dyninst/loader: fix bug with circuit breaker stats (#43323)

### What does this PR do?

Before this change we were interpretting the number of nanoseconds as the number of hits. This lead to quite incorrect logic with regards to the circuit breaker firing (it would fire too often).

### Motivation

I noticed the circuit breaker tripping when it absolutely should not have been.

### Describe how you validated your changes

Added a test.

### Additional Notes

Relates to https://datadoghq.atlassian.net/browse/DEBUG-4202.

Co-authored-by: andrew.werner <[email protected]>

Author: ajwerner

pkg/dyninst/loader/loader.go

@@ -182,12 +182,12 @@ func (p *Program) Close() {
 
 // RuntimeStats are cumulative stats aggregated throughout program lifetime.
 type RuntimeStats struct {
+	// Aggregated cpu time spent in probe execution (excluding interrupt overhead).
+	CPU time.Duration
 	// Number of probe hits.
 	HitCnt uint64
 	// Number of probe hits that skipped data capture due to throttling.
 	ThrottledCnt uint64
-	// Aggregated cpu time spent in probe execution (excluding interrupt overhead).
-	CPU time.Duration
 }
 
 // RuntimeStats returns the per-core runtime stats for the program.
@@ -198,11 +198,14 @@ func (p *Program) RuntimeStats() []RuntimeStats {
 	}
 	entries := statsMap.Iterate()
 	var key uint32
-	var stats []RuntimeStats
-	if !entries.Next(&key, &stats) {
-		return stats
-	}
-	return stats
+	var stats []stats
+	_ = entries.Next(&key, &stats)
+	// This is safe because these two structs have the same layout.
+	// See TestRuntimeStatsHasSameLayoutAsStats for more details.
+	return unsafe.Slice(
+		(*RuntimeStats)(unsafe.Pointer(unsafe.SliceData(stats))),
+		len(stats),
+	)
 }
 
 const defaultRingbufSize = 1 << 20 // 1 MiB

pkg/dyninst/loader/stats_test.go

@@ -0,0 +1,33 @@
+// Unless explicitly stated otherwise all files in this repository are licensed
+// under the Apache License Version 2.0.
+// This product includes software developed at Datadog (https://www.datadoghq.com/).
+// Copyright 2016-present Datadog, Inc.
+
+//go:build linux_bpf
+
+package loader
+
+import (
+	"crypto/rand"
+	"testing"
+	"time"
+	"unsafe"
+
+	"github.com/stretchr/testify/require"
+)
+
+// Verify that the RuntimeStats struct has the same layout as the stats struct
+// used in C.
+func TestRuntimeStatsHasSameLayoutAsStats(t *testing.T) {
+	require.Equal(t, unsafe.Sizeof(RuntimeStats{}), unsafe.Sizeof(stats{}))
+	bytes := make([]byte, unsafe.Sizeof(stats{}))
+	rand.Read(bytes)
+	cStats := *(*stats)(unsafe.Pointer(&bytes[0]))
+	runtimeStats := RuntimeStats{
+		CPU:          time.Duration(cStats.Cpu_ns),
+		HitCnt:       cStats.Hit_cnt,
+		ThrottledCnt: cStats.Throttled_cnt,
+	}
+	require.Equal(t, runtimeStats, *(*RuntimeStats)(unsafe.Pointer(&cStats)))
+	require.Equal(t, cStats, *(*stats)(unsafe.Pointer(&runtimeStats)))
+}