[trace][intel pt] Simple detection of infinite decoding loops

The low-level decoder might fall into an infinite decoding loop for
various reasons, the simplest being an infinite direct loop reached due
to wrong handling of self-modified code in the kernel, e.g. it might
reach

```
0x0A: pause
0x0C: jump to 0x0A
```

In this case, all the code is sequential and requires no packets to be
decoded. The low-level decoder would produce an output like the
following

```
0x0A: pause
0x0C: jump to 0x0A
0x0A: pause
0x0C: jump to 0x0A
0x0A: pause
0x0C: jump to 0x0A
... infinite amount of times
```

These cases require stopping the decoder to avoid infinite work and signal this
at least as a trace error.

- Add a check that breaks decoding of a single PSB once 500k instructions have been decoded since the last packet was processed.
- Add a check that looks for infinite loops after certain amount of instructions have been decoded since the last packet was processed.
- Add some `settings` properties for tweaking the thresholds of the checks above. This is also nice because it does the basic work needed for future settings.
- Add an AnomalyDetector class that inspects the DecodedThread and the libipt decoder in search for anomalies. These anomalies are then signaled as fatal errors in the trace.
- Add an ErrorStats class that keeps track of all the errors in a DecodedThread, with a special counter for fatal errors.
- Add an entry for decoded thread errors in the `dump info` command.

Some notes are added in the code and in the documention of the settings,
so please read them.

Besides that, I haven't been unable to create a test case in LLVM style, but
I've found an anomaly in the thread #12 of the trace
72533820-3eb8-4465-b8e4-4e6bf0ccca99 at Meta. We have to figure out how to
artificially create traces with this kind of anomalies in LLVM style.

With this change, that anomalous thread now shows:

```
(lldb)thread trace dump instructions 12 -e -i 23101

thread #12: tid = 8
    ...missing instructions
    23101: (error) anomalous trace: possible infinite loop detected of size 2
  vmlinux-5.12.0-0_fbk8_clang_6656_gc85768aa64da`panic_smp_self_stop + 5 [inlined] rep_nop at processor.h:13:2
    23100: 0xffffffff81342785    pause
  vmlinux-5.12.0-0_fbk8_clang_6656_gc85768aa64da`panic_smp_self_stop + 7 at panic.c:87:2
    23099: 0xffffffff81342787    jmp    0xffffffff81342785        ; <+5> [inlined] rep_nop at processor.h:13:2
  vmlinux-5.12.0-0_fbk8_clang_6656_gc85768aa64da`panic_smp_self_stop + 5 [inlined] rep_nop at processor.h:13:2
    23098: 0xffffffff81342785    pause
  vmlinux-5.12.0-0_fbk8_clang_6656_gc85768aa64da`panic_smp_self_stop + 7 at panic.c:87:2
    23097: 0xffffffff81342787    jmp    0xffffffff81342785        ; <+5> [inlined] rep_nop at processor.h:13:2
  vmlinux-5.12.0-0_fbk8_clang_6656_gc85768aa64da`panic_smp_self_stop + 5 [inlined] rep_nop at processor.h:13:2
    23096: 0xffffffff81342785    pause
  vmlinux-5.12.0-0_fbk8_clang_6656_gc85768aa64da`panic_smp_self_stop + 7 at panic.c:87:2
    23095: 0xffffffff81342787    jmp    0xffffffff81342785        ; <+5> [inlined] rep_nop at processor.h:13:2
```

It used to be in an infinite loop where the decoder never stopped.

Besides that, the dump info command shows

```
(lldb) thread trace dump info 12

 Errors:
    Number of individual errors: 32
      Number of fatal errors: 1
      Number of other errors: 31
```

and in json format

```
(lldb) thread trace dump info 12 -j

 "errors": {
      "totalCount": 32,
      "libiptErrors": {},
      "fatalErrors": 1,
      "otherErrors": 31
    }
```

Differential Revision: https://reviews.llvm.org/D136557

lldb/include/lldb/Core/PluginManager.h

@@ -342,7 +342,8 @@ class PluginManager {
       llvm::StringRef name, llvm::StringRef description,
       TraceCreateInstanceFromBundle create_callback_from_bundle,
       TraceCreateInstanceForLiveProcess create_callback_for_live_process,
-      llvm::StringRef schema);
+      llvm::StringRef schema,
+      DebuggerInitializeCallback debugger_init_callback);
 
   static bool
   UnregisterPlugin(TraceCreateInstanceFromBundle create_callback);
@@ -487,6 +488,10 @@ class PluginManager {
       Debugger &debugger, const lldb::OptionValuePropertiesSP &properties_sp,
       ConstString description, bool is_global_property);
 
+  static bool CreateSettingForTracePlugin(
+      Debugger &debugger, const lldb::OptionValuePropertiesSP &properties_sp,
+      ConstString description, bool is_global_property);
+
   static lldb::OptionValuePropertiesSP
   GetSettingForObjectFilePlugin(Debugger &debugger, ConstString setting_name);
 

lldb/source/Core/PluginManager.cpp

@@ -1051,9 +1051,10 @@ struct TraceInstance
       llvm::StringRef name, llvm::StringRef description,
       CallbackType create_callback_from_bundle,
       TraceCreateInstanceForLiveProcess create_callback_for_live_process,
-      llvm::StringRef schema)
+      llvm::StringRef schema, DebuggerInitializeCallback debugger_init_callback)
       : PluginInstance<TraceCreateInstanceFromBundle>(
-            name, description, create_callback_from_bundle),
+            name, description, create_callback_from_bundle,
+            debugger_init_callback),
         schema(schema),
         create_callback_for_live_process(create_callback_for_live_process) {}
 
@@ -1072,10 +1073,10 @@ bool PluginManager::RegisterPlugin(
     llvm::StringRef name, llvm::StringRef description,
     TraceCreateInstanceFromBundle create_callback_from_bundle,
     TraceCreateInstanceForLiveProcess create_callback_for_live_process,
-    llvm::StringRef schema) {
+    llvm::StringRef schema, DebuggerInitializeCallback debugger_init_callback) {
   return GetTracePluginInstances().RegisterPlugin(
       name, description, create_callback_from_bundle,
-      create_callback_for_live_process, schema);
+      create_callback_for_live_process, schema, debugger_init_callback);
 }
 
 bool PluginManager::UnregisterPlugin(
@@ -1506,6 +1507,7 @@ CreateSettingForPlugin(Debugger &debugger, ConstString plugin_type_name,
 static const char *kDynamicLoaderPluginName("dynamic-loader");
 static const char *kPlatformPluginName("platform");
 static const char *kProcessPluginName("process");
+static const char *kTracePluginName("trace");
 static const char *kObjectFilePluginName("object-file");
 static const char *kSymbolFilePluginName("symbol-file");
 static const char *kJITLoaderPluginName("jit-loader");
@@ -1559,6 +1561,14 @@ bool PluginManager::CreateSettingForProcessPlugin(
                                 properties_sp, description, is_global_property);
 }
 
+bool PluginManager::CreateSettingForTracePlugin(
+    Debugger &debugger, const lldb::OptionValuePropertiesSP &properties_sp,
+    ConstString description, bool is_global_property) {
+  return CreateSettingForPlugin(debugger, ConstString(kTracePluginName),
+                                ConstString("Settings for trace plug-ins"),
+                                properties_sp, description, is_global_property);
+}
+
 lldb::OptionValuePropertiesSP
 PluginManager::GetSettingForObjectFilePlugin(Debugger &debugger,
                                              ConstString setting_name) {

lldb/source/Plugins/Trace/intel-pt/CMakeLists.txt

@@ -13,6 +13,14 @@ lldb_tablegen(TraceIntelPTCommandOptions.inc -gen-lldb-option-defs
   SOURCE TraceIntelPTOptions.td
   TARGET TraceIntelPTOptionsGen)
 
+lldb_tablegen(TraceIntelPTProperties.inc -gen-lldb-property-defs
+  SOURCE TraceIntelPTProperties.td
+  TARGET TraceIntelPTPropertiesGen)
+
+lldb_tablegen(TraceIntelPTPropertiesEnum.inc -gen-lldb-property-enum-defs
+  SOURCE TraceIntelPTProperties.td
+  TARGET TraceIntelPTPropertiesEnumGen)
+
 add_lldb_library(lldbPluginTraceIntelPT PLUGIN
   CommandObjectTraceStartIntelPT.cpp
   DecodedThread.cpp
@@ -38,4 +46,7 @@ add_lldb_library(lldbPluginTraceIntelPT PLUGIN
   )
 
 
-add_dependencies(lldbPluginTraceIntelPT TraceIntelPTOptionsGen)
+add_dependencies(lldbPluginTraceIntelPT
+  TraceIntelPTOptionsGen
+  TraceIntelPTPropertiesGen
+  TraceIntelPTPropertiesEnumGen)

lldb/source/Plugins/Trace/intel-pt/DecodedThread.cpp

@@ -170,34 +170,36 @@ DecodedThread::GetNanosecondsRangeByIndex(uint64_t item_index) {
   return prev(next_it)->second;
 }
 
+uint64_t DecodedThread::GetTotalInstructionCount() const {
+  return m_insn_count;
+}
+
 void DecodedThread::AppendEvent(lldb::TraceEvent event) {
   CreateNewTraceItem(lldb::eTraceItemKindEvent).event = event;
   m_events_stats.RecordEvent(event);
 }
 
 void DecodedThread::AppendInstruction(const pt_insn &insn) {
   CreateNewTraceItem(lldb::eTraceItemKindInstruction).load_address = insn.ip;
+  m_insn_count++;
 }
 
 void DecodedThread::AppendError(const IntelPTError &error) {
   CreateNewTraceItem(lldb::eTraceItemKindError).error =
       ConstString(error.message()).AsCString();
+  m_error_stats.RecordError(/*fatal=*/false);
 }
 
-void DecodedThread::AppendCustomError(StringRef err) {
+void DecodedThread::AppendCustomError(StringRef err, bool fatal) {
   CreateNewTraceItem(lldb::eTraceItemKindError).error =
       ConstString(err).AsCString();
+  m_error_stats.RecordError(fatal);
 }
 
 lldb::TraceEvent DecodedThread::GetEventByIndex(int item_index) const {
   return m_item_data[item_index].event;
 }
 
-void DecodedThread::LibiptErrorsStats::RecordError(int libipt_error_code) {
-  libipt_errors_counts[pt_errstr(pt_errcode(libipt_error_code))]++;
-  total_count++;
-}
-
 const DecodedThread::EventsStats &DecodedThread::GetEventsStats() const {
   return m_events_stats;
 }
@@ -207,6 +209,29 @@ void DecodedThread::EventsStats::RecordEvent(lldb::TraceEvent event) {
   total_count++;
 }
 
+uint64_t DecodedThread::ErrorStats::GetTotalCount() const {
+  uint64_t total = 0;
+  for (const auto &[kind, count] : libipt_errors)
+    total += count;
+
+  return total + other_errors + fatal_errors;
+}
+
+void DecodedThread::ErrorStats::RecordError(bool fatal) {
+  if (fatal)
+    fatal_errors++;
+  else
+    other_errors++;
+}
+
+void DecodedThread::ErrorStats::RecordError(int libipt_error_code) {
+  libipt_errors[pt_errstr(pt_errcode(libipt_error_code))]++;
+}
+
+const DecodedThread::ErrorStats &DecodedThread::GetErrorStats() const {
+  return m_error_stats;
+}
+
 lldb::TraceItemKind
 DecodedThread::GetItemKindByIndex(uint64_t item_index) const {
   return static_cast<lldb::TraceItemKind>(m_item_kinds[item_index]);

lldb/source/Plugins/Trace/intel-pt/DecodedThread.h

@@ -61,15 +61,6 @@ class DecodedThread : public std::enable_shared_from_this<DecodedThread> {
 public:
   using TSC = uint64_t;
 
-  // Struct holding counts for libipts errors;
-  struct LibiptErrorsStats {
-    // libipt error -> count
-    llvm::DenseMap<const char *, int> libipt_errors_counts;
-    size_t total_count = 0;
-
-    void RecordError(int libipt_error_code);
-  };
-
   /// A structure that represents a maximal range of trace items associated to
   /// the same TSC value.
   struct TSCRange {
@@ -125,16 +116,38 @@ class DecodedThread : public std::enable_shared_from_this<DecodedThread> {
     bool InRange(uint64_t item_index) const;
   };
 
-  // Struct holding counts for events;
+  // Struct holding counts for events
   struct EventsStats {
     /// A count for each individual event kind. We use an unordered map instead
     /// of a DenseMap because DenseMap can't understand enums.
-    std::unordered_map<lldb::TraceEvent, size_t> events_counts;
-    size_t total_count = 0;
+    ///
+    /// Note: We can't use DenseMap because lldb::TraceEvent is not
+    /// automatically handled correctly by DenseMap. We'd need to implement a
+    /// custom DenseMapInfo struct for TraceEvent and that's a bit too much for
+    /// such a simple structure.
+    std::unordered_map<lldb::TraceEvent, uint64_t> events_counts;
+    uint64_t total_count = 0;
 
     void RecordEvent(lldb::TraceEvent event);
   };
 
+  // Struct holding counts for errors
+  struct ErrorStats {
+    /// The following counters are mutually exclusive
+    /// \{
+    uint64_t other_errors = 0;
+    uint64_t fatal_errors = 0;
+    // libipt error -> count
+    llvm::DenseMap<const char *, uint64_t> libipt_errors;
+    /// \}
+
+    uint64_t GetTotalCount() const;
+
+    void RecordError(int libipt_error_code);
+
+    void RecordError(bool fatal);
+  };
+
   DecodedThread(
       lldb::ThreadSP thread_sp,
       const llvm::Optional<LinuxPerfZeroTscConversion> &tsc_conversion);
@@ -194,12 +207,22 @@ class DecodedThread : public std::enable_shared_from_this<DecodedThread> {
   ///     The load address of the instruction at the given index.
   lldb::addr_t GetInstructionLoadAddress(uint64_t item_index) const;
 
+  /// \return
+  ///     The number of instructions in this trace (not trace items).
+  uint64_t GetTotalInstructionCount() const;
+
   /// Return an object with statistics of the trace events that happened.
   ///
   /// \return
   ///   The stats object of all the events.
   const EventsStats &GetEventsStats() const;
 
+  /// Return an object with statistics of the trace errors that happened.
+  ///
+  /// \return
+  ///   The stats object of all the events.
+  const ErrorStats &GetErrorStats() const;
+
   /// The approximate size in bytes used by this instance,
   /// including all the already decoded instructions.
   size_t CalculateApproximateMemoryUsage() const;
@@ -221,7 +244,14 @@ class DecodedThread : public std::enable_shared_from_this<DecodedThread> {
   void AppendError(const IntelPTError &error);
 
   /// Append a custom decoding.
-  void AppendCustomError(llvm::StringRef error);
+  ///
+  /// \param[in] error
+  ///   The error message.
+  ///
+  /// \param[in] fatal
+  ///   If \b true, then the whole decoded thread should be discarded because a
+  ///   fatal anomaly has been found.
+  void AppendCustomError(llvm::StringRef error, bool fatal = false);
 
   /// Append an event.
   void AppendEvent(lldb::TraceEvent);
@@ -289,10 +319,16 @@ class DecodedThread : public std::enable_shared_from_this<DecodedThread> {
   /// TSC -> nanos conversion utility.
   llvm::Optional<LinuxPerfZeroTscConversion> m_tsc_conversion;
 
+  /// Statistics of all tracing errors.
+  ErrorStats m_error_stats;
+
   /// Statistics of all tracing events.
   EventsStats m_events_stats;
   /// Total amount of time spent decoding.
   std::chrono::milliseconds m_total_decoding_time{0};
+
+  /// Total number of instructions in the trace.
+  uint64_t m_insn_count = 0;
 };
 
 using DecodedThreadSP = std::shared_ptr<DecodedThread>;