usdt: Use uprobe refcnt if available in usdt C++ API

src/cc/api/BPF.cc

@@ -20,9 +20,12 @@
 #include <cstdio>
 #include <cstring>
 #include <exception>
+#include <fcntl.h>
 #include <iostream>
 #include <memory>
 #include <sstream>
+#include <sys/stat.h>
+#include <sys/types.h>
 #include <utility>
 #include <vector>
 
@@ -39,6 +42,37 @@
 
 #include "BPF.h"
 
+namespace {
+/*
+ * Kernels ~4.20 and later support specifying the ref_ctr_offset as an argument
+ * to attaching a uprobe, which negates the need to seek to this memory offset
+ * in userspace to manage semaphores, as the kernel will do it for us.  This
+ * helper function checks if this support is available by reading the uprobe
+ * format for this value, added in a6ca88b241d5e929e6e60b12ad8cd288f0ffa
+*/
+bool uprobe_ref_ctr_supported() {
+  const char *ref_ctr_pmu_path =
+      "/sys/bus/event_source/devices/uprobe/format/ref_ctr_offset";
+  const char *ref_ctr_pmu_expected = "config:32-63\0";
+  char ref_ctr_pmu_fmt[64];  // in Linux source this buffer is compared vs
+                             // PAGE_SIZE, but 64 is probably ample
+  int fd = open(ref_ctr_pmu_path, O_RDONLY);
+  if (fd < 0)
+    return false;
+
+  int ret = read(fd, ref_ctr_pmu_fmt, sizeof(ref_ctr_pmu_fmt));
+  close(fd);
+  if (ret < 0) {
+    return false;
+  }
+  if (strncmp(ref_ctr_pmu_expected, ref_ctr_pmu_fmt,
+              strlen(ref_ctr_pmu_expected)) == 0) {
+    return true;
+  }
+  return false;
+}
+} // namespace
+
 namespace ebpf {
 
 std::string uint_to_hex(uint64_t value) {
@@ -268,15 +302,16 @@ StatusTuple BPF::attach_uprobe(const std::string& binary_path,
 
 StatusTuple BPF::attach_usdt_without_validation(const USDT& u, pid_t pid) {
   auto& probe = *static_cast<::USDT::Probe*>(u.probe_.get());
-  if (!probe.enable(u.probe_func_))
+  if (!uprobe_ref_ctr_supported() && !probe.enable(u.probe_func_))
     return StatusTuple(-1, "Unable to enable USDT %s" + u.print_name());
 
   bool failed = false;
   std::string err_msg;
   int cnt = 0;
   for (const auto& loc : probe.locations_) {
     auto res = attach_uprobe(loc.bin_path_, std::string(), u.probe_func_,
-                              loc.address_, BPF_PROBE_ENTRY, pid);
+                             loc.address_, BPF_PROBE_ENTRY, pid, 0,
+                             probe.semaphore_offset());
     if (!res.ok()) {
       failed = true;
       err_msg += "USDT " + u.print_name() + " at " + loc.bin_path_ +
@@ -507,7 +542,7 @@ StatusTuple BPF::detach_usdt_without_validation(const USDT& u, pid_t pid) {
     }
   }
 
-  if (!probe.disable()) {
+  if (!uprobe_ref_ctr_supported() && !probe.disable()) {
     failed = true;
     err_msg += "Unable to disable USDT " + u.print_name();
   }