Fix BPF tests on linux v6.8 by ensuring NSEC_PER_SEC is defined on 6.8 and later kernels

Signed-off-by: Dom Del Nano <ddelnano@gmail.com>

Author: ddelnano

src/stirling/bpf_tools/bcc_bpf/task_struct_utils.h

@@ -25,6 +25,14 @@
 
 #include <linux/sched.h>
 
+// Between linux v6.1 and v6.8, NSEC_PER_SEC is no longer defined by including
+// linux/sched.h. This ifndef covers newer kernels that won't have it defined.
+// libbpf provides a trace_helpers.h header that provides NSEC_PER_SEC, so it seems
+// tools are moving towards defining their own instead of using a linux header for it.
+#ifndef NSEC_PER_SEC
+#define NSEC_PER_SEC 1000000000ULL
+#endif
+
 #include "src/stirling/bpf_tools/bcc_bpf/utils.h"
 
 // This is how Linux converts nanoseconds to clock ticks.

src/stirling/source_connectors/dynamic_tracer/dynamic_tracing/code_gen.cc

@@ -896,6 +896,11 @@ std::vector<std::string> GenIncludes() {
   return {
       // For struct task_struct.
       "#include <linux/sched.h>",
+      // NSEC_PER_SEC is not defined within linux/sched.h for
+      // 6.x kernels, so we define it here.
+      "#ifndef NSEC_PER_SEC",
+      "#define NSEC_PER_SEC 1000000000ULL",
+      "#endif",
   };
 }