perf_event_output() it is convenient to return the errors, basically
-ENOSPC.

Currently bpf_perf_event_output() returns an error indication, the last
thing it does, which is to push it to the ring buffer is that can fail
and if so, this failure won't be reported back to its users, fix it.

Reported-by: Jamal Hadi Salim <jhs@mojatatu.com>
Tested-by: Jamal Hadi Salim <jhs@mojatatu.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/r/20190118150938.GN5823@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

kernel/events/core.c

@@ -6489,7 +6489,7 @@ void perf_prepare_sample(struct perf_event_header *header,
 		data->phys_addr = perf_virt_to_phys(data->addr);
 }
 
-static __always_inline void
+static __always_inline int
 __perf_event_output(struct perf_event *event,
 		    struct perf_sample_data *data,
 		    struct pt_regs *regs,
@@ -6499,13 +6499,15 @@ __perf_event_output(struct perf_event *event,
 {
 	struct perf_output_handle handle;
 	struct perf_event_header header;
+	int err;
 
 	/* protect the callchain buffers */
 	rcu_read_lock();
 
 	perf_prepare_sample(&header, data, event, regs);
 
-	if (output_begin(&handle, event, header.size))
+	err = output_begin(&handle, event, header.size);
+	if (err)
 		goto exit;
 
 	perf_output_sample(&handle, &header, data, event);
@@ -6514,6 +6516,7 @@ __perf_event_output(struct perf_event *event,
 
 exit:
 	rcu_read_unlock();
+	return err;
 }
 
 void
@@ -6532,12 +6535,12 @@ perf_event_output_backward(struct perf_event *event,
 	__perf_event_output(event, data, regs, perf_output_begin_backward);
 }
 
-void
+int
 perf_event_output(struct perf_event *event,
 		  struct perf_sample_data *data,
 		  struct pt_regs *regs)
 {
-	__perf_event_output(event, data, regs, perf_output_begin);
+	return __perf_event_output(event, data, regs, perf_output_begin);
 }
 
 /*

kernel/trace/bpf_trace.c

@@ -431,8 +431,7 @@ __bpf_perf_event_output(struct pt_regs *regs, struct bpf_map *map,
 	if (unlikely(event->oncpu != cpu))
 		return -EOPNOTSUPP;
 
-	perf_event_output(event, sd, regs);
-	return 0;
+	return perf_event_output(event, sd, regs);
 }
 
 BPF_CALL_5(bpf_perf_event_output, struct pt_regs *, regs, struct bpf_map *, map,