[lldb] [Process/NetBSD] Fix segfault when handling watchpoint

[mgorny]

Summary

Fix the watchpoint/breakpoint code to search for matching thread entry
in m_threads explicitly rather than assuming that it will be present
at specified index. The previous code segfault since it wrongly assumed
that the index will match LWP ID which was incorrect even for a single
thread (where index was 0 and LWP ID was 1).

While fixing that off-by-one error would help for this specific task,
I believe it is better to be explicit in what we are searching for.

[mgorny]

Ping.

[labath]

LGTM. I don't know what Kamil is up to these days. It might be better to have him review patches like this, but if he's unable to do that, I'm happy to stamp them too...

[mgorny]

I'm going to ask Kamil to stamp those patches anyway but I always appreciate your advice wrt LLDB coding style and general integration. After all, we all want LLDB codebase to be more unified.

[krytarowski]

lldb/source/Plugins/Process/NetBSD/NativeProcessNetBSD.cpp

250 ↗

(On Diff #206528)

We should always have a fallback and whenever we register an event for an unknown thread, we shall create it in place. There are potential races in the kernel that might lead to this order of reporting events. This is less likely for DB Registers, but still we shall be prepared for it just in case.

I'm fine to leave this for later once we will handle debuggees with multiple threads.

lldb/trunk/source/Plugins/Process/NetBSD/NativeProcessNetBSD.cpp

case TRAP_EXEC: {
     // Let our delegate know we have just exec'd.
     NotifyDidExec();
 
     for (const auto &thread : m_threads)
       static_cast<NativeThreadNetBSD &>(*thread).SetStoppedByExec();
     SetState(StateType::eStateStopped, true);
   } break;
   case TRAP_DBREG: {
+    // Find the thread.
+    NativeThreadNetBSD* thread = nullptr;
+    for (const auto &t : m_threads) {
+      if (t->GetID() == info.psi_lwpid) {
+        thread = static_cast<NativeThreadNetBSD *>(t.get());
+        break;
+      }
+    }
+    if (!thread) {
+      LLDB_LOG(log,
+               "thread not found in m_threads, pid = {0}, LWP = {1}",
+               GetID(), info.psi_lwpid);
+      break;
+    }
+
     // If a watchpoint was hit, report it
-    uint32_t wp_index;
-    Status error = static_cast<NativeThreadNetBSD &>(*m_threads[info.psi_lwpid])
-                       .GetRegisterContext()
-                       .GetWatchpointHitIndex(
-                           wp_index, (uintptr_t)info.psi_siginfo.si_addr);
+    uint32_t wp_index = LLDB_INVALID_INDEX32;
+    Status error = thread->GetRegisterContext().GetWatchpointHitIndex(
     if (error.Fail())
       LLDB_LOG(log,
                "received error while checking for watchpoint hits, pid = "
                "{0}, LWP = {1}, error = {2}",
                GetID(), info.psi_lwpid, error);
     if (wp_index != LLDB_INVALID_INDEX32) {
       for (const auto &thread : m_threads)
         static_cast<NativeThreadNetBSD &>(*thread).SetStoppedByWatchpoint(
             wp_index);
       SetState(StateType::eStateStopped, true);
       break;
     }
 
     // If a breakpoint was hit, report it
-    uint32_t bp_index;
-    error = static_cast<NativeThreadNetBSD &>(*m_threads[info.psi_lwpid])
-                .GetRegisterContext()
-                .GetHardwareBreakHitIndex(bp_index,
-                                           (uintptr_t)info.psi_siginfo.si_addr);
+    uint32_t bp_index = LLDB_INVALID_INDEX32;
+    error = thread->GetRegisterContext().GetHardwareBreakHitIndex(
+        bp_index, (uintptr_t)info.psi_siginfo.si_addr);
     if (error.Fail())
       LLDB_LOG(log,
                "received error while checking for hardware "
                "breakpoint hits, pid = {0}, LWP = {1}, error = {2}",
                GetID(), info.psi_lwpid, error);
     if (bp_index != LLDB_INVALID_INDEX32) {
       for (const auto &thread : m_threads)
         static_cast<NativeThreadNetBSD &>(*thread).SetStoppedByBreakpoint();